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. 2023 Jan 24;21(1):e3001939. doi: 10.1371/journal.pbio.3001939

Towards a great ape dictionary: Inexperienced humans understand common nonhuman ape gestures

Kirsty E Graham 1,*, Catherine Hobaiter 1,*
Editor: Frans B M de Waal2
PMCID: PMC9873169  PMID: 36693024

Abstract

In the comparative study of human and nonhuman communication, ape gesturing provided the first demonstrations of flexible, intentional communication outside human language. Rich repertoires of these gestures have been described in all ape species, bar one: us. Given that the majority of great ape gestural signals are shared, and their form appears biologically inherited, this creates a conundrum: Where did the ape gestures go in human communication? Here, we test human recognition and understanding of 10 of the most frequently used ape gestures. We crowdsourced data from 5,656 participants through an online game, which required them to select the meaning of chimpanzee and bonobo gestures in 20 videos. We show that humans may retain an understanding of ape gestural communication (either directly inherited or part of more general cognition), across gesture types and gesture meanings, with information on communicative context providing only a marginal improvement in success. By assessing comprehension, rather than production, we accessed part of the great ape gestural repertoire for the first time in adult humans. Cognitive access to an ancestral system of gesture appears to have been retained after our divergence from other apes, drawing deep evolutionary continuity between their communication and our own.

All nonhuman great apes share a large repertoire of gestures, but there are many challenges to observing these gestures in humans. This study, by showing people videos of ape gestures and testing their understanding, reveals that humans may share these gestures too.

Introduction

Regarded by philosophers and scientists alike as the cognitive capacity most critical to human uniqueness [1], the apparent discontinuity between human language and nonhuman communication has been argued to present an evolutionary puzzle. However, more and more research has started to unveil language’s deep phylogenetic roots: from the way other species combine signals to change the meaning (we use “meaning” in this article to refer to signal functions and Apparently Satisfactory Outcomes; [2,3]) of an utterance [4]; to their use of social inference in communication [5]; to how behavioural and social contexts seem to disambiguate signal meanings [6]. Nevertheless, many species’ communication is based on the exchange of specific, detailed information: Alarm calls, for example, can encode combinatory information on both the type and proximity of a predator [7,8]. While a rich source of information, these signals typically exist as a fixed response to stimuli, produced irrespective of a recipient’s attention or interest, or even whether a recipient is there [9]. Humans produce these types of signals too. Picking up a too-hot pan from the cooker, we might give an involuntary yelp, shake our hand, and/or make a facial grimace of pain. Any potential recipients around receive useful information from these signals: The pan is hot! But we did not yelp, shake, or grimace with the goal of communicating, we’d have done it whether someone was there or not. Language is different. We choose whether to tell someone who was out of the room to “watch out, the pan is hot.” We can use it in the absence of the stimuli that we were originally responding to. We would stop using it once our recipient indicated that they understood. We might even use it to talk to ourselves.

Fundamentally, with language, we do more than broadcast information; we intend to communicate a goal to a partner we recognise as having their own behaviour, goals, and knowledge. Human languages’ intentional nature takes it beyond sharing information: It communicates meaning [10–13]. This fundamental property is very rarely observed in other species [9,14], and when it is, it is typically restricted to one or two signals used in a highly specified way [15,16]. Nevertheless, the emergence of intentional communication through a single recent genetic leap in the human lineage remains implausible; instead, precursor abilities were likely present in the communication of our evolutionary ancestors and should be shared among modern ape species today [17].

Strikingly, great ape gestures are used in this language-like way: Rich systems of over 80 signals deployed communicate everyday goals (for example, [2,18–25]), and ape gesturing has been suggested to be an important scaffold in the evolution of human language [26,27]. Great ape repertoires show substantial overlap across species, including overlap among ape species more distantly related than chimpanzees, bonobos, and humans [28–32]. As a result, we would expect humans to retain the use of this system of ape gestural communication; but, to date, the use of naturalistic ape gestures appeared to be absent in human communication. Humans are highly gestural, deploying deictic, iconic, conventional, co-speech/co-sign, among other kinds of gestures. However, this itself is part of what makes studying gestural overlap between adult humans and other apes challenging. Gestures shared with other apes may be masked by the myriad ways that people signal with their hands and body. From pointing to pantomime, language-competent humans regularly employ gestures that accompany [33] and may even create [34] language; highly variable across cultures, they are rarely used to independently convey the core goal of the communication and do not map closely onto those employed by nonhuman apes. Unpicking gestures from the great ape repertoire in naturalistic adult human gesturing may not be impossible, but it will take a substantial collaborative effort to one day do so. In the meantime, there are other methods at our disposal. A recent study suggested that gestures from the “ape repertoire” may not be completely absent: Before language emerges, preverbal 1- to 2-year-old human infants were found to deploy over 50 gestures from the ape repertoire [35]. Given the available movements and body parts, there are well over 1,000 potential gesture forms that could be produced with the ape body, but apes only use approximately 12% of these [36]. Thus, any overlap between species is very unlikely to be trivial. Here, we provide the first test of the hypothesis that language-competent adult humans still share access to “family-typical” great ape gesture.

We employ a method regularly used in studies of nonhuman primate communication, a “play-back” experiment, in which recipient behaviour is analysed following exposure to a signal [37,38]. This type of comprehension study has historically been employed to test nonhuman species on comprehension of human language [39,40], but here we flip the paradigm to test humans on nonhuman communication. Of course, our experimental paradigm is more conventional in the human psychology literature and has the advantages that, with humans rather than nonhumans, we are able to conduct tests with untrained participants and to use text responses in this match-to-sample type paradigm. While language-competent humans seem to no longer typically produce gestures from the ape repertoire (or that these gestures may be masked by other common human-typical gesturing), the presence of a signal in an individual’s communicative repertoire can also be shown through their comprehension of it [30]. We conducted an online experiment to crowdsource whether adult human subjects understand the meaning of gestures produced by nonhuman apes. The experiment was presented in Gorilla.sc (www.gorilla.sc; [41]); a full preview and all importable sheets are available through Gorilla Open Materials (https://app.gorilla.sc/openmaterials/344409); and video data files are available at the Great Ape Dictionary on YouTube (https://tinyurl.com/greatapedictionary). Participants were randomly allocated to two conditions: those who viewed gesture videos only (Video only), and those who viewed gesture videos with a brief, one-line description of context (Context). Each video was accompanied by a simple illustration of the gesture to assist inexperienced viewers in identifying the gesture action (https://greatapedictionary.ac.uk/gesture-videos2/). From a set of 40 videos, each participant saw 20 videos with examples of ape gesture (10 chimpanzee, 10 bonobo). Videos were cut to show only the gesture, eliminating any behaviour before or after communication.

