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Nature Ecoogy & Evoution

nature ecology & evolution

https://doi.org/10.1038/s41559-024-02420-wArticle

African elephants address one another with

individually specific name-like calls

Michael A. Pardo 

, Kurt Fristrup 

, David S. Lolchuragi

, Joyce H. Poole

Petter Granli

, Cynthia Moss

, Iain Douglas-Hamilton

& George Wittemyer 

1,3

Personal names are a universal feature of human language, yet few analogues

exist in other species. While dolphins and parrots address conspecics by

imitating the calls of the addressee, human names are not imitations of

the sounds typically made by the named individual. Labelling objects or

individuals without relying on imitation of the sounds made by the referent

radically expands the expressive power of language. Thus, if non-imitative

name analogues were found in other species, this could have important

implications for our understanding of language evolution. Here we present

evidence that wild African elephants address one another with individually

specic calls, probably without relying on imitation of the receiver. We

used machine learning to demonstrate that the receiver of a call could be

predicted from the call’s acoustic structure, regardless of how similar the

call was to the receiver’s vocalizations. Moreover, elephants dierentially

responded to playbacks of calls originally addressed to them relative to calls

addressed to a dierent individual. Our ndings oer evidence for individual

addressing of conspecics in elephants. They further suggest that, unlike

other non-human animals, elephants probably do not rely on imitation of

the receiver’s calls to address one another.

A hallmark of spoken human language is the use of vocal labels: learned

sounds that refer to an object or individual (the ‘referent’)

. Many spe-

cies produce functionally referential calls for food and predators

2,3

, but

the production of these calls is typically innate

. Learned vocal labels

expand the expressive scope of communication by making it possible to

establish labels for new referents. Thus, they increase the sophistication

of cooperative behaviour and are central to humans’ ability to articulate

symbolic thought

. Personal names are a type of vocal label that refers to

another individual. Names must involve vocal learning, as an individual

cannot be born knowing the names for all its future social affiliates.

Thus, non-human analogues of personal names are highly relevant

to understanding the evolution of language and complex cognition.

Most human words, including names, are arbitrary: they are not

imitations of sounds typically made by the referent or tied to its physi-

cal properties

. Arbitrariness is crucial to language because it enables

communication about referents that do not make any imitable sound.

However, clear evidence for arbitrary names in other species is lacking.

Bottlenose dolphins (Tursiops truncatus) and orange-fronted parakeets

(Eupsittula canicularis) address individual conspecifics by imitating the

receiver’s ‘signature’ call, a sound that is most commonly produced by

the receiver to broadcast their identity

7,8

. While considered arbitrary

when used for self-identification

, it may be argued that copied signa-

ture calls used to address the call’s owner are iconic (non-arbitrary)

labels since they are an imitation of a sound most often produced by

the individual to whom the call refers. Non-imitative learned vocal

labelling may be more cognitively demanding than imitative labelling,

as it requires individuals to make an abstract connection between a

sound and referent. Evidence that arbitrary vocal labelling is not unique

to humans would expand the breadth of models for the evolution of

language and cognition.

Received: 24 October 2023

Accepted: 22 April 2024

Published online: xx xx xxxx

Check for updates

Department of Fish, Wildlife, and Conservation Biology, Colorado State University, Fort Collins, CO, USA.

Department of Electronic and Computer

Engineering, Colorado State University, Fort Collins, CO, USA.

Save The Elephants, Nairobi, Kenya.

ElephantVoices, Sandejord, Norway.

Amboseli Elephant Research Project, Nairobi, Kenya. e-mail: map385@cornell.edu

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

call pairs with same receiver, 179 pairs with different receivers, χ

= 13.0,

P = 0.0003, partial η

= 0.063) (Fig. 1 and Extended Data Table 4). This

indicates that rumbles contain information specific to the individual

receiver, not merely to the caller or to the type of relationship between

the caller and receiver (Table 1, hypothesis 1, prediction 2).

Vocal labels more likely in certain contexts and

age classes

For 87.4% of calls, receiver ID was predicted consistently correctly or

consistently incorrectly across >95% of random forest iterations. We

used logistic regression to assess factors influencing the probability

of correct classification. Contact (n = 138, 42.0% correct) and caregiv-

ing rumbles (n = 62, 46.8% correct) were more likely to be correctly

classified than greeting rumbles (n = 127, 3.9% correct) (care/contact:

P = 0.264, odds ratio 6.4; care/greeting: P = 0.014, odds ratio 48.9;

contact/greeting: P = 0.047, odds ratio 7.6) (Extended Data Table 5).

Calls from adult females (n = 274, 32.8% correct) were more likely to

be predicted correctly than calls from juveniles (n = 53, 3.8% correct)

(χ

= 6.5, P = 0.011, odds ratio 0.067). Calls that occurred later in the

bout were more likely to be predicted correctly (χ

= 3.8, P = 0.0498,

odds ratio 2.8), as were calls addressed to receivers with more total

calls in our dataset (χ

= 7.6, P = 0.006, odds ratio 1.4).

No evidence for imitation of receiver in vocal

labels

Elephants are not known to produce discrete ‘signature’ calls like dol-

phins and parrots; instead, the caller specificity of elephant rumbles

is probably a product of voice characteristics

12,13

. If elephants address

individual receivers by imitating the receiver’s voice, they should sound

more like the receiver when addressing her than when addressing other

individuals. Among the calls for which we had recordings of the receiver

and recordings of the caller addressing other individuals (n = 236),

59.7% were divergent from the receiver’s calls; that is, less similar to the

Elephants are among the few mammals capable of mimicking

novel sounds, although the function of this vocal learning ability is

unknown

10,11

. The most common elephant call type is the rumble, a har-

monically rich, low-frequency sound that is individually distinct

12,13

and

distinguishable

and is produced across most behavioural contexts

Contact rumbles are long-distance calls produced when the caller is

out of sight and more than ~50 m from one or more social affiliates and

attempting to reinitiate contact. Greeting rumbles are affiliative calls

produced when one individual approaches another to within touching

distance

. Caregiving rumbles are affiliative calls produced by an adult

or adolescent female while suckling, comforting or rousing a calf

In this Article, we analysed contact, greeting and caregiving rum-

bles from female–offspring groups of wild African savannah elephants

(Loxodonta africana) to assess whether they contain individual vocal

labels. We investigated (1) if elephants address conspecifics using

receiver-specific vocal labels, (2) if the labels are imitative of the receiv-

er’s calls or arbitrary, (3) if different callers share the same label for

the same receiver and (4) if playbacks to the assumed receiver elicit

behavioural responses indicating label recognition (Table 1).

For contact calls, we defined the receiver as the only adult mem-

ber of the family group separated (>50 m) from the caller or the only

individual who responded to the call by vocalizing or approaching.

For greeting calls, the receiver was the individual who approached or

was approached by the caller. For caregiving calls, the receiver was

the calf being suckled, comforted or roused by the caller. We excluded

calls with uncertain or multiple recipients. Given the complexity of

elephant vocalizations, it was not clear what acoustic features were

optimal for capturing the relevant variation in the calls. Thus, we ran

models separately for two different sets of features measured on each

call (spectral and cepstral; Extended Data Fig. 1 and Extended Data

Table 1). The results reported in the text and figures are for the spectral

features (see tables for cepstral results, which were similar).

Calls were specific to individual receivers

We ran a random forest

with sevenfold cross-validation to predict the

receiver of each rumble as a function of the acoustic features. Call struc-

ture varied with the identity of the targeted receiver (Extended Data

Figs. 2 and 3) as expected if elephants vocally label other individuals.

Our model correctly identified the receiver for 27.5% of calls analysed,

a significantly greater proportion than achieved by models with ran-

domly permuted acoustic features (permutation test, mean ± standard

deviation (s.d.) accuracy for 10,000 permuted models: 8.0 ± 0.66%

correct, one-tailed P < 0.0001) (Fig. 1 and Extended Data Table 2). This

indicated that receivers of calls could be correctly identified from call

structure statistically significantly better than chance (Table 1, hypoth-

esis 1, prediction 1).

As caller ID and receiver ID were partially aliased in our dataset

(Supplementary Table 1), the random forest could theoretically use

acoustic cues to caller ID

to predict receiver ID, even if the calls did

not contain any vocal label. To assess this possibility, we compared the

mean similarity of pairs of calls with the same caller and receiver to the

mean similarity of pairs of calls with the same caller and different receiv-

ers, using proximity scores derived from the random forest as a metric

of call similarity

. If the random forest relied entirely on cues to caller ID

to predict receiver ID, there should be no difference in proximity score

between ‘same caller/same receiver’ pairs and ‘same caller/different

receivers’ pairs. To control for the possibility that calls were specific to

the type of relationship between the caller and receiver rather than to

individual receivers, we categorized social relationship on the basis of

relatedness and age (Extended Data Table 3) and only considered pairs

of calls with the same type of relationship between caller and receiver.

Calls with the same caller and receiver were significantly more similar

(higher proximity scores) than calls with the same caller and different

receivers, even after controlling for social relationship, behavioural

context and recording date (rank-transformed linear model, n = 1,105

Table 1 | Hypotheses and predictions tested in this study and

whether they were supported

Hypotheses Predictions Supported?

1. Elephants vocally

label individual

conspeciics

1. Receiver ID can be predicted from

call structure

1. Yes

2. Calls with same caller and same

receiver will be more similar than

calls with same caller and different

receivers, while controlling for

caller–receiver relationship type

2. Yes

3. Elephants will respond more

strongly to playback of call originally

addressed to them than to playback

of call from same caller originally

addressed to another individual

3. Yes

2. Vocal labels

are arbitrary (not

imitative of receiver’s

calls)

1. Receiver can be predicted from

call structure regardless of whether

calls are convergent or divergent

from receiver’s calls relative to other

calls by the same caller

1. Yes

2. Calls from caller A to receiver B

will be no more similar to receiver B’s

calls than calls from caller A to other

receivers are to receiver B’s calls

2. Yes

3. Different callers

use same label for

same receiver

1. Calls with different callers and

same receiver will be more similar

than calls with different callers and

different receivers

1. Yes

2. Receiver ID can be predicted

from call structure independently of

caller ID

2. No

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

receiver’s calls than typical for that caller. The random forest’s predic-

tion accuracy was significantly better than baseline expectations for

both convergent and divergent calls (Table 1, hypothesis 2, prediction 1)

(permutation test; convergent calls: 20.1% correct, permuted models

mean ± s.d. accuracy of 7.7 ± 1.3%, n = 95 calls, one-tailed P < 0.0001;

divergent calls: 32.6% correct, permuted models mean ± s.d. accuracy

of 17.9 ± 1.6%, n = 141 calls, one-tailed P < 0.0001) (Fig. 2 and Extended

Data Table 2).