We selected the 10 most common gesture types for which we were previously able to confirm “meaning” in both chimpanzees and bonobos, determined by recipient responses that consistently satisfy the signaller [19]. Chimpanzees and bonobos are humans’ closest living relatives (we are also theirs, with the split from humans more recent than the last common ancestor shared between Pan and Gorilla; [42]). While in principle, given the overlap in gesture repertoires across all apes [28], we would predict that gorilla and orangutan gestures may also be salient to humans, the meanings for gestures in these ape species are not yet established.

Some gestures are used towards a single meaning (i.e., recipients consistently respond in the same way to that gesture), whereas others are used towards two or more meanings [2,19]. For example, the Big Loud Scratch is used to initiate grooming (meaning = “Groom me”), while Object Shake is used to initiate copulation (meaning = “Let’s have sex”), to initiate grooming (meaning = “Groom me”), and to increase distance between signaller and recipient (meaning = “Move away”). The correct meaning for a gesture video stimulus was assigned based on the specific meaning used for that instance of communication, rather than in general for that gesture type. Six of the chimpanzee and 7 of the bonobo gesture types had a single meaning, and 4 chimpanzee and 3 bonobo gestures types had multiple meanings. For these ambiguous gesture types, participants viewed one instance where the correct outcome was the primary meaning (the most common recipient response to that gesture type), and one instance where the correct outcome was the alternate meaning (the second most common recipient response), and in both cases were given the primary and alternate meanings as possible answers. Some of the gesture types, for example, Directed Push, have different primary and alternate meanings, for example, “Climb on my back” for bonobos and “Move to a new position” for chimpanzees. For these, as for ambiguous gestures, we expect participants to answer with the correct response for the specific video.

Results

A total of 17,751 people participated. We analysed n = 112,648 responses (Video only, n = 59,001; Context, n = 53,647) from n = 5,656 participants who completed the full set of videos (Video only, n = 2,962; Context, n = 2,694). Participants correctly interpreted the meanings of chimpanzee and bonobo gestures with or without additional Contextual information (Context: Success rate mean = 57.3 ± 11.9%; binomial, n = 53,647, p < 0.0001; Video only: Success rate mean = 52.1 ± 11.0%; binomial, n = 59,001, p < 0.0001) significantly higher than expected by chance (0.25). Participants were above chance across all but one (“Object shake”) gesture type (S1 Table and Fig 1).

Fig 1.

Fig 1

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The distribution of correct responses for each gesture type, with the legend differentiating ambiguous and unambiguous gesture types (see https://greatapedictionary.ac.uk/gesture-videos2/ for illustrations and video examples of each gesture type; the data underlying this figure can be found at https://doi.org/10.5281/zenodo.7347608). *BLS, Big Loud Scratch.

Across gesture types, the addition of information on behavioural Context had an, at best, marginal positive effect on participant success (full-null model comparison: X2 = 5.746, df = 2, p = 0.057; Table 1). More specifically, only the interaction between Context and Ambiguity showed any possible effect, with an again weak nonsignificant trend towards improved participant success (X2 = 2.791, df = 1, p = 0.095) where gesture meaning was classed as Ambiguous and information on Context was available (S1 Fig and Table 1). Participants showed a small but significant increase in confidence in their responses for gestures with a single correct meaning (mean = 6.05, SD = 2.16), than for Ambiguous gestures (mean = 5.88, SD = 2.31; t test: t(81,512) = −166, p = 0.049).

Table 1. Results of the glmm (estimates, standard errors, and significance tests).

Term Estimate Std. Error Chisq df P
(Intercept) 0.724 0.794 (1)
Context_Video only −0.054 0.222
Ambiguity_yes −0.757 0.706
Context_Video only: Ambiguity_yes −0.569 0.314 2.791 1 0.095
Meaning_Climb on my back(2) −0.571 0.741 5.649 7 0.581
Meaning_Give me that food 2.927 1.738
Meaning_Groom me −0.302 0.751
Meaning_Let’s be friendly −0.680 0.891
Meaning_Let’s have sex 2.168 0.922
Meaning_Move away from me 2.769 1.198
Meaning_Move into a new position −0.606 1.008
Species_chimpanzee 0.192 0.312 0.359 1 0.549
z.Trial Number(3) 0.031 0.037 0.684 1 0.408
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(1)Not indicated because of very limited interpretability.

(2)Test statistic refers to the overall effect of meaning.

(3)z-transformed; mean and standard deviation of the original trial number were 10.490 and 5.765, respectively.

Within Ambiguous gestures, where participants failed to select the correct meaning for this specific instance of communication (primary meaning), results were mixed as to whether they were more likely to select the secondary meaning for this gesture (alternate meaning, correct in other instances of use) than an incorrect meaning not associated with this gesture type (Fig 2). In 3 out of 5 gesture types, participants did not select the alternate meaning significantly above chance (S1 Table). Notably, the “Object shake” gesture is the only gesture type for which participants failed to assign either the primary or the alternate meanings.

Fig 2.

Fig 2

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These five gesture types were ambiguous, having a primary (correct in this instance of use) and an alternate (correct in other uses) meaning included in the response options. This figure shows the percentage of responses for success selecting the primary meaning, success selecting the alternate, and failure to select either meaning for a given gesture type. The data underlying this figure can be found at https://doi.org/10.5281/zenodo.7347608.