Proximity scores for pairs of calls in which the receiver of one

call made the other call were marginally higher than for pairs in

which this was not the case, but this was not statistically significant

(rank-transformed linear model, n = 943 call pairs where receiver of

one call made the other call, 1,553 pairs where this was not the case,

= 3.7, P = 0.056, partial η

= 0.001) (Fig. 2 and Extended Data Table 6).

This suggests that calls addressed to a given receiver were no more con-

vergent with the receiver’s calls than with calls from other individuals

(Table 1, hypothesis 2, prediction 2). Collectively, the evidence suggests

that vocal labelling in elephants probably does not rely on imitation

of the receiver’s calls. However, a definitive conclusion about the role

of imitation will require exhaustively sampling the vocal repertoire

of each caller.

Mixed evidence for shared labels across callers

In humans and bottlenose dolphins, different callers generally use

the same label for a given receiver. To determine if elephants do the

same, we further examined call proximity scores. Calls from differ-

ent callers to the same receiver were significantly more similar than

calls from different callers to different receivers (Table 1, hypothesis

3, prediction 1) (rank-transformed linear model, n = 693 call pairs with

same receiver, 7,522 pairs with different receivers, χ

= 10.7, two-tailed

P = 0.001, partial η

= 0.004) (Fig. 3 and Extended Data Table 7). This

suggests that there was some vocal convergence among different call-

ers addressing the same receiver.

We then ran a random forest structured to predict receiver ID

from different callers than the model was trained on (n = 437 calls)

(Table 1, hypothesis 3, prediction 2). This model correctly classified

1.1% of calls, no better than the corresponding models with randomly

permuted acoustic features (permutation test, mean ± s.d. accuracy of

permuted models 1.4 ± 0.33% correct, one-tailed P = 0.896) (Fig. 3 and

Extended Data Table 2). Therefore, the random forest was not able to

predict receiver ID independently of caller ID, suggesting convergence

across callers was weak.

Playback confirms receiver recognition of vocal

labels

To determine if elephants perceive and respond to the vocal labels

in calls addressed to them (Table 1, hypothesis 1, prediction 3), we

compared reactions of 17 wild elephants to playback of a call that was

originally addressed to them (test) relative to playback of a call from

the same caller that was originally addressed to a different individual

(control). By using test and control stimuli from the same caller, we

controlled for the possibility of the caller’s relationship to the subject

influencing the results. To control for the possibility that calls were spe-

cific to the type of relationship between the caller and receiver rather

than to the individual receiver, we included the type of relationship

between the caller and the original receiver as a factor in the analysis.

Further supporting the existence of vocal labels, subjects approached

the speaker more quickly (Cox regression, χ

= 6.8, P = 0.009, hazards

ratio 8.77), vocalized more quickly (Cox regression, χ

= 7.9, P = 0.005,

hazards ratio 7.45) and produced more vocalizations (Poisson regres-

sion, χ

= 6.7, P = 0.009, rate ratio 2.41) in response to test playbacks

than control playbacks (Fig. 4 and Table 2). In trials where an approach

or vocalization occurred, the mean ± s.d. latency to the first approach

or vocalization was 99.7 ± 161.4 s.

Discussion and conclusions

Very few species are known to address conspecifics with vocal labels.

Our discovery of individual vocal labels in a species that diverged from

both the primate and cetacean lineages ~90–100 million years ago

provides an important opportunity to study the convergent evolu-

tion of unusually sophisticated communication

. Moreover, where

evidence for vocal labels has been found in non-human species, they are

either clearly imitative

7,8

or of unknown structure

19–21

. Our data suggest

that elephants may label conspecifics without relying on imitation of

the receiver’s calls, a phenomenon previously known to occur only in

human language. If further research supports the absence of receiver

imitation in elephant vocal labels, then investigating the social context,

acoustic structure and ontogeny of vocal labels in elephants may shed

light on why elephants and humans developed non-imitative vocal

labels in contrast to other species known to vocally label conspecifics.

Our results also have significant implications for elephant cognition,

as inventing or learning sounds to address one another suggests the

capacity for some degree of symbolic thought.

The existence of individual vocal labelling in elephants is sup-

ported by multiple lines of evidence that exclude simpler alternative

explanations. Receiver ID could be predicted from call structure sig-

nificantly better than chance. Moreover, analysis of random forest

proximity scores showed that calls from the same caller to the same

receiver were significantly more similar than calls from the same caller

to two different receivers who had the same type of relationship with

the caller. This ruled out the alternative explanations that call structure

predicted receiver ID because of the correlation between caller ID and

receiver ID in our dataset or that call structure reflected only the type

of relationship between caller and receiver and not the individual

0.275

500

1,000

1,500

0.1 0.2

Classification accuracy

Same caller pair type

0.3

Frequency

500

1,000

1,500

Same caller

same receiver

Same caller

dierent receivers

Rank-transformed proximity score

Fig. 1 | Evidence that calls are specific to individual receivers within a caller.

Left: the classification accuracy of a random forest predicting receiver ID from

acoustic features (red line) was significantly higher than the classification

accuracies of 10,000 models predicting receiver ID from randomized acoustic

features (black histogram) (n = 437 calls, permutation test, one-tailed

P = 0.0000). Cross-validation folds were stratified so that the model was

trained and tested on the same combinations of caller and receiver; thus, the

classification accuracy represents the receiver specificity of calls within a caller.

Right: calls with the same caller and same receiver were significantly more similar

(higher proximity score) than calls with the same caller and different receivers

who had the same type of relationship to the caller (n = 1,105 call pairs with same

receiver, 179 pairs with different receivers, ANOVA on ranks, χ

= 13.0, d.f. 1, two-

tailed P = 0.0003, partial η

= 0.063). Boxplot centre lines, medians; box limits,

25th and 75th quantiles; whiskers, 1.5× interquartile range.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

identity of the receiver. We also controlled for behavioural context and

recording date in the proximity score analysis, ensuring that receiver

specificity was not an artefact of context-related cues or autocorrela-

tion among calls from the same day. The results did not change when

two individuals that accounted for a disproportionate number of calls

in the dataset (M6 and M6.99) were excluded, indicating that our results

were not driven by a few highly influential individuals (Supplementary

Information). Most importantly, elephants responded more strongly

to playback of calls addressed to them than to playback of calls from

the same caller addressed to a different receiver, indicating that the

calls contained receiver-specific information that was salient to the

elephants. The difference in response to test and control trials was

often pronounced. For example, subject R26 vocalized eight times and

approached the speaker in response to the test playback but vocalized

only once and did not approach the speaker in response to the control

playback. Only one subject exhibited an unambiguously stronger

response to the control playback than to the test playback. These

results are particularly notable in that we could not be certain that all

playback stimuli contained vocal labels.

The social behaviour and ecology of elephants create an environ-

ment in which individual vocal labelling may be particularly advanta-

geous. Elephants maintain lifelong differentiated social bonds with

many individuals, and due to their fission–fusion social dynamics are

often separated from their closely bonded social partners

22,23

. In contact

calls, where the caller and receiver are separated, vocal labels probably

allow elephants to attract the attention of a specific distant receiver.

In close-distance calls such as greeting and caregiving rumbles, vocal

labels may help strengthen social bonds, similar to the way in which

humans experience a positive affective response and increased willing-

ness to cooperate when someone remembers their name

Our random forest model correctly predicted receiver ID for

slightly over a quarter of calls (albeit significantly better than ran-

dom), suggesting that vocal labels may not be necessary in all or even

most contexts. Indeed, both humans and bottlenose dolphins only use

individual vocal labels (that is, names or imitated signature whistles) in

a small percentage of utterances

. We found that receiver ID was more

likely to be correctly predicted for contact and caregiving rumbles

than for greeting rumbles, which suggests that vocal labels may be

used more in the former two contexts. Vocally identifying the intended

receiver seems especially likely to be beneficial in contact calls, where

the caller and receiver are out of visual and tactile contact. It is some-

what surprising, however, that caregiving rumbles were more likely to

be correctly classified than greeting rumbles, as both are close-distance

affiliative calls. Perhaps labels are included in caregiving rumbles to

help calves learn the labels with which others address them or because

hearing the label is comforting for calves. Calls made by adult females

were also more likely to be correctly classified than calls made by

juveniles. This suggests that adult females may use vocal labels more

than calves, possibly because the behaviour takes years to develop.

Elephant rumbles are highly complex and simultaneously encode

multiple messages, including but not limited to caller identity, age,

sex, emotional state and behavioural context

12,15,26,27

. The top acoustic

features for predicting receiver ID were not those that explained the

most variation in the calls (Supplementary Discussion), suggesting that

0.201

1,000

2,000

3,000

0.05 0.10 0.15 0.20

0.326

500

1,000

1,500

2,000

Frequency

Rank-transformed proximity score

0.1 0.2 0.3

Classification accuracy

Imitation pair type

Convergent calls

Divergent calls

1,000

2,000

3,000

Call A receiver

is Call B caller

Call A receiver

not Call B caller

Fig. 2 | Evidence that vocal labelling probably did not rely on imitation of the

receiver’s calls. Random forest predicted receiver ID significantly better than

models with randomly permuted features both among calls that were identified

as convergent to the receiver’s calls (top left) (n = 95 calls, permutation test,

one-tailed P = 0.0000) and divergent from the receiver’s calls (bottom left)

(n = 141 calls, permutation test, one-tailed P = 0.0000). The red lines represent

classification accuracy of the original random forest model, and the black

histograms represent the distribution of classification accuracies of null models

with randomized acoustic features. Right: pairs of calls in which the receiver of

one call made the other call did not differ significantly in mean proximity score

from pairs of calls in which the receiver of one call did not make the other call

(n = 943 call pairs where receiver of one call made the other call, 1,553 pairs where

this was not the case, ANOVA on ranks, χ

= 3.7, d.f. 1, P = 0.056, partial η

= 0.001).

Boxplot centre lines, medians; box limits, 25th and 75th quantiles; whiskers, 1.5×

interquartile range.

2,500

5,000

7,500

10,000

Dierent callers

same receiver

Dierent callers

dierent receivers

Dierent caller pair type

Classification accuracy

Rank-transformed proximity score

Frequency

0.011

1,000

2,000

3,000

0.01 0.02 0.03

Predicting receiver across

callers

Fig. 3 | Mixed evidence that different callers use similar labels for the same

receiver. Left: pairs of calls with different callers and the same receiver were

significantly more similar (higher proximity score) than pairs of calls with

different callers and different receivers, indicating some convergence among

callers addressing the same receiver (n = 693 call pairs with same receiver,

7,522 pairs with different receivers, ANOVA on ranks, χ

= 10.7, d.f. 1, two-tailed

P = 0.001, partial η

= 0.004). Boxplot centre lines, medians; box limits, 25th and

75th quantiles; whiskers, 1.5× interquartile range. Right: classification accuracy

(red line) of random forest designed to predict receiver ID from acoustic features

independently of caller ID (all calls with the same caller and receiver allocated to

the same cross-validation fold) was not significantly different from classification

accuracies of models with randomized acoustic features (black histogram),

indicating that receiver ID could not be predicted independently of caller ID

(n = 437 calls, permutation test, one-tailed P = 0.896). The fact that elephant calls

contain multiple messages and are structurally highly complex may account for

the model’s poor generalization of receiver ID across callers.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

vocal labels account for only a small fraction of the total variation in

rumbles. This appears to contrast with human names, in which the vocal

label accounts for most of the acoustic variation in the signal, even

though information such as the identity, age, sex and emotional state

of the speaker is also encoded in the speaker’s voice characteristics

Whereas human language conveys complex messages via sequential

encoding of information, elephants may rely more on simultaneous

encoding, packing more information into a single vocalization than

humans typically do.

The richness in the information content of elephant vocaliza-

tions makes it difficult to identify the specific acoustic parameters

that encode receiver ID, although the variable importance scores from

the random forest suggest possible candidate features (Supplementary

Discussion). Unlike dolphin and parrot signature calls

20,25,29

, elephant

vocal labels cannot be discerned by visual inspection of the spectrogram

and are probably encoded by a complex and subtle interaction among

many acoustic parameters. As a result, we employed machine learning

in this analysis, but innovative approaches in signal processing may

be necessary to isolate the aspects of rumbles encoding vocal labels.

We found mixed support for the hypothesis that different callers

use the same label to address the same receiver. While the random

forest failed to predict receiver ID independently of caller ID, analysis

of proximity scores indicated at least some convergence among differ-

ent callers addressing the same receiver. It is possible that all callers

within a family group use the same label for the same receiver and the

poor performance of the random forest was due to limitations of our

data. The dense information content and high variability of rumbles

coupled with the small number of calls per receiver in our dataset may

have prevented the random forest from learning cues to receiver ID

that generalized across callers. Moreover, as the acoustic features we

extracted were based on the mel frequency scale, which was inspired by

human vocal tract models

, it is possible that they provided peripheral

measures of the principal modes of label encoding. Acoustic features

more closely tailored to the properties of the elephant vocal tract might

result in a higher classification accuracy for receiver ID.

Alternatively, it is possible that callers only partially share labels

for a given receiver. Such a system would greatly increase the number

of labels that elephants need to understand, although partial overlap

in the labels addressed to a given receiver could mitigate the difficulty

of this task. Nonetheless, partial convergence among labels might be

favoured if it is easier for receivers to learn to respond to multiple labels

than it is for callers to learn to produce the exact same label for a given

100 200 300

Seconds after playback

400 500 600

0.2

0.4

Cumulative probability of approach

0.6

100 200 300

Seconds after playback

400 500 600

0.2

0.4

Cumulative probability of call

Mean number of vocalizations

0.6

0.8

Test

Control

Test Control

Treatment

Test Control

Fig. 4 | Response to playbacks of test stimuli (calls originally addressed to

the subject) versus control stimuli (calls from the same caller originally

addressed to a different individual). Left: subjects approached the speaker

more quickly (n = 17 individuals, Cox regression, χ

= 6.8, d.f. 1, two-tailed

P = 0.009, hazards ratio 8.77) in response to test playbacks than controls.

Centre: subjects vocalized more quickly in response to test playbacks than

controls (n = 17 individuals, Cox regression, χ

= 7.9, d.f. 1, two-tailed P = 0.005,

hazards ratio 7.45). Right: subjects produced more vocalizations in response to

test playbacks than controls (n = 17 individuals, Poisson generalized linear model,

= 6.7, d.f. 1, two-tailed P = 0.009, hazards ratio 2.41). The shaded areas in the left

and centre panels represent 95% confidence intervals around survival curves.

Boxplot centre line, median; box limits, 25th and 75th quantiles; whiskers, 1.5×

interquartile range; grey squares, location of outliers; black circles, all individual

data points. The median and the 25th quantile of the control box are both 0. No

corrections were done for multiple comparisons as the analyses presented in this

figure were three distinct models with different response variables.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

receiver. This seems possible, as modifying the structure of calls based

on auditory experience (vocal production learning) requires more spe-

cialized neural circuitry than modifying the context in which calls are

produced (usage learning)

. Spectacled parrotlets (Forpus conspicil-

latus) and budgerigars (Melopsittacus undulatus) reportedly address

individual conspecifics with vocal labels that are not shared across call-

ers

19,20

, although this could reflect imperfect imitation of the receiver’s

calls rather than discrete ‘nicknames’

. Further work to identify how

vocal labels are encoded in elephant calls is necessary to determine to

what degree different callers use the same label for the same receiver.

Isolating the labels for individual elephants will allow investigation of

questions such as whether elephants understand the labels used by

third parties or even refer to third parties in their absence.

Both African and Asian elephants have a demonstrated capacity

for vocal mimicry in captivity, but no study has documented a function

of this ability in the wild

10,11

. Depending on whether callers share labels

for the same receiver, vocal labelling in elephants could rely on either

vocal production learning or vocal innovation combined with usage

learning. However, given the evidence for partial convergence among

callers, it seems likely that production learning is involved. Dolphins

and parrots, which show evidence for individual vocal addressing

via imitation of the receiver, are adept vocal learners. Another vocal

learner, the Egyptian fruit bat (Rousettus aegyptiacus), produces calls

that are specific to individual receivers and may be vocal labels as well,

although it is currently unknown if the bats perceive this information

Humans, dolphins, parrots, bats and elephants all form long-term

social bonds and live in groups with a high degree of fission–fusion

dynamics

22,32–35

. A mechanism to direct communication to individual

conspecifics could be especially beneficial for animals that frequently

separate and rejoin with bonded social partners. This raises the possibil-

ity that social selection pressures creating a need to address individual

conspecifics may have led to multiple independent origins of vocal

production learning, a precursor for language.

The use of learned arbitrary labels is part of what gives human

language its uniquely broad range of expression

. Our results sug-

gesting possible use of arbitrary vocal labels in elephants provide an

opportunity to investigate the selection pressures that may have led

to the evolution of this rare ability in two divergent lineages. Moreo-

ver, these findings raise intriguing questions about the complexity

of elephant social cognition, considering the potential relevance of

symbolic communication to their social decision-making.

Methods

Field recording

We collected audio recordings of wild female–calf groups in Amboseli

National Park, Kenya in 1986–1990 and 1997–2006 and Samburu and

Buffalo Springs National Reserves (hereafter, Samburu), Kenya in

November 2019 to March 2020 and June 2021 to April 2022. Both

populations have been continuously monitored for decades, and all

individuals can be individually identified by external ear morphol-

ogy

22,36

. We recorded calls from a vehicle during daylight hours with

all-occurrence sampling

using an Earthworks QTC1 microphone

(4 Hz to 40 kHz ± 1 dB) with a Nagra IV-SJ reel-to-reel tape recorder or

an HHB PDR 1000 DAT recorder in Amboseli, and an Earthworks QTC40

microphone (3 Hz to 40 kHz ± 1 dB) with a Sound Devices MixPre3 or

MixPre3-II digital recorder in Samburu. Recordings were recorded at

a 48 kHz sampling rate with 16 bits of amplitude resolution and stored

at 2 kHz in Amboseli and recorded and stored at 44.1 kHz with 24 or

32 bits of amplitude resolution in Samburu.

When possible, we recorded for each call the identity of the caller,

the behavioural context and the identity of the receiver (criteria for

identifying receiver defined in the main text). The caller was identi-

fied using behavioural and contextual cues, such as an open mouth,

flapping ears or being the only individual of the right age class in the

immediate vicinity (calls made by young calves are audibly shorter

and higher pitched than adult calls)

. Behavioural observations were

recorded by a single observer at each field site (M.A.P. in Samburu,

J.H.P. in Amboseli). Since the observations at each field site were con-

ducted without accompanying video in most cases, there was no way

to calculate inter-observer reliability.

Scoring behavioural context

For this study, we only used rumbles produced in the contexts of ‘con-

tact calling’, ‘greeting’ and ‘caregiving’, as these are the contexts in

which vocal labelling seems most likely to be beneficial

. We did not

include rumbles from other behavioural contexts as these typically

either involve multiple simultaneous receivers (for example, ‘let’s go’

rumbles) or occur in contexts where vocal labelling is less likely to be

necessary (for example, ‘begging’, ‘protest’, ‘oestrus’ and ‘musth’ rum-

bles)

. Nonetheless, there was a great deal of variation in the precise

social context surrounding the production of each call and the age and

internal state of the callers. As elephant rumbles vary with behavioural

context, age and the emotional state of the caller

12,15,27

, this contextual

Table 2 | Results for type III analyses of deviance on playback experiment models

Response

variable (model

type)

Subject ID

(s.d.)