Discussion

Until now, humans have presented a problematic gap in the study of great ape gesture, with comparative observational methods limited to early development because of the feasibility of observing gesture production in humans after the onset of language [35]. By deploying a play-back method that flips the paradigm from the study of gesture production to gesture comprehension, we have accessed great ape gestural communication in adult humans for the first time. Participants were substantially above chance at assigning the “correct” meanings to chimpanzee and bonobo gestures across types, suggesting that humans may have retained their understanding of core features of a gestural system present in our last common ancestor with the Pan genus 6 to 7 million years ago [43]. This ability was present across both the functionally more fixed and the flexible gestures that are deployed with more than one meaning. Participants were highly successful at detecting the meaning for which gestures were used in the specific instance of communication that they saw. Where gestures had alternate meanings, these were also detected more often than chance in two gesture types. That our participants were able to interpret primate signals complements recent findings that suggest humans may be able to perceive affective cues in primate vocalisations [44].

The underlying mechanism that makes gestural communication comprehensible across great ape species, now including humans, remains unresolved. Humans use of gesture as intertwined with language in diverse ways makes detecting gesture types from the ape repertoire difficult. It remains unknown whether the great ape repertoire itself is biologically inherited [28], or whether apes—now including humans—share an underlying ability to produce and interpret naturally meaningful signals that are mutually understandable because of general intelligence and shared body plans and social goals, or the resemblance of gestures to the actions that they aim to elicit. These are not the only possible explanations, for example, gestures could be biologically inherited in nonhuman apes but understood by humans through other cognitive mechanisms, and we need to continue to develop innovative methods such as these video playbacks to address remaining unknowns.

Despite the importance of context in the interpretation of human communication [45] (and see [6] for bonobos), comprehension of great ape gestures was only marginally impacted by whether gestures had multiple meanings or whether participants were given the behavioural context in which the communication occurred. However, there were some gesture types for which we could not completely remove contextual information because it overlapped with gesture production, for example, the presence of food in some Mouth Stroke gestures. Future experiments with artificial stimuli may be able to test the limit of gesture comprehension by manipulating the amount and nature of information available, for example, stripping back situational context or exploring whether similar movements share a semantic core.

Our findings add a substantial new thread of evidence to the continuity of communication throughout our hominid lineage, and we propose that this novel citizen-science play-back approach will become a powerful and fruitful tool for bridging gaps in the study of comparative communication.

Materials and methods

The experiment was presented in Gorilla.sc (www.gorilla.sc); a full preview and all importable sheets are available through Gorilla Open Materials (https://app.gorilla.sc/openmaterials/344409); and video data files are available at the Great Ape Dictionary on YouTube (https://tinyurl.com/greatapedictionary). Participants were recruited using a combination of online social and traditional media. The experiment ran from 20 July 2017 to 23 October 2017. The study was given ethical approval by the University of St Andrews University Teaching and Research Ethics Committee, under code PS12558.

Participants

Each participant was asked for their year of birth to determine which version of the experiment they would be taken to, we had (a) a child-friendly game version for under-12s in which no data were collected; (b) a child-friendly experiment for 12- to 15-year-olds; and (c) an adult experiment for participants aged 16 years and over. Our two experimental cohorts were then taken to age-appropriate consent forms. Where consent was provided, we collected demographic data about age (12 to 15, 16 to 20, 21 to 30, 31 to 40, 41 to 50, 51 to 60, 61 to 70, 71+), gender (Female, Male, Other, Prefer not to say), and experience working with nonhuman primates (No, Yes (0 to 2 years), Yes (3 to 5 years), Yes (over 5 years)), as well as whether the participants had done the experiment before (Yes, No). Demographic data were used to exclude participants who were too young to consent, who had experience with nonhuman primates, and who had done the experiment before. Participants were shown a set of instructions (Gorilla Open Materials: https://app.gorilla.sc/openmaterials/344409) before beginning the experiment. The adult group’s experiment contained videos of gestures with the meaning “Let’s have sex,” while the child-friendly game version, and the adolescent group’s experiment did not.

Participant exclusions

A total of 17,538 over 15-year-olds and 213 twelve- to 15-year-olds participated. Data from the adolescent group (213), from adults who stated that they had any experience of working with nonhuman primates (480), from participants who stated that they had done the experiment before (143), and from participants who didn’t complete the full experiment were excluded from the analyses (11,259).

Design

Participants were randomly allocated to two conditions—those who viewed gesture videos only (Video only), and those who viewed gesture videos with a brief, one-line description of context (Gorilla Open Materials: https://app.gorilla.sc/openmaterials/344409). Each condition was divided into a further two groups, with each group being shown a different set of videos. We showed one example of each gesture type for both species (20 videos) to half of the participants, and a different example of each gesture type for both species to the other half (20 videos). Subgroups were split a final time into a further 4 random groups so that the position of the correct answer in each of the 4 box locations underneath the video varied among participants (Gorilla Open Materials: https://app.gorilla.sc/openmaterials/344409).

Videos were cut to show only the gesture, eliminating any behaviour before or after the signal. Each video showed the gesture once at regular speed and once in slow motion. Video lengths ranged from 7 to 33 seconds and could be watched as often as required before the answer was selected. A 500-millisecond fixation point was presented in the centre of the screen prior to each gesture video, videos were presented together with a Bonobo-bot illustration to highlight the gestural action within each video (Gorilla Open Materials: https://app.gorilla.sc/openmaterials/344409), and four possible meaning answers. The one-line descriptive text in the Context condition was presented below the video. After selecting an answer, participants were taken to a page and asked to rate their confidence in their answer using a sliding scale from not at all confident to 100% confident. At the end of the experiment, participants were provided with a numeric score, but no feedback on which questions were correctly answered.

We selected 10 gesture types for which we were previously able to confirm meaning in both chimpanzee and bonobos [19]. Gesture meanings were originally established using the Apparently Satisfactory Outcome: the response by the recipient that stopped the signaller from continuing to gesture [2,19]. Some gestures are used towards a single meaning, whereas others can be used towards two or more meanings. The correct meaning for a gesture video stimulus was assigned based on the specific meaning used for that instance of communication, rather than in general for that gesture type.