Treatment

(d.f. 1)

Relationship of

caller to original

receiver (d.f. 4)

Distance (d.f. 1) dBC (d.f. 1) Other adults

(d.f. 1)

Speaker

location

(d.f. 1)

Cumulative

playback

exposure (d.f. 1)

Latency to

approach (Cox)

3.43 χ

=6.8,

P=0.009,

RR 8.77

=1.7,

P=0.80

=2.4,

P=0.12,

RR 0.79

=0.65,

P=0.42,

RR 1.38

=0.41,

P=0.52,

RR 3.13

=0.59,

P=0.44,

RR 4.62

=0.11,

P=0.73,

RR 0.88

Latency to

vocalize (Cox)

2.84 χ

=7.9,

P=0.005,

RR 7.45

=6.4,

P=0.17

=0.97,

P=0.32,

RR 0.87

=0.02,

P=0.90,

RR 0.96

=0.64,

P=0.42,

RR 3.25

=0.20,

P=0.66,

RR 2.02

=0.10,

P=0.75,

RR 0.91

Number of

calls (Poisson)

– χ

=6.7,

P=0.009,

RR 2.41

=20.2,

P=0.0005

=0.32,

P=0.57,

RR 0.98

=0.54,

P=0.46,

RR 1.09

=0.72,

P=0.40,

RR 1.54

=0.13,

P=0.72,

RR 0.84

=0.01,

P=0.91,

RR 0.99

Latency to

vigilance (Cox)

0.02 χ

=3.1,

P=0.08,

RR 2.07

=10.1,

P=0.038

=1.8,

P=0.18,

RR 0.93

=1.9,

P=0.16,

RR 0.84

=5.5,

P=0.019,

RR 4.24

=0.55,

P=0.46,

RR 0.64

=0.02,

P=0.88,

RR 0.99

Vigilance

duration after–

before (linear)

9.95 χ

=0.06,

P=0.81,

β=1.70

=2.1,

P=0.72

=4.0,

P=0.045,

β=−1.98

=0.02,

P=0.89,

β=−0.30

=0.43,

P=0.51,

β=7.58

=0.33,

P=0.56,

β=6.68

=0.83,

P=0.36,

β=−1.73

Subject ID was included as a random effect in all models except the Poisson regression for number of calls, because it had a variance of 0 for this model. Values in the ‘Subject ID’ column

represent the square root of the variance explained by that random effect. Signiicant P values are in bold. Latency to vigilance exhibited a non-signiicant trend towards faster onset of vigilance

in response to test playbacks. In addition to the d.f., χ

statistic and two-tailed P value from the analysis of deviance, this table includes the hazard or rate ratios (RR) for the Cox and Poisson

models and the estimated slope parameters (β) for the linear model. Ratios and slopes are not shown for relationship of caller to original receiver, as this covariate had more than two levels.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

heterogeneity of the recordings probably added substantial noise to

the data.

Following published methodology

, we defined contact rumbles

as calls produced by or addressed to an individual who was separated

from the receiver by >~50 m and apparently attempting to reinitiate

contact. Our category of ‘greeting’ rumbles encompasses two different

categories distinguished by Poole

: ‘little-greeting’ and ‘greeting’. Both

call types are produced when one individual approaches another in an

affiliative manner, but Poole’s ‘greeting rumbles’ are produced after a

greater period of separation than ‘little-greeting rumbles’, are more

likely to involve a face-to-face approach and typically involve greater

emotive behaviour such as temporal gland streaming and pirouetting

to stand in parallel

. The context of ‘caregiving’ in our study is primarily

synonymous with ‘coo rumbles’ described by Poole

, which are rumbles

produced by adult or adolescent females to a calf when gently touch-

ing or suckling the calf or in an apparent attempt to reassure a calf who

exhibited distress (for example, being pushed by another elephant,

being separated from its mother and so on). We also included in this

category two calls from adult females attempting to rouse a calf who

was sleeping when the group began to move off.

Scoring certainty of caller ID, behavioural context and

receiver ID

In Samburu, we recorded the certainty with which we knew caller ID,

behavioural context and receiver ID as 1 over the number of possible

alternatives

. For example, in cases where we thought the call was

plausibly addressed to a single individual but there were two possible

candidates for who the receiver was, we designated one of the two indi-

viduals as the putative receiver and assigned the certainty of receiver

ID a value of 0.5. In Amboseli, certainty of caller ID and behavioural

context were scored as ‘certain’, ‘fairly confident’, ‘educated guess’ or

‘no idea’. The certainty of receiver ID was not systematically recorded

in Amboseli, but sometimes the field notes specified that the receiver

ID was uncertain.

Call selection

For all analyses in this paper, we only used rumbles with the highest pos-

sible certainty for receiver ID (that is, certainty of 1 for Samburu calls,

no notes indicating uncertain receiver ID for Amboseli calls). We also

required rumbles to have the first two formants clearly visible in the

spectrogram with no significant overlap with other calls or loud sounds

in the same frequency range. This dataset consisted of 469 calls, 101

unique callers and 117 unique receivers, with 1–36 (median 2) calls per

caller, 1–40 (median 2) calls per receiver, 1–7 (median 2) receivers per

caller and 1–7 (median 1) callers per receiver (Supplementary Table 1).

There were 32 calls for which the receiver ID was certain but the

caller ID was not. We used these calls in the random forest model that

was used to generate the proximity score matrix and the conditional

inference forest used to calculate variable importance scores for pre-

dicting receiver ID, as caller ID was irrelevant to these models. However,

for all other analyses, including the linear mixed models with proximity

score as a response variable, we only used calls where the caller ID was

known for certain (certainty of 1 for Samburu, ‘certain’ for Amboseli).

For analyses that examined behavioural context (linear mixed

models, logistic regression), we required the certainty of behavioural

context to be 1 in Samburu or ‘certain’ in Amboseli. For analyses that did

not explicitly include behavioural context, we also included calls with

uncertain contexts as long as the only possible options were contact,

greeting or caregiving.

Call segmentation

In Amboseli, we wrote down the elapsed time on the recorder and

contextual information for each call heard in the field; in Samburu, we

recorded verbal annotations onto a second channel of the recorder in

real time using a Martel Stenomask, which isolated the sound of the

observer’s voice from the Earthworks microphone

. We manually drew

a selection box around the spectrogram of each call in Raven Pro 1.5

(Cornell Lab of Ornithology, Ithaca, NY), with a buffer of approximately

1 s on either side of the call (Samburu (44.1 kHz sampling rate): Hann

window, 50% overlap, window 11,878 samples, Discrete Fourier Trans-

form 16,384 samples; Amboseli (2 kHz sampling rate): Hann window,

50% overlap, window 312 samples, Discrete Fourier Transform 512 sam-

ples). This automatically generated a selection table in .txt format with

the file name and start and end times of each selection box, to which

we added caller ID, receiver, ID, behavioural context and the certainty

of each. We performed all further acoustic and statistical analyses in

R version 4.1.3 (ref. 39).

To determine the precise onset and offset of each call, we low-pass

filtered the calls (Butterworth filter, order 5, cut-off 490 Hz), downsam-

pled them to 2,000 Hz if not already at that sampling rate, applied a

high-pass filter (Butterworth filter, order 10, cut-off 30 Hz) and normal-

ized them to 70% of max amplitude and 16 bits of amplitude resolution

using the packages seewave

and tuneR

. We then used the function

segment() in the package soundgen

to detect the onset and offset of

each call based on the amplitude envelope. We verified the automati-

cally detected start and end time for each call by visual inspection of

the amplitude envelope and spectrogram and manually adjusted the

times when necessary.

Acoustic measurements

We trimmed the original unfiltered sound clips to the automatically

detected start and end times, low-pass filtered the clips (Butterworth

filter, order 5, cut-off 800 Hz), downsampled them to 2,000 Hz if not

already at that sampling rate, applied a high-pass filter (Butterworth

filter, order 2, cut-off 4 Hz) and finally normalized them to 70% of the

max amplitude and 16 bits of amplitude resolution. For each call, we

measured the smoothed Hilbert amplitude envelope (moving average

window, window length 350 ms, overlap 90%) and two alternative sets

of features: normalized mel spectrogram and mel-frequency cepstral

coefficients (MFCCs).

A mel spectrogram is similar to a traditional spectrogram (raster

plot with time on the x axis, frequency on the y axis, and amplitude indi-

cated by pixel darkness) but with frequency transformed to the loga-

rithmic mel scale

. While the mel scale was designed to approximate

human hearing sensitivity, most other mammals, including elephants,

perceive frequency on a similar logarithmic scale

. We calculated a mel

spectrogram for each call using the audspec() function of the tuneR

package (26 mel-frequency bands between 0 Hz and 500 Hz, 350 ms

Hamming window, 90% overlap). We then normalized the mel spectro-

gram by dividing the energy value in each cell of the spectrogram by

its column sum so that the energies would be a proportion of the total

energy in each time window, and logit-transformed these proportional

energies so the values would not be limited between 0 and 1. We also

calculated delta and delta–delta values for each mel spectral band,

with delta values being the differences between successive energy

values in the mel spectral band (that is, the change in energy over

time within a mel spectral band) and delta–delta values being the dif-

ferences between successive delta values (that is, the acceleration of

energy over time within a mel spectral band) (Extended Data Fig. 1). We

saved the vector of energies in each mel spectral band and their corre-

sponding delta and delta–delta values as acoustic contours for further

processing. While mel spectral bands have not previously been used as

acoustic features for analysing elephant calls, they describe more of the

variation in the call than commonly used features such as fundamental

frequency and formants, while remaining easily interpretable.

We also calculated MFCCs for each call, which are less interpretable

than mel spectral bands but have been previously used successfully to

classify elephant vocalizations

13,27,44

. MFCCs are calculated by applying

a discrete cosine transform to each time window of a mel spectro-

gram, with the coefficients of the discrete cosine transform being the

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cepstral coefficients

. Each cepstral coefficient can be thought of as

representing the degree of modulation of the spectrum at a different

period, with lower numbered coefficients representing slower periods

of modulation. Since MFCCs are calculated for each time window of

the mel spectrogram, the output is a vector of values for each cepstral

coefficient. We calculated MFCCs using the melfcc() function in the

tuneR package, with a time window of 350 ms with 90% overlap, 40

mel-frequency bands between 0 Hz and 500 Hz, and a pre-emphasis

filter with a cut-off frequency of 10 Hz, and kept the first 12 coefficients

(12 vectors per call) for further processing. We also calculated delta

and delta–delta values for the first 12 cepstral coefficient contours.

Extraction of derived features from acoustic contours

We extracted derived acoustic features separately for the spectral

acoustic contours + amplitude envelope and the cepstral acoustic

countours + amplitude envelope. We rescaled each set of acoustic con-

tours by arranging them in a matrix with each contour in a separate row,

and then subtracting the column median from each value and dividing

the result by the column mean average deviation. We decorrelated

the contours with robust principal components analysis in the rpca

package in R, which separates the data into a low-rank matrix of robust

principal components without outliers, and a sparse matrix containing

the outlier values (λ = 0.00996)

. Robust principal component analysis

(PCA) has the advantage over standard PCA of being more resilient to

noisy data. We extracted four measurements from the sparse matrix to

use for statistical analysis: median, robust skewness and two measures

of spread: minimum extent and equivalent statistical extent. We also

calculated the means of the first n low-rank principal components

required to explain 99.9% of the variation (74 for spectral features, 12

for cepstral features).

We used multi-taper spectral estimation

to derive the frequency

spectra of the low-rank principal components that explained 99.9% of

the variation (treating each principal component as if it were a wave-

form) and calculated an F ratio for each point in each spectrum, test-

ing the null hypothesis that the spectral value in question could have

been derived from a random waveform. We calculated the mean of

the F ratios at each point across the aligned spectra and selected the

four largest peaks in the series of mean F ratios. We sorted these peaks

in order of increasing frequency and calculated the frequency and

magnitude of each peak.

We calculated the same metrics on spectra that were weighted

according to the proportion of variation that was explained by the

principal component from which the spectrum was derived. We mul-

tiplied the F ratios in each of the spectra by the proportion of variation

in the data explained by the principal component in question, summed

the weighted F ratios at each point in the aligned spectra and then

calculated the frequencies and magnitudes of the four largest peaks

in the summed F ratios, sorted in order of increasing frequency. The

final acoustic features used in our models are summarized in Extended

Data Table 1. We ran all subsequent statistical analyses separately for

the spectral and cepstral acoustic features.

Statistical analysis of acoustic data

Unless otherwise specified, all statistical tests were two-tailed and all

measurements were taken from distinct samples. The significance

level was set to 0.05 for all tests. We used partial η

as a measure of

effect size for linear models, calculated according to the formula

partial η

SSE

−SSE

SSE

, where SSE

is the sum of the variances for all the

error terms (random effects and residual error) in the full model and

SSE

is the sum of the variances for all the error terms in the same model

minus the fixed effect of interest

. For all regression models, we calcu-

lated P values for the fixed effects using type III analysis of deviance.

Are calls speciic to individual receivers (hypothesis 1)? We ran a sev-

enfold cross-validated random forest model in the R package ranger

to predict the identity of the receiver of each call (receiver ID) as a

function of the acoustic features (Table 1, hypothesis 1, prediction 1).

We stratified the cross-validation folds by caller ID and receiver ID to

ensure as even a distribution as possible of all caller–receiver dyads

across all folds. Thus, if calls contain acoustic cues to receiver ID, this

model was expected to predict receiver ID better than chance regard-

less of whether the label for a given receiver is shared across callers

(Table 1, hypothesis 1, prediction 1). We only used calls where caller ID

was known for certain (n = 437 calls). The model used 500 trees, 6 vari-

ables per node, 60% of observations per tree, a minimum node size of

1 and no maximum tree depth. To increase the stability of the model’s

classification accuracy, we ran the model 2,000 times and used the

mean classification accuracy across the 2,000 runs. To determine if

the model predicted receiver ID better than expected by chance, we

ran the model 10,000 times with the acoustic features randomly per-

muted and compared the classification accuracy of the original model

(averaged across 2,000 runs) with the null distribution of classification

accuracies generated by the 10,000 models with randomized acoustic

features (one-tailed permutation test).

To disentangle the effects of caller ID and receiver ID on call struc-

ture, we compared the mean pairwise similarities between pairs of calls

with the same caller and receiver and pairs with the same caller and

different receivers (same caller pair type). As a metric of call similarity,

we extracted a proximity score for each pairwise combination of calls

from a random forest trained to predict receiver ID as a function of the

acoustic features on the full dataset (469 training observations, 8,000

trees, other hyperparameters same as above). The proximity score for

a given pair of calls was the proportion of trees in which both calls were

classified in the same terminal node, corrected for the size of each node

and represented the degree of similarity between the two calls in terms

of the acoustic features most relevant to predicting receiver ID

. If calls

are specific to individual receivers within a given caller, then pairs of

calls with the same caller and same receiver should be more similar

(have higher proximity scores) than pairs of calls with the same caller

and different receivers (Table 1, hypothesis 1, prediction 2).

Previous work has shown that elephants alter the structure of their

rumbles when interacting with more dominant conspecifics

. To rule

out the possibility that calls were specific to the type of relationship

between caller and receiver rather than to individual receivers per se,

we restricted the analysis of same caller pair type to pairs of calls that

had the same type of relationship between caller and receiver. We

defined the caller–receiver relationship using 12 categories based on

sex, family group membership, relative age and mother–offspring

relationship, reflecting the fact that dominance in elephants is primar-

ily determined by age

50,51

and that mother–calf bonds are the strongest

social bonds in elephants

22,52

(Extended Data Table 3). As calls from

different behavioural contexts differ in acoustic structure

, we cat-

egorized each pair of calls according to whether the two calls had the

same or different behavioural contexts (‘same context’) and included

this variable as a factor in the analysis. We also included a binary factor

indicating whether the two calls were recorded on the same date, as

exploratory analyses indicated that calls recorded on the same date

were more similar than calls recorded on different dates. We only used

calls in this model for which the caller ID and behavioural context were

known for certain.

The proximity scores were highly skewed to the right, so

we rank-transformed them and ran a linear mixed model with

rank-transformed proximity score as the response variable and same

caller pair type, same context and same date as fixed effects. To account

for the fact that there were multiple call pairs with the same combina-

tion of callers and receivers, we included ‘pair ID’ (a unique identifier

for each caller–receiver–caller–receiver combination) as a random

effect. We excluded pair IDs with only one observation as it was not

possible to estimate within-class variability for these pair IDs (final

n = 1,284 call pairs).

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Article https://doi.org/10.1038/s41559-024-02420-w

Which calls are most likely to contain vocal labels? Vocal labels might

be more likely to occur in certain behavioural contexts than others.

Similarly, callers may only use a vocal label in some of the calls within a

bout, as it would be redundant to include the same information in all the

calls. To assess whether behavioural context or position within a bout

influenced the likelihood of a call containing a vocal label, we calculated

the proportion of the 2,000 iterations of the random forest in which the

receiver ID was correctly predicted for each call (probability of correct

classification). We designated calls that were correctly predicted in

≥95% of iterations as ‘correct’ and calls that were correctly predicted in

≤5% of iterations as ‘incorrect’ and excluded all calls that did not meet

these criteria, as well as all calls with uncertain caller ID or behavioural

context, and receivers that occurred only once after applying the previ-

ous criteria (n = 327). Then, we ran a mixed-effects logistic regression

with prediction outcome (1 or 0) as the response, receiver ID as a random

effect, and behavioural context, caller age class, position within the bout

and the total number of calls addressed to the receiver in question as

fixed effects. The latter effect was included because receivers with more

calls in our dataset were expected to be predicted with greater accuracy,

as there were more training opportunities for the random forest to learn

them. Caller age class was defined as juvenile (<10 years old for females,

not yet dispersed from natal group for males) or adult (>10 years old for

females). There were no adult male callers in our dataset. We defined

a bout as calls produced by the same caller within the same sound file

with no more than 30 s between successive calls.

Are vocal labels based on imitation of the receiver’s calls

(hypothesis 2)? To assess whether imitation of the receiver’s calls was

necessary for vocal labelling, we examined the calls in the dataset for

which we had at least one recording of the receiver’s calls and at least

one recording of the caller addressing someone other than the receiver

(n = 236 calls). For each of these calls, we calculated its mean proximity

score to all the calls made by the receiver (mean proximity to targeted

receiver). We also calculated the mean proximity score between the

same caller and receiver when the caller was addressing other individu-

als (mean proximity when targeting others). Calls in which the mean

proximity to targeted receiver was greater than the mean proximity

when targeting others were classified as ‘convergent’ (n = 95) and diver-

gent otherwise (n = 141). We then examined the proportion of conver-

gent and divergent calls that were classified correctly by the random

forest model with receiver ID and the acoustic features as input vari-

ables, and cross-validation folds stratified by caller ID and receiver ID.

If vocal labelling relies on imitation of the receiver’s calls, we expected

only the convergent calls to be classified correctly more often than by

the null model, but if imitation is not necessary for vocal labelling, we

expected both convergent and divergent calls to be classified correctly

more often than by the null model (Table 1, hypothesis 2, prediction 1).

If elephants imitate the calls of the receiver that they are addressing,

then callers should sound more like a given conspecific when they are

addressing her than when they are addressing someone else (Table 1,

hypothesis 2, prediction 2). To assess whether this was the case, we clas-

sified each pair of calls into one of two types (hereafter, ‘imitation pair

type’): pairs in which the receiver of one call was the caller of the other

call, and pairs in which this was not the case. We separately classified each

call pair according to whether the two calls had the same relationship

between caller and receiver (hereafter, ‘same relationship’). We also cre

ated a categorical variable caller dyad ID, which was an identifier for each

unique combination of callers that composed a call pair. We ran a linear

mixed model with rank-transformed proximity score as the response

variable, imitation pair type, same relationship, same context and same

date as fixed effects, and caller dyad ID and pair ID as random effects.

By including caller dyad ID as a random effect, we assessed the effect

of imitation pair type within a given pair of callers, that is, whether calls

from caller A to receiver B were more similar to receiver B’s calls than

calls from caller A addressed to other receivers were to receiver B’s calls.

We excluded pairs of calls with the same caller or receiver, uncertain caller

ID or behavioural context for either call, that were recorded from different

family groups, for which caller dyad ID did not occur with both levels of

imitation pair type, or for which pair ID occurred only once (n = 2,360 call

pairs). Pairs of calls from different family groups were excluded because

they comprised a small percentage of pairs where the receiver of one call

was the caller of the other, and because it is possible that different families

have different vocal signatures, which would influence call similarity.

Do different callers use the same label for the same receiver

(hypothesis 3)? If different callers use similar labels for the same

receiver, then pairs of calls with different callers and the same receiver

should be more similar than pairs of calls with different callers and dif-

ferent receivers (Table 1, hypothesis 3, prediction 1). To test whether this

was the case, we ran another linear mixed model with rank-transformed

proximity score as the response variable, different caller pair type (dif-

ferent callers/same receiver or different callers/different receivers),

same relationship and same context as fixed effects, and pair ID as a

random effect. As before, we excluded calls with uncertain caller ID

or behavioural context, pairs of calls recorded from different family

groups, and levels of pair ID that occurred only once (n = 8,215 call pairs).