For gesture types with a single primary meaning (gestures used towards a single meaning in 80% or more of cases; see S1 Table), both clips within a species showed gesture instances that went on to achieve that meaning (although note that this outcome could not be seen on the video stimuli presented). For gesture types with multiple meanings (each meaning used in at least 30% to 80% of cases; see S1 Table), one clip showed one meaning and one clip showed the other. For gestures regularly used with two meanings, the second meaning (incorrect for this specific instance of communication) was always included among possible answers.

The remaining response options were randomly selected from among the 8 meanings that were correct at some point in the experiment, and 3 meanings that are regularly achieved by apes with their gestures but not with the gesture types used in this experiment (“Follow me”; “Move closer to me”; “Stop doing that”). The answers were randomly selected, but if there was a repeat, we replaced it by skipping to the next randomly selected meaning so that an answer could only appear once among the 4 response buttons.

Data exclusions

Data where all participant values were identical across all variables (Video only n = 453; Context n = 368) were eliminated as apparent upload error duplicates from page refreshing. Data where response time was shorter than 3 seconds (the minimum time required to watch the shortest real-time section of a gesture video) were excluded. Data where response time exceeded 3 standard deviations from the mean within each data set were also excluded.

Data analyses

All analyses were conducted in R (version 3.5.3) [46]. We estimated the effect of gesture Context on participant success, by fitting a Generalised Linear Mixed model using the glmer function in package lme4 (version 1.1–27.1) with a binomial error structure and logit link function. We included Condition (Video only, Context), Ambiguity (yes, no), and their interaction, as well as Meaning, Species, and Trial number as fixed effects, and Participant ID, Gesture type, and Video ID as random effects. We included all possible random slopes, but correlations among random intercepts and slopes were not computationally feasible. As an overall test of the fixed effects, we compared the full model with a null model that was identical except for the exclusion of Condition. All significance tests were conducted using a likelihood ratio test [47].

The sample for this model included n = 5,656 Participant IDs, 40 Video IDs, and 10 Gesture types, with a total n = 112,648 responses. Prior to fitting the model, we z-transformed Trial number to a mean of 0 and a standard deviation of 1. All factors entering the random effects as random slopes were manually dummy coded and then centred. Confidence intervals were not computationally feasible.

Full model results

We found a weak nonsignificant effect of Condition on participant success (X2 = 5.746, df = 2, p = 0.057). More specifically, only the interaction between Condition and Ambiguity showed any possible effect, with an again weak nonsignificant trend towards improved participant success (X2 = 2.791, df = 1, p = 0.095) where gesture meaning was classed as Ambiguous and information on Context was available (S1 Table).

Supporting information

S1 Table. Gesture types used, with the meanings for which they are used in both bonobos and chimpanzees.

(DOCX)

Click here for additional data file. (17.6KB, docx)
S1 Fig. Probability of success, separately for each combination of Context and Ambiguity.

(DOCX)

Click here for additional data file. (204.5KB, docx)

Acknowledgments

We thank the staff of the Budongo Conservation Field Station and the Wamba, Luo Scientific Reserve, where the gestural video data were collected, as well as the Ugandan National Council for Science and Technology, the Uganda Wildlife Authority, the WCBR, CREF, and the Ministère de la Recherche Scientifique et Technologie (Democratic Republic of Congo) for permission to work at these sites and in these countries. Video data were collected under UNCST Research Permit NS179 (Uganda) and Permis de Recherche No MIN.ESURS/SG-RST/002.2014 and Permis de Recherche No 002/MIN.RST/SG/180/002/2015 (DRC). We thank Professor Richard Byrne and Professor Takeshi Furuichi for their support and discussions. We thank Roger Mundry for support in running the code for analyses that would have otherwise been computationally nonfeasible.

Data Availability

Data and code are available in an open access repository at: https://doi.org/10.5281/zenodo.7347608. The experiment was presented in Gorilla.sc (www.gorilla.sc); a full preview and all importable sheets are available through Gorilla Open Materials (https://app.gorilla.sc/openmaterials/344409); and video data files are available at the Great Ape Dictionary on Youtube (https://tinyurl.com/greatapedictionary).

Funding Statement

This research received funding from the European Union’s 8th Framework 287 Programme, Horizon 2020, under grant agreement no 802719 to CH (https://ec.europa.eu/info/research-and-innovation/funding/funding-opportunities/funding-programmes-and-open-calls/horizon-2020_en). This work was supported by Gorilla Awards in Behavioural Science who provided the Gorilla.sc licensing fee and an unlimited participant award to KG (https://gorilla.sc/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