To determine if receiver ID could be predicted independently of

caller ID, which would be possible only if callers use similar labels for

a given receiver, (Table 1, hypothesis 3, prediction 2), we ran another

sevenfold cross-validated random forest model to predict receiver ID as

a function of the acoustic features but partitioned the cross-validation

folds such that all calls with the same caller and receiver were always

allocated to the same fold (observations and hyperparameters same as

first model). We averaged the classification accuracy of the model across

2,000 runs and compared this value with the distribution of classifica-

tion accuracies generated by 10,000 iterations of the same model with

the acoustic features randomly permuted (one-tailed permutation test).

Checking model assumptions. For all rank-transformed linear mixed

models, we checked the assumption of normality by visually examin-

ing histograms of the residuals. We checked the assumption of equal

variances by visually examining boxplots of all groups. The residuals

for all models exhibited only minor deviations from normality, with

the absolute values of skewness and excess kurtosis being less than 1

for all models. As linear models have been shown to be robust even to

severe deviations from normality with skewness as high as 2 and excess

kurtosis as high as 6 (a normal distribution has a skewness of 0 and

excess kurtosis of 0)

, we deemed the choice of model appropriate.

Boxplots indicated similar variances across groups.

How are labels encoded in calls? To investigate which acoustic fea-

tures encode receiver ID and caller ID, we extracted variable importance

scores (Supplementary Table 2) from a conditional inference random

forest model in the R package ‘party’

trained on the full dataset to

predict the response variable in question (receiver ID or caller ID)

as a function of the acoustic features (469 training observations for

receiver ID, 437 for caller ID; 1,000 trees; all other hyperparameters

same as other random forests). We used a conditional inference forest

because, unlike traditional random forest, it is not biased towards cor-

related variables

. We only calculated variable importance scores for

the spectral features, as cepstral coefficients are difficult to interpret

intuitively. To assess the relative importance of the original acoustic

contours, we weighted the loadings of the acoustic contours on each

principal component by the variable importance score of the mean

of the principal component in question and then calculated the sum

of the absolute values of these weighted loadings for each acoustic

contour (Supplementary Table 3). Acoustic contours with a higher

sum of the absolute values of the weighted loadings were deemed

more important. This weighting process only considered the means

of low-rank principal components.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

Playback experimental design

To determine if elephants respond more strongly to calls addressed to

them (Table 1, hypothesis 1, prediction 3), we played back rumbles with

known adult (>10-year-old) female callers and known receivers to 17

elephants (15 adult females, one 9-year-old female, one 9–10-year-old

male) in the Samburu study area. Fourteen subjects received one ‘test’

playback of a call that was originally addressed to them and one ‘control’

playback of a call from the same caller that was originally addressed to

another individual. One subject received two sets of test and control

playbacks from two different callers, one received only a test playback,

and one received only a control playback (Supplementary Table 4). Most

stimuli functioned as the test stimulus for one subject and the control

stimulus for another, but no stimulus was used as the same experimental

condition for more than one subject. The order of presentation was

balanced across subjects, and we waited at least 7 days (mean ± s.d.,

29.5 ± 27.1 days) between successive playbacks to the same subject.

Playback stimuli

Playback stimuli were recorded in Samburu and Buffalo Springs

between January 2020 and March 2022 from adult female callers. In

all but two cases, the playback stimuli were contact calls. In one case

we used a loud greeting call (similar in original amplitude to a typical

contact call but produced at a much closer distance), and in one case

we used a call that was produced in a similar context to contact calls

(caller and receiver >100 m apart and out of sight of each other) but was

lower in original amplitude than a typical contact call and was part of a

lengthy antiphonal exchange between two individuals and, therefore,

was probably a ‘cadenced rumble’

. These non-contact calls were used

to complete a pair of test and control stimuli because we were unable

to obtain contact calls to two different receivers from the same caller.

Three playback stimuli were elicited by another playback, and we

assumed that the individual whose call was broadcast from the speaker

was the intended receiver of the call that was produced in response to

that playback. We identified the receiver of natural calls as the only

adult member of the family group who was separated from the caller

during the call or the only individual who responded to the call. In one

case, there were two adult females separated from the caller, and we

assumed the receiver was the older of the two females who was in the

lead and who rejoined the caller first. We note that there was no mecha-

nism to ensure the playback stimulus contained a vocal label, and it is

possible not all stimuli were labelled. We prepared all playback stimuli

in Audacity 3.0.2. Each stimulus consisted of a single rumble preceded

by one second of background noise with a fade-in and followed by 1 s of

background noise with a fade-out. In three cases, we applied a high-pass

(5 Hz cut-off, 6 dB roll-off) or low-pass filter (1,000 Hz cut-off, 6 dB

roll-off) to remove excessive noise.

Playback system and volume

We played back all stimuli as .wav files (uncompressed audio) from

an iPhone SE (Apple) attached to a QLXD1 wireless bodypack trans-

mitter (Shure) transmitting to a custom-built loudspeaker (Bag End

Loudspeakers). The cord connecting the playback device to the wire-

less transmitter had to be replaced three times over the course of the

experiment, each time changing the output level of the speaker. Thus,

depending on which cord was in use, we normalized the stimuli to −24,

−22.5 or −18 dB in Audacity 3.0.2 to ensure a functionally equivalent

normalization level across all trials.

The speaker’s frequency response was flat from 10 Hz to 500 Hz up

to a given maximum output level (maximum output 89 dB sound pres-

sure level (SPL) at 10 Hz, 101 dB SPL at 20 Hz and 113 dB SPL at 40 Hz).

If the signal exceeded the maximum output at a given frequency, the

speaker automatically reduced the level of the frequencies in question

to avoid damage. Reported amplitudes for natural contact calls range

from 94 to 115 dB SPL (extrapolated value at 1 m from source)

15,55

. We

did not have access to an SPL meter with a flat frequency response

at low frequencies, but our playback stimuli ranged from 96.2 to

104.3 dBC (decibels with a C-weighting) at 1 m measured with a Protmex

PT6708 sound level meter (Protech International Group Co.) or 93.4

to 102.9 dB SPL at 1 m measured with the SoundMeter 10.5.8 iPhone

application (Faber Acoustical). Mean measured volume did not differ

between test and control stimuli (dBC: t-test, t

32.0

= 0.03, P = 0.97; dB

SPL: t-test, t

32.0

= 0.15, P = 0.88).

Playback trial protocol

We placed the speaker 40.2–59.0 m from the subject (mean 49.1 ± 4.2 m),

either on the ground in front of a tree or shrub and covered by cam-

ouflage netting or on the edge of the rear seat of a Toyota double cab

Landcruiser facing the door with all four doors and windows and both

roof hatches open. Rerecordings at 50 m revealed no obvious differ-

ence between sounds played with the speaker on the ground or inside

the vehicle. We conducted playbacks only when the original caller and

‘alternate receiver’ (the other subject receiving playbacks from the same

caller) were >180 m from and out of sight of the subject (>270 m from the

alternate receiver if she had not yet received all her playbacks). When

the original caller’s location was known (19/34 trials) the speaker was

placed in approximately the same direction relative to the subject as

the original caller. In the remaining trials, the caller could not be located

after searching a ~300 m radius around the subject. Trials were redone

after at least 7 days if the speaker malfunctioned, the subject moved her

head out of sight right before the playback started or we discovered after

the playback that the speaker was not in the correct location relative to

the subject and the original caller (Supplementary Table 4). During each

trial, we filmed the subject from inside the vehicle for at least 1 min before

the playback, then played the stimulus once and continued filming for at

least another 10 min. We also recorded audio with an Earthworks QTC40

microphone and Sound Devices MixPre3-II recorder. The observers

were blind to the playback condition (test or control) until all trials were

complete, and all videos and audio recordings were scored.

Statistical analysis of playback data

From the video and audio recordings of each playback trial, we meas-

ured the subject’s latency to approach the speaker, latency to vocalize,

number of calls produced within 10 min following the playback, latency

to vigilance and change in vigilance duration in the minute following

the playback compared with the minute preceding the playback. Laten-

cies were defined as the time from the start of the playback until the

behaviour of interest occurred and were censored when the subject

moved out of sight or at 10 min, whichever came first. Vigilance was

defined as lifting head above shoulder level, moving head from side

to side, holding ears away from body without flapping, or lifting trunk

while sniffing towards speaker

. We ran a separate model for each

response variable with subject ID as a random effect and treatment

and the following covariates/factors as fixed effects: caller–original

receiver relationship (relationship between the caller and the original

receiver of the call; Extended Data Table 3), distance (distance in metres

between the speaker and the subject), dBC (amplitude of the playback

stimulus in dBC at 1 m), other adults (whether other adults were within

50 m of subject during playback), speaker location (whether speaker

was on ground or in vehicle) and cumulative playback exposure (cumu-

lative number of playbacks to which subject was exposed at distance

of 300 m or less, including trials that were redone and playbacks to

other subjects). We used Cox proportional hazards regression in the

coxme package

for the latency variables, a generalized linear model

with a Poisson error distribution in the lme4 package

for number of

calls, and a linear model for change in vigilance duration. We applied

analysis of deviance with type III sums of squares to each model to

calculate a two-tailed P value for each fixed effect. For the Poisson

regression modelling number of calls, the random effect of subject ID

had a variance of 0, resulting in a near singular fit, so we removed the

random effect from this model.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

For the Cox regression models, we checked the assumption of

proportional hazards with a Schoenfeld test, which tests the null

hypothesis that there is no relationship between the scaled Schoen-

feld residuals and time. This test was non-significant (P > 0.05) for all

models, indicating no violation of the proportional hazards assump-

tion. For the Poisson regression model, we checked for overdispersion

using the AER package in R

. The dispersion parameter was estimated

to be 1.1, which did not differ significantly from the ideal value of 1

(P = 0.26), indicating that a Poisson distribution was appropriate. For

the linear regression model used to examine the change in vigilance

duration before versus after playbacks, visual inspection of the histo-

gram of the residuals indicated that the residuals were approximately

normally distributed. For treatment, distance, dBC, speaker location

and cumulative playback exposure, visual inspection of boxplots or

residual plots indicated approximate homoscedasticity. Relationship

of caller to original receiver and other adults were heteroscedastic.

However, regardless of whether these covariates were included, treat-

ment was not significant, so any potential issues with this model had

no bearing on the conclusions of our study.