References

  • 1.Chomsky N. Language architecture and its import for evolution. Neurosci Biobehav Rev. 2017;81:295–300. doi: 10.1016/j.neubiorev.2017.01.053 [DOI] [PubMed] [Google Scholar]
  • 2.Hobaiter C, Byrne RW. The meanings of chimpanzee gestures. Curr Biol. 2014;24(14):1596–1600. doi: 10.1016/j.cub.2014.05.066 [DOI] [PubMed] [Google Scholar]
  • 3.Smith WJ. Message, meaning, and context in ethology. Am Nat. 1965;99(908):405–409. [Google Scholar]
  • 4.Engesser S, Townsend SW. Combinatoriality in the vocal systems of nonhuman animals. Wiley Interdiscip Rev Cogn Sci. 2019;10(4):e1493. doi: 10.1002/wcs.1493 [DOI] [PubMed] [Google Scholar]
  • 5.Warren E, Call J. Inferential communication: bridging the gap between intentional and ostensive communication in non-human primates. Front Psychol. 2021;12:718251. doi: 10.3389/fpsyg.2021.718251 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 6.Graham KE, Furuichi T, Byrne RW. Context, not sequence order, affects the meaning of bonobo (Pan paniscus) gestures. Gesture. 2020;19(2–3):335–364. [Google Scholar]
  • 7.Manser MB. The acoustic structure of suricates’ alarm calls varies with predator type and the level of response urgency. Proc R Soc B. 2001;268:2315–2324. doi: 10.1098/rspb.2001.1773 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 8.Ouattara K, Lemasson A, Zuberbühler K. Campbell’s monkeys concatenate vocalizations into context-specific call sequences. Proc Natl Acad Sci. 2009;106(51):22026–22031. doi: 10.1073/pnas.0908118106 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 9.Seyfarth RM, Cheney DL. Signalers and receivers in animal communication. Annu Rev Psychol. 2003;54(1):145–173. doi: 10.1146/annurev.psych.54.101601.145121 [DOI] [PubMed] [Google Scholar]
  • 10.Bates E, Camaioni L, Volterra V. The acquisition of performatives prior to speech. Merrill Palmer Q Behav Dev. 1975;21(3):205–226. [Google Scholar]
  • 11.Dennett DC. Intentional systems in cognitive ethology: The “Panglossian paradigm” defended. Behav Brain Sci. 1983;6(3):343–390. [Google Scholar]
  • 12.Dennett DC. The intentional stance. Cambridge, Massachusetts: MIT press; 1987. [Google Scholar]
  • 13.Grice HP. Meaning. Philos Rev. 1957;66:377–388. [Google Scholar]
  • 14.Rendall D, Owren MJ, Ryan MJ. What do animal signals mean? Anim Behav. 2009;78:233–240. [Google Scholar]
  • 15.Schel AM, Townsend SW, Machanda Z, Zuberbühler K, Slocombe KE. Chimpanzee alarm call production meets key criteria for intentionality. PLoS ONE. 2013;8(10):e76674. doi: 10.1371/journal.pone.0076674 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 16.Vail AL, Manica A, Bshary R. Referential gestures in fish collaborative hunting. Nat Commun. 2013;4:1765. doi: 10.1038/ncomms2781 [DOI] [PubMed] [Google Scholar]
  • 17.Lieberman P. Language did not spring forth 100,000 years ago. PLoS Biol. 2015;13(2):e1002064. doi: 10.1371/journal.pbio.1002064 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 18.Fröhlich M, Wittig RM, Pika S. Play-solicitation gestures in chimpanzees in the wild: flexible adjustment to social circumstances and individual matrices. R Soc Open Sci. 2016;3:160278. doi: 10.1098/rsos.160278 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 19.Graham KE, Hobaiter C, Ounsley J, Furuichi T, Byrne RW. Bonobo and chimpanzee gestures overlap extensively in meaning. PLoS Biol. 2018;16(2):e2004825. doi: 10.1371/journal.pbio.2004825 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 20.Liebal K, Pika S, Tomasello M. Gestural communication of orang-utans (Pongo pygmaeus). Gesture. 2006;6:1–36. [Google Scholar]
  • 21.Moore R. Social cognition, Stag Hunts, and the evolution of language. Biol Philos. 2017;32:797–818. [Google Scholar]
  • 22.Pika S, Liebal K, Tomasello M. Gestural communication in young gorillas (Gorilla gorilla): Gestural repertoire, learning and use. Am J Primatol. 2003;60:95–111. [DOI] [PubMed] [Google Scholar]
  • 23.Plooij FX. Some basic traits of human language in wild chimpanzees. In: Locke Andy, editor. Action, gesture and symbol: The emergence of language. London, UK: Academic Press; 1978. [Google Scholar]
  • 24.Tomasello M, George BL, Kruger AC, Jeffrey M, Evans A. The development of gestural communication in young chimpanzees. J Hum Evol. 1985;14:175–186. [Google Scholar]
  • 25.Roberts AI, Vick SJ, Buchanan-Smith HM. Usage and comprehension of manual gestures in wild chimpanzees. Anim Behav. 2012;84(2), 459–70. [Google Scholar]
  • 26.Corballis MC. The gestural origins of language. The origins of language. Tokyo: Springer; 2008. p. 11–23. [Google Scholar]
  • 27.Pollick AS, de Waal FBM. Ape gestures and language evolution. Proc Natl Acad Sci. 2007;104:8184–8189. doi: 10.1073/pnas.0702624104 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 28.Byrne RW, Cartmill E, Genty E, Graham KE, Hobaiter C, Tanner J. Great ape gestures: intentional communication with a rich set of innate signals. Anim Cogn. 2017;20(4):755–769. doi: 10.1007/s10071-017-1096-4 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 29.Genty E, Breuer T, Hobaiter C, Byrne RW. Gestural communication of the gorilla (Gorilla gorilla): repertoire, intentionality and possible origins. Anim Cogn. 2009;12(3):527–546. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 30.Graham KE, Furuichi T, Byrne RW. The gestural repertoire of the wild bonobo (Pan paniscus): a mutually understood communication system. Anim Cogn. 2017;20(2):171–177. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 31.Hobaiter C, Byrne RW. The gestural repertoire of the wild chimpanzee. Anim Cogn. 2011;14(5):745–767. doi: 10.1007/s10071-011-0409-2 [DOI] [PubMed] [Google Scholar]
  • 32.Schneider C, Call J, Liebal K. What Role Do Mothers Play in the Gestural Acquisition of Bonobos (Pan paniscus) and Chimpanzees (Pan troglodytes)? Int J Primatol. 2011;33(1):246–262. [Google Scholar]
  • 33.Goldin-Meadow S. The role of gesture in communication and thinking. Trends Cogn Sci. 1999;3(11):419–429. doi: 10.1016/s1364-6613(99)01397-2 [DOI] [PubMed] [Google Scholar]
  • 34.Fay N, Walker B, Ellison TM, Blundell Z, De Kleine N, Garde M, et al. Gesture is the primary modality for language creation. Proc R Soc B. 2022;289:20220066. doi: 10.1098/rspb.2022.0066 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 35.Kersken V, Gómez JC, Liszkowski U, Soldati A, Hobaiter C. A gestural repertoire of 1- to 2-year-old human children: in search of the ape gestures. Anim Cogn. 2019;22(4):577–595. doi: 10.1007/s10071-018-1213-z [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 36.Hobaiter C, Byrne RW. What is a gesture? A meaning-based approach to defining gestural repertoires. Neurosci Biobehav Rev. 2017;82:3–12. doi: 10.1016/j.neubiorev.2017.03.008 [DOI] [PubMed] [Google Scholar]
  • 37.Fischer J, Noser R, Hammerschmidt K. Bioacoustic field research: a primer to acoustic analyses and playback experiments with primates. Am J Primatol. 2013;75(7):643–663. doi: 10.1002/ajp.22153 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 38.Radick G. Primate Language and the Playback Experiment, in 1890 and 1980. J Hist Biol. 2005;38:461–493. [Google Scholar]
  • 39.Herman LM, Richards DG, Wolz JP. Comprehension of sentences by bottlenosed dolphins. Cognition. 1984;16(2);129–219. doi: 10.1016/0010-0277(84)90003-9 [DOI] [PubMed] [Google Scholar]
  • 40.Sevcik RA, Savage-Rumbaugh ES. Language comprehension and use by great apes. Lang Commun. 1994;14(1):37–58. [Google Scholar]
  • 41.Anwyl-Irvine AL, Massonié J, Flitton A, Kirkham NZ, Evershed JK. Gorilla in our midst: an online behavioural experiment builder. Behav Res Methods. 2019;52:388–407. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 42.Hobolth A, Christensen OF, Mailund T, Schierup MH. Genomic relationships and speciation times of human, chimpanzee, and gorilla inferred from a coalescent hidden Markov model. PLoS Genet. 2007;3(2):e7. doi: 10.1371/journal.pgen.0030007 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 43.Young NM, Capellini TD, Roach NT, Alemseged Z. Fossil hominin shoulders support an African ape-like last common ancestor of humans and chimpanzees. Proc Natl Acad Sci. 2015;112(38):11829–11834. doi: 10.1073/pnas.1511220112 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 44.Debracque C, Clay Z, Grandjean D, Gruber T. Humans recognize affective cues in primate vocalizations: Acoustic and phylogenetic perspectives. bioRxiv 2022.01.26.477864 [Preprint]. 2022. [cited 2022 Sept 20]. Available from: https://www.biorxiv.org/content/10.1101/2022.01.26.477864v1 [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 45.Tabossi P, Zardon F. Processing ambiguous words in context. J Mem Lang. 1993;32(3):359–372. [Google Scholar]
  • 46.R Core Team. R: A Language and Environment for Statistical Computing. Vienna, Austria; 2020. Available from: https://www.R-project.org/
  • 47.Dobson AJ. An Introduction to Generalized Linear Models. Boca Raton; Chapman Hall/CRC; 2002. [Google Scholar]

Decision Letter 0

Roland G Roberts

30 Jun 2022

Dear Kirsty,

Thank you for submitting your manuscript entitled "A great ape dictionary: language-using naïve humans understand non-human ape gestures" for consideration as a Research Article by PLOS Biology.

Your manuscript has now been evaluated by the PLOS Biology editorial staff, as well as by an academic editor with relevant expertise, and I'm writing to let you know that we would like to send your submission out for external peer review. Sorry for the delay incurred while we consulted an external expert.

IMPORTANT:

a) The Academic Editor found your study interesting, but was concerned about the interpretation and framing, and strongly recommends that you address these before we send it out for review. Specifically, the Academic Editor said:

"The authors conclude that humans understand the meaning of ape gestures and that this has implications for theories of language evolution. How exactly it relates to language is not entirely clear, and I would have preferred an actual study of how humans use hand gestures and interpret them in real life, but it is interesting that they understand ape gestures. They probably do so, not because they can reach back in evolutionary time, but because these gestures are still employed by our species.

"There exists a rich literature on ape gestures and multi-modal communication such as by Slocombe, Pollick, Liebal, Pika, Call, and others, but curiously little of it is mentioned here. Even the 2008 paper by Michael Corballis “The gestural origins of language” remains unmentioned.

"If the paper is shortened and the link with language de-emphasized (because unsubstantiated), and the paper focuses more on the universal nature of primate gestures to the point that humans understand those of apes, we may be getting close to a publishable paper. The study supports the shared gestural heritage of hominids in the same way that the literature often emphasizes the shared facial communication of hominids." [comments lightly edited]

b) I'm not convinced that you need to actually *shorten* your already concise paper, but you should address the above concerns and re-upload the manuscript when you upload the addition metadata (see next paragraph-but-one). Please also select "Short Reports" as the article type when you do so.

c) I've set your deadline to do all this as next Wednesday. If you need more time than this, do get in touch and we can discuss how to do that procedurally.

However, before we can send your manuscript to reviewers, we need you to complete your submission by providing the metadata that is required for full assessment. To this end, please login to Editorial Manager where you will find the paper in the 'Submissions Needing Revisions' folder on your homepage. Please click 'Revise Submission' from the Action Links and complete all additional questions in the submission questionnaire.

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If your manuscript has been previously peer-reviewed at another journal, PLOS Biology is willing to work with those reviews in order to avoid re-starting the process. Submission of the previous reviews is entirely optional and our ability to use them effectively will depend on the willingness of the previous journal to confirm the content of the reports and share the reviewer identities. Please note that we reserve the right to invite additional reviewers if we consider that additional/independent reviewers are needed, although we aim to avoid this as far as possible. In our experience, working with previous reviews does save time.

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Feel free to email us at plosbiology@plos.org if you have any queries relating to your submission.

Kind regards,

Roli

Roland Roberts, PhD

Senior Editor

PLOS Biology

rroberts@plos.org

Decision Letter 1

Roland G Roberts

2 Sep 2022

Dear Dr Graham,

Thank you for your patience while your manuscript "A great ape dictionary: language-using naïve humans understand non-human ape gestures" went through peer-review at PLOS Biology. Your manuscript has now been evaluated by the PLOS Biology editors, an Academic Editor with relevant expertise, and by three independent reviewers (note that reviewer #1's comments are in an attachment).