Reporting summary

Further information on research design is available in the Nature

Portfolio Reporting Summary linked to this article.

Data availability

Data are available at https://doi.org/10.5061/dryad.hmgqnk9nj (ref. 60).

Code availability

Code is available at https://doi.org/10.5281/zenodo.10576772 (ref. 61).

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Acknowledgements

We thank the Oice of the President of Kenya, the Samburu, Isiolo

and Kajiado County governments, the Wildlife Research & Training

Institute of Kenya, and Kenya Wildlife Service for permission to

conduct ieldwork in Kenya. We thank Save The Elephants and the

Amboseli Trust for Elephants for logistical support in the ield,

J. M. Leshudukule, D. M. Letitiya and N. Njiraini for assistance with the

ieldwork, G. Pardo for blinding the playback stimuli and S. Pardo for

input on the statistical analyses. We thank J. Berger, W. Koenig and

A. Horn for comments on the manuscript. This project was funded

by a Postdoctoral Research Fellowship in Biology to M.A.P. from the

National Science Foundation (award no. 1907122) and grants to

J.H.P. and P.G. from the National Geographic Society, Care for the Wild,

and the Crystal Springs Foundation. Fieldwork was supported by Save

the Elephants.

Author contributions

M.A.P. conceived the study. M.A.P. and D.S.L. collected the data in

Samburu, and J.H.P. and P.G. collected the data in Amboseli. M.A.P.

and K.F. performed the statistical analysis, and M.A.P. created the

igures. M.A.P. drafted the manuscript, and K.F., J.H.P. and G.W. edited

it. C.M., I.D.-H. and G.W. provided resources and access to long-term

datasets, and G.W. supervised the study.

Competing interests

The authors declare no competing interests.

Additional information

Extended data is available for this paper at

https://doi.org/10.1038/s41559-024-02420-w.

Supplementary information The online version

contains supplementary material available at

https://doi.org/10.1038/s41559-024-02420-w.

Correspondence and requests for materials should be addressed to

Michael A. Pardo.

Peer review information Nature Ecology & Evolution thanks Kenna

Lehmann and the other, anonymous, reviewer(s) for their contribution

to the peer review of this work. Peer reviewer reports are available.

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terms of such publishing agreement and applicable law.

2024

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Extended Data Fig. 1 | Schematic illustrating how spectral acoustic features

were measured. First, a spectrogram was calculated by applying a Fast Fourier

Transform to the signal (Hamming window, 700 samples, 90% overlap). Then

a mel filter bank with 26 overlapping triangular filters between 0-500 Hz was

applied to each window of the spectrogram to produce a mel spectrogram. The

mel spectrogram was then normalized by dividing the energy value in each cell

by the total energy in that time window and these proportional energies were

logit-transformed so they would not be limited to between 0 and 1. As features for

the robust principal components analysis, we used the vector of energy in each of

the 26 mel frequency bands as well as the vectors of delta and delta-delta values

for each frequency band (representing the change and acceleration in energy

over time, respectively). In the spectrogram and mel spectrogram in this figure,

warmer colors indicate higher amplitudes (greater energy).

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

Extended Data Fig. 2 | Scatterplots illustrating the separation in 3D space

between calls from the same caller to different receivers. Axes are the first

three principal coordinates extracted from the proximity scores of a random

forest trained to predict receiver ID. Each plot represents a single caller, each

point is a single call, and receiver IDs are coded by both color and shape. This

figure only includes calls where caller ID was known for certain, where the call

was predicted correctly in at least 25% of random forest iterations, and where the

caller made at least two such calls each to at least two different receivers.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

Extended Data Fig. 3 | Scatterplot illustrating the clustering in 3D space

of calls from different callers to the same receiver. Axes are the first three

principal coordinates extracted from the proximity scores of a random forest

trained to predict receiver ID. Each shape represents a different receiver and each

color represents a different caller. This figure only includes calls where caller ID

was known for certain, where the call was predicted correctly in at least 25% of

random forest iterations, and where the receiver received at least one such call

each from at least two different callers.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

Extended Data Table 1 | Acoustic features used in the random forest models

All acoustic features were derived from either the sparse matrix or low-rank matrix of a robust principal components analysis performed on multiple acoustic contours of equal length that

were measured directly from the signal. For the spectral acoustic features, the acoustic contours were the Hilbert amplitude envelope, the vector of energies in each of the 26 bands of

a mel spectrogram, and the delta and delta-delta values of the mel spectral bands. For the cepstral acoustic features, the acoustic contours were the Hilbert amplitude envelope, irst 12

mel-frequency cepstral coeficients, and the delta and delta-delta values of the irst 12 cepstral coeficients. The principal components analysis was performed on a matrix of all the contours

for each call stacked end-to-end.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

Extended Data Table 2 | Results of random forest models predicting receiver ID as a function of the acoustic features

All random forests had 500 trees, 6 variables per node, 60% of observations per tree, minimum node size = 1, no maximum tree depth, and 7-fold cross-validation. Classiication accuracies

were averaged across 2000 runs of the model to improve stability. To determine if the classiication accuracy was higher than expected by chance, the model was run 10,000 times with

randomly permuted acoustic variables, and the original classiication accuracy was compared to the distribution of classiication accuracies for these 10,000 permuted models. P-values are

one-tailed.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

Extended Data Table 3 | Deinitions of social relationship categories between caller and receiver

Categories were deined based on sex, age, and mother-offspring status, the most important factors inluencing dominance and bond strength within an elephant family group. Females were

deined as adults if ≥10 years old, and males were deined as adults if independent from their natal group. All non-adults under this deinition were classiied as juveniles. Six years was chosen

as the cutoff for different age classes because it is between 1-2x the average inter-birth interval, so a female ≥6 years older than another individual could have been that individual’s allomother.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

Extended Data Table 4 | Results for linear mixed model assessing whether calls are speciic to individual receivers or the

type of relationship between caller and receiver

Each observation was a pair of calls and the response variable was rank-transformed proximity score. Same Caller Pair Type = whether the two calls in a pair had the same caller and receiver

(reference level) or same caller and different receivers with the same type of relationship to the caller; Same Context = whether the two calls in a pair had the same behavioral context

(reference level = no); Same Date = whether the two calls in a pair were recorded on the same day (reference level = no); Pair ID = unique combination of callers and receivers (random effect).

Pairs of calls recorded from different groups and levels of Pair ID that only occurred once were excluded (n=1105 call pairs with same receiver, 179 with different receivers who had the same

type of relationship to the caller). P-values are two-tailed.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

Extended Data Table 5 | Results for mixed effects logistic regression modeling the probability of a call being correctly

classiied

Odds ratios, χ

statistics, degrees of freedom, two-tailed P-values, reported for ixed effects. Standard deviations (square root of the variance explained) reported for the random effect. Odds

ratios for Context were calculated from the estimated marginal means. χ

statistics, degrees of freedom, two-tailed P-values were calculated from Type III Analysis of Deviance on the full

model. Receivers that only occurred once were excluded. Cepstral features model had warning message indicating convergence issues when Caller age class was included. Context: n=138

contact rumbles, 127 greeting rumbles, 62 caregiving rumbles. Caller age class: n=274 calls from adults, 53 juvenile calls from juveniles.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

Extended Data Table 6 | Results for linear mixed model assessing whether calls addressed to a receiver imitate the

receiver’s calls

Each observation was a pair of calls and the response variable was rank-transformed proximity score. Imitation Pair Type = whether the receiver of one call in a pair was the caller of the other

call (reference level = yes); Same Relationship = whether the callers of both calls in a pair had the same type of relationship to their respective receivers (reference level = no); Caller Dyad ID

= unique combination of callers (random effect). Same Context, Same Date, and Pair ID same as in Extended Data Table 4. Pairs of calls recorded from different groups, pairs with the same

caller or receiver, levels of Caller Dyad ID that only occurred with one level of Imitation Pair Type, and levels of Pair ID that only occurred once were excluded (n=943 call pairs where receiver

of one call was the caller of the other, 1553 where this was not the case). P-values are two-tailed.

Nature Ecoogy & Evoution

Article https://doi.org/10.1038/s41559-024-02420-w

Extended Data Table 7 | Results for linear mixed model assessing whether different callers use similar labels for same

receiver

Each observation was a pair of calls and the response variable was rank-transformed proximity score. Different Caller Pair Type = whether the two calls in a pair had different callers and the

same receiver (reference level) or different callers and different receivers; Same Relationship, Same Context, Same Date, and Pair ID same as in Extended Data Tables 4 and 6. Pairs of calls

recorded from different groups and levels of Pair ID that only occurred once were excluded (n=693 call pairs with same receiver, 7522 with different receivers). P-values are two-tailed.

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Corresponding author(s):

Michael A. Pardo

Last updated by author(s):

Apr 2, 2024

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Data analysis

Rough segmentation of calls was performed in Raven Pro 1.5 (Cornell Lab of Ornithology, Ithaca, NY, USA). All other acoustic and statistical

analyses were performed in R version 4.1.3. The following R packages were used:

AER: testing overdispersion of Poisson GLM

car: type III ANOVA

caret: data partitioning for machine learning

coxme: mixed-effects Cox regression

data.table: data wrangling

dplyr: data wrangling

emmeans: post-hoc comparisons

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multitaper: multi-taper spectral estimation (for deriving some acoustic features)

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party: conditional inference random forest (for variable importance scores)

ranger: fast random forest

robustbase: calculating robust skewness

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rsvd: robust principal components

Rraven: importing Raven Pro selection tables into R

rsvd: robust principal components analysis (for derived acoustic features)

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Ecological, evolutionary & environmental sciences study design

All studies must disclose on these points even when the disclosure is negative.

Study description

We investigated the hypothesis that elephants address individual members of their family group with name-like calls. We recorded

contact and greeting calls from wild African elephants in Samburu & Buffalo Springs National Reserves, northern Kenya and Amboseli

National Park, southern Kenya, noting when possible the identity of the caller and the identity of the receiver.