You'll see that the reviewers are broadly positive about your study, but each of them has a number of requests to improve the manuscript. In particular, reviewers #1 and #2 have significant concerns about your interpretation of the results, and these are shared by the Academic Editor, who said [lightly edited]:

"I must agree with both reviewers 1 and 2 that something in the interpretation of results seems off. The study itself is impressive and well-conducted, and the reviewers have good suggestions for improvement that the authors should follow. But to argue that humans “retain” an understanding of gestural communication, as a reference to a hypothetical past in which our ancestors still used ape-like gestures, and the parallel claim that our current gestural repertoires barely overlap with those of the apes (the overlap is called “trivial”) seems problematic. As reviewer #2 writes, there is hardly any support for the last statement, and as reviewer #1 writes there are other ways of guessing what a gesture may mean. My own hunch is that human and ape gestural repertoires have plenty of overlap and that this explains the success of human subjects guessing gesture meaning, but there is also quite a bit of contextual help in this study to make these guesses. The overall interpretation of results needs to be revised to be more cautious about both a) what we know about human gestures compared to those of the apes and b) avoid references to a “retained understanding” and the implication that we humans have some evolutionary memory of past communication systems."

In light of the reviews, which you will find at the end of this email, we are pleased to offer you the opportunity to address the comments from the reviewers in a revision that we anticipate should not take you very long. We will then assess your revised manuscript and your response to the reviewers' comments with our Academic Editor aiming to avoid further rounds of peer-review, although might need to consult with the reviewers, depending on the nature of the revisions.

We expect to receive your revised manuscript within 1 month. Please email us (plosbiology@plos.org) if you have any questions or concerns, or would like to request an extension.

At this stage, your manuscript remains formally under active consideration at our journal; please notify us by email if you do not intend to submit a revision so that we withdraw the manuscript.

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To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols

Thank you again for your submission to our journal. We hope that our editorial process has been constructive thus far, and we welcome your feedback at any time. Please don't hesitate to contact us if you have any questions or comments.

Sincerely,

Roli Roberts

Roland Roberts, PhD

Senior Editor

PLOS Biology

rroberts@plos.org

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REVIEWERS' COMMENTS:

Reviewer #1:

IMPORTANT: See attached file for review, which contains formatting and images!!!

Reviewer #2:

This was a very nice and interesting study reporting a novel citizen science experiment that examined people's comprehension of great ape (chimpanzee and bonobo) gestures. It was well written and well structured. The study used a simple but elegant design of presenting members of the public with videos of great ape gestures whose meanings were known, which they had to judge the meaning of in a forced-choice design. Participants were better than chance at judging the correct meaning of ape gestures, and interestingly context made only a relatively small difference. The authors are to be congratulated on the very impressive sample size: managing to engage over 17 000 particpants, of which 5600 provided useable data is a great achievement. Although assessing human perception in detecting animal signals is not itself new, this citizen science study is the first of its kind, including to explore it on such a large scale. The public engagement for the study is a great achievement.

The study was well designed, well executed. Nevertheless, there are some aspects that warrant attention, particularly the framing, rationale as well as the interpretation. There are also a few omissions in the analyses which need attention (detailed below).

A key premise of the paper is that it is a conundrum that human gestures do not map onto great ape gestures (except in infants) (L65). There are two issues with this stance, First, the statement itself seems evolutionarily counter-intuitive, given evidence of extensive overlaps among other great ape gesture repertoires (making it most likely that humans overlap too), and there is also evidence of considerable continuities in the production and perception of communicative expressions of humans and other great apes in other modalities too (facial expressions and vocalisations). In this sense, it should be expected that humans can perceive gestures of apes too, above chance. This doesnt undermine their result, it just means its consistent with what one would expect. however, the statement that he producton of human gestures don't map onto ape gestures (at least from what is reported here) appears to be based on an absence of evidence, rather than evidence of absence. There is no study reported in the paper which addresses this, its only implied in various statements. Clarifying this is important as otherwise 'the conundrum' could more be just a case that production hasn't yet been adequately investigated. A study focussing on production would be a very exciting complement to this study.

Also regarding framing, the paper starts out by resting on rather a strawman argument regarding a discontinuity between language and non-human communication (L30). As the authors themselves will be aware, evidence is revealing greater continuities than previously assumed. The authors contrast ape gestures to the apparently 'fixed' nature of primate alarm calls (L34-36); however there are multiple studies that now challenge this, for instance, studies of chimpanzee and orangutan alarm calls showing that caller's flexibly adjust their signals according to audience presence and knowledge (also potentially in other primates too e.g. audience effects in Thomas langur calls). Likewise, the next paragraph (L42) states that, by contast to animal signals, language 'does more than broadcast information'. Yet we now know that many animal signals also do more than broadcast information, as well as non-linguistic signals in humans. This part would warrant adjustment to avoid undermining progress that has recently been made to move away from such argumentation.

The analogy made in L37-41 seems a little over-simplistic. Even in the absence of an audience, a signal may still be produced with some intention or be based upon a context which ordnarily would have a communicative goal. Linguistic signals also can also be used in this way, not just nonlingustic ones. E.g. When picking up a hot pan alone, one might still say "that was really hot!, That hurt.' - this may contain intentionality even in the absence of an audience. Moreover, there is also evidence that signalling to oneself may also serve other cognitive functions for the signaller. A signal even in the absence of an immediate audience may still have evolved to be communicative, even in the absence of an audience at that given moment. There is a lot to unpack here

Methodology: The experimenters collected data on age, gender and experience with primates yet these variables didn't seem to be included in the analyses? It would be interesting and worthwhile to examine age and experience effects in particular, especially as the authors allude that perhaps human gestures get less 'ape-like' with age? It would also be interesting to see the demographic information about the sample population, was this collected?

Minor comments:

L33: One of the references used to support a sentence on primate alarm calls is actually about meerkats (Manser (2001)

L156: There is also evidence that humans can perceive the emotional content of primate calls, so this finding is not just limited to gestures.

L236: For gestures with multiple meanings, there is a bit of a bias introduced in the experimental design which could increase chances to get at least one right if the 'second meaning' was always added as one of the options. To ensure chances remain at 0.25 per trial, it would have been good to have done a counterbalanced design where only one meaning option (first or second) was available. It's possible this was somehow controlled for and I missed it.