We measured a suite of acoustic features on each call (n=469 calls) and used a random forest model to show that calls could be

assigned to individual receivers based on acoustic structure with greater than chance accuracy. To determine if elephants rely on

imitation of the receiver's calls to address receiver, we examined random forest classification accuracies separately for calls that

were more similar to the receiver's calls than typical for that caller (convergent calls, n=95) and calls that were less similar to the

receiver's calls than typical for that caller (divergent calls, n=141). We found that calls could be assigned to receiver ID with greater

than chance accuracy regardless of whether they were convergent with or divergent from the receiver's calls. We calculated pairwise

proximity scores between each call in the dataset and ran an ANOVA which showed that call pairs with the same caller and same

receiver were more similar on average than call pairs with the same caller and different receivers who had the same type of

relationship with the caller. We ran a logistic regression to assess the factors influencing the probability that the random forest would

correctly predict the receiver for a call. We found that the receiver was more likely to be correctly predicted for contact rumbles and

caregiving rumbles than for greeting rumbles and more likely to be correctly predicted for adult callers than for juvenile callers. This

suggests that contact and caregiving rumbles may be more likely to contain a vocal label than greeting rumbles and adults may be

more likely than juveniles to use vocal labels.

To determine if elephants imitated the calls of the receiver they were addressing, we ran another ANOVA to test if call pairs in which

the receiver of one call produced the other call had higher proximity scores than call pairs in which this was not the case. There was

no significant difference, indicating no evidence for imiation. To determine if different callers use the same label to address a given

receiver (i.e., if calls could be assigned to receiver ID independent of caller ID), we ran a second random forest with the training and

test sets partitioned so the model was trained and tested on different callers. This random forest failed to assign calls to receiver ID

any better than chance, suggesting that different callers do not use the same label for the same receiver. However, an ANOVA

showed that call pairs with different callers and the same receiver were more similar (had higher proximity scores) on average than

call pairs with different callers and different receivers, suggesting that different callers do use similar labels for the same receiver.

Finally, we conducted a playback experiment to determine if elephants perceive and respond to putative labels in their calls. We

played 17 elephants a recording of a call that was originally addressed to them (test) and a recording of a call from the same caller

that was originally addressed to someone else (control). One subject received two different sets of test and control playbacks, one

subject received just 1 test playback (no control) and one subject received just one control playback (no test). All other subjects

received exactly one test playback and one control playback each. Subjects approached the speaker more quickly, vocalized more

quickly, and produced more vocalizations in response to test playbacks than controls, further supporting the hypothesis that calls are

specific to individual receivers.

Research sample

Subjects were wild African savannah elephants (Loxodonta africana) from two Kenyan populations: Samburu & Buffalo Springs

(northern Kenya) and Amboseli (southern Kenya). Acoustic analyses were conducted on 371 rumbles from 52 adult females, 16

juvenile females, 2 females recorded as both juveniles and adults (cutoff for adulthood was 10 years of age), and 14 juvenile males in

Samburu, as well as 98 rumbles from 13 adult females, 3 juvenile females, and 1 juvenile male in Amboseli. Playbacks were

conducted to 17 individuals in Samburu (15 adult females, 1 adolescent female, and 1 adolescent male).

Sampling strategy

Calls were recorded using all-occurrence sampling. There was no predetermined sample size as we attempted to record as many calls

as possible. Subjects for playbacks were chosen based on which individuals we were able to record a test stimulus and control

stimulus for. We did not predetermine the sample size for playbacks and instead did as many playbacks as we were able to given

what recordings were available.

Data collection

Calls were recorded during daylight hours from a vehicle using a handheld Earthworks microphone. Callers and receivers were

identified using behavioral cues, and elephants were identified individually using naturally-occurring marks on the ears and other

distinct physical features. Playbacks were conducted from 50 meters away from a loudspeaker placed on the ground or in a

Landcruiser with all the doors and windows open. Data in Samburu (recordings and playbacks) were collected by MP and DL. Data in

Amboseli (recordings) were collected by JP and PG.

Timing and spatial scale

Calls were recorded in Samburu in Nov 2019-Mar 2020 and Jun 2021-Apr 2022. Calls were recorded in Amboseli in 1986-1990 and

1997-2006. Playbacks were conducted from Oct 2021 to Apr 2022. Playbacks to the same subject were spaced apart by at least 7

days which previous studies on elephants have suggested as a rule of thumb to minimize the risk of habituation. Samburu and Buffalo

Springs National Reserves cover an area of about 296 km2 and Amboseli covers an area of about 392 km2.

Data exclusions

We only analyzed rumbles that were produced in the contexts of contact calling , greeting, and caregiving. We also only included calls

with minimal overlapping sounds, a high enough signal-to-noise ratio for the first two formants to be clearly visible in the

spectrogram. Finally, we only included calls where the identity of the receiver was known for certain and for which there was only

one receiver. For analyses involving caller ID or behavioral context, we also made sure that the identity of the caller/behavioral

context was known for certain.

Reproducibility

Due to the logistical constraints of conducting this type of experiment in the field and the time constraints of available funding, we

did not attempt to replicate the experiment.

Randomization

For the playback experiment, we attempted to conduct both a test playback and control playback to each individual (within-subjects

design), only failing to do so for 2/17 subjects. The order of presentation of test and control playbacks was balanced across subjects.

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Subjects were randomly assigned to receive the test or control playback first, with the constraint that 50% of subjects should receive

the test first and 50% should receiver the control first.

Blinding

The experimenters were blind to the condition of each playback trial until after all playback trials had been conducted and all videos

of those trials were scored. The same observer (MP) conducted the playback trials and scored the videos.

Did the study involve field work?

Yes No

Field work, collection and transport

Field conditions

The habitat of both field sites is a mixture of open grassland, bushy shrubs, and patches of woodland and permanent swamp. Both

sites are semi-arid, receiving an average of about 350 mm of rain per year with peaks in November and April. Fieldwork was

conducted in both wet and dry seasons. Average annual temperature is about 21.6 degrees Celsius in Amboseli and 24.5 degrees

Celsius in Samburu.

Location

Samburu & Buffalo Springs: (0.61 N, 37.5 E), 800-1230 m above sea level

Amboseli National Park: (2.7 S, 37.3 E), 1100-1200 m above sea level.

Access & import/export

Permits were obtained from the Wildlife Research & Training Institute (WRTI) of Kenya and the National Commission for Science,

Technology, and Innovation (NACOSTI) of Kenya, in consultation with local county governments (Samburu, Isiolo, and Kajiado

counties). Permit numbers: NACOSTI/P/19/2735, WRTI-0061-06-21, NACOSTI/P/21/14091.

Disturbance

Elephants were not physically handled as part of this study. They may have been temporarily and slightly disturbed by playback

stimuli. To minimize potential disturbance, we only played back a single call in any given trial and waited a minimum of 7 days

between playbacks to the same subjects. Subjects did not always exhibit any response to playbacks, and when they did, they

returned to baseline behavior in <10 min. The elephants in Samburu and Amboseli are habituated to research vehicles so it is unlikely

that they were disturbed in any substantial way by our presence. To avoid damage to vegetation, we only drove off road when

absolutely necessary to access the elephants and returned to an existing road as soon as possible.

Reporting for specific materials, systems and methods

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Research

Laboratory animals

This study did not involve laboratory animals.

Wild animals

This study involved wild African savannah elephants (Loxodonta africana). No elephants were captured or handled as part of this

study. We used audio recordings from 65 adult females, 19 juvenile females, and 15 juvenile males, as well as 2 females who were

considered juveniles (<10 yo) in earlier recordings and adults (>10 yo) in later recordings. Playbacks were conducted to 17 individuals

in Samburu (15 adult females, 1 adolescent female, and 1 adolescent male).

Reporting on sex

We focused on female-calf groups for this study because females and calves are much more vocal than adult males in elephants. As

most of the elephants (and all the adults) in our study were female, these results may only be applicable to females. We did not

conduct a sex-based analysis because we did not have sufficient data from males to consider them separately from females.

Field-collected samples

This study did not involve samples collected from the field (only audio and video recordings)

Ethics oversight

This study was approved by the Institutional Animal Care and Use Committee of Colorado State University (protocol #19-9229A)

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Note that full information on the approval of the study protocol must also be provided in the manuscript.

Discussion

>> "Both African and Asian elephants have a demonstrated capacity for vocal mimicry in captivity, but no study has documented a function of this ability in the wild Depending on whether callers share labels for the same receiver, vocal labelling in elephants could rely on either vocal production learning or vocal innovation combined with usage learning. However, given the evidence for partial convergence among callers, it seems likely that production learning is involved. Dolphins and parrots, which show evidence for individual vocal addressing via imitation of the receiver, are adept vocal learners. Another vocal learner, the Egyptian fruit bat (Rousettus aegyptiacus), produces calls that are specific to individual receivers and may be vocal labels as well, although it is currently unknown if the bats perceive this information. Humans, dolphins, parrots, bats and elephants all form long-term social bonds and live in groups with a high degree of fission–fusion dynamics. A mechanism to direct communication to individual conspecifics could be especially beneficial for animals that frequently separate and rejoin with bonded social partners. This raises the possibility that social selection pressures creating a need to address individual conspecifics may have led to multiple independent origins of vocal production learning, a precursor for language." This article gained quite a bit press covereage: - https://www.smithsonianmag.com/smart-news/african-elephants-may-call-each-other-by-name-180984521/ - https://www.nationalgeographic.com/premium/article/african-elephants-names-communication > "Very few species are known to address conspecifics with vocal labels. Our discovery of individual vocal labels in a species that diverged from both the primate and cetacean lineages ~90–100 million years ago provides an important opportunity to study the convergent evolution of unusually sophisticated communication." Learn more about the park and reserve. Amboseli National Park: https://en.wikipedia.org/wiki/Amboseli_National_Park Samburu: https://en.wikipedia.org/wiki/Samburu_National_Reserve >> "Contact rumbles are long-distance calls produced when the caller is out of sight and more than ~50 m from one or more social affiliates and attempting to reinitiate contact. Greeting rumbles are affiliative calls produced when one individual approaches another to within touching distance. Caregiving rumbles are affiliative calls produced by an adult or adolescent female while suckling, comforting or rousing a calf." > >"Here we present evidence that wild African elephants address one another with individually specific calls, probably without relying on imitation of the receiver. We used machine learning to demonstrate that the receiver of a call could be predicted from the call’s acoustic structure, regardless of how similar the call was to the receiver’s vocalizations. Moreover, elephants diferentially responded to playbacks of calls originally addressed to them relative to calls addressed to a different individual. Our findings offer evidence for individual addressing of conspecifics in elephants. They further suggest that, unlike other non-human animals, elephants probably do not rely on imitation of the receiver’s calls to address one another."

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