Figures: The figures are nicely presented, just the fonts could be adjusted to be more legible

Table 1: Wasn't clear what the third column in the table was (unlabelled)

Reviewer #3:

[identifies herself as Heidi Lyn]

This article is an extremely interesting and modernized take on an older method of studying linguistic competence, comprehension. The authors present ape gestural data online and human participants indicate their understanding of the meaning of the gestures, to great success. I think the study is acceptable, with just a few clarifications to improve readibility, mostly.

Lines 92 - 96 - some examples of gestures with single and multiple meanings, as well as examples of primary and secondary meanings would be helpful. Also a description of how those meanings were determined.

Line 102 - why is the completion rate so low? Is it possible there is some bias introduced here?

Line 107 - are these binomials based on a 50% chance rate? But the participants chose from 4 possible answers? It would be helpful to clarify that chance would actually be much lower than 50% in the results, otherwise the percentage correct does not look impressive - because your numbers of trials are so high, a marginal improvement over chance would still be significant. Have you considered analyzing individuals' mean scores with a one-way t-test? Then you could also report on whether individual participants were able to recognize the entire repertoire.

Page 6 - I find both Figure 2 and Table 2 hard to interpret, although Figure 2 has me more puzzled. Is each gesture being added to the glmm as a separate predictor(Fixed effect)? Or a moderating variable? And you state there are 10 gestures, but only 7 meanings in the table? And their estimates in the glmm seem to vary widely from negative to positive. I'm not sure these visuals aid in the understanding of your findings or distract from them as the reader tries to interpret.

Line 133 - this is a really interesting finding that I think should be emphasized more. When there was an alternate meaning, participants averaged closer to 75% choosing one of the possible meanings. In other words, choosing a completely unrelated meaning was much less likely than the initial percentages correct may suggest.

Discussion and Introduction: It would be great if there were some mention of the long history of comprehension as an earlier/better measure for language or symbolic competence in many species: e.g.

Children: Shipley, E. F., Smith, C. S., & Gleitman, L. R. (1969). A Study in the Acquisition of Language: Free Responses to Commands. Language, 45(2), 322-342. https://doi.org/10.2307/411663),

marine mammals: Herman, L. M., Richards, D. G., & Wolz, J. P. (1984). Comprehension of sentences by bottlenosed dolphins. Cognition, 16(2), 129-219. https://doi.org/10.1016/0010-0277(84)90003-9

and apes: Sevcik, R. A., & Savage-Rumbaugh, E. S. (1994). Language comprehension and use by great apes. Language & Communication, 14(1), 37-58.

An Extended Data Table is mentioned several times, but I don't see it?

Attachment

Submitted filename: Graham and Hobaiter-Great Ape Dictionary-PLOS ONE.pdf

Click here for additional data file. (1.3MB, pdf)

Decision Letter 2

Roland G Roberts

17 Nov 2022

Dear Dr Graham,

Thank you for your patience while we considered your revised manuscript "Towards a great ape dictionary: naïve humans understand non-human ape gestures" for publication as a Short Report at PLOS Biology. This revised version of your manuscript has been evaluated by the PLOS Biology editors, the Academic Editor, and two of the original reviewers.

Based on the reviews and our Academic Editor's assessment of your responses to reviewer #2 (who was not able to re-review), we are likely to accept this manuscript for publication, provided you satisfactorily address the remaining points raised by the reviewers. Please also make sure to address the following data and other policy-related requests.

IMPORTANT:

a) Regarding the Title, 1. we tend to avoid punctuation in Titles, so we suggest that you remove the first five words (though we would understand if you wanted to keep them); 2. You'll see that reviewer #1 thinks that including "some" in the Title would be more accurate, and you should consider this; 3. One of my colleagues felt that "naïve" often has pejorative connotations, and should be avoided; however, I couldn't think of a suitable substitute.

b) Please attend to the remaining points from reviewer #1.

c) Please address my Data Policy requests below; specifically, we need you to supply the numerical values underlying Figs 1, 2 and S2, either as a supplementary data file or as a permanent DOI’d deposition. I also note that your main data and code deposition is in Github; please could you make a permanent DOI’d copy (e.g. in Zenodo) and cite this URL in the paper?

d) Please cite the location of the data clearly in all relevant main and supplementary Figure legends, e.g. “The data underlying this Figure can be found in S1 Data” or “The data underlying this Figure can be found in https://doi.org/XXXX”

As you address these items, please take this last chance to review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the cover letter that accompanies your revised manuscript.

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Please do not hesitate to contact me should you have any questions.

Sincerely,

Roli Roberts

Roland Roberts, PhD

Senior Editor,

rroberts@plos.org,

PLOS Biology

------------------------------------------------------------------------

DATA POLICY:

You may be aware of the PLOS Data Policy, which requires that all data be made available without restriction: http://journals.plos.org/plosbiology/s/data-availability. For more information, please also see this editorial: http://dx.doi.org/10.1371/journal.pbio.1001797

Note that we do not require all raw data. Rather, we ask that all individual quantitative observations that underlie the data summarized in the figures and results of your paper be made available in one of the following forms:

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DATA NOT SHOWN?

- Please note that per journal policy, we do not allow the mention of "data not shown", "personal communication", "manuscript in preparation" or other references to data that is not publicly available or contained within this manuscript. Please either remove mention of these data or provide figures presenting the results and the data underlying the figure(s).

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REVIEWERS' COMMENTS:

Reviewer #1:

I am happy to recommend publication of this very, very interesting paper (kudos to the authors for paying great attention to the objections), but I would suggest that some additional clarifications/improvements be made.

1. Optionally: ***this should be left to the authors***, but personally I would prefer a title that's less generic, e.g. with 'some': e.g. "Towards a great ape dictionary: naïve humans understand some non-human ape gestures."

The generic suggests that naïve humans *generally* understand ape gestures, and this seems to me to be an overstatement. Something less generic would be more accurate given the small number of gestures investigated. I do understand that this might take away from the impact of the article, so I'll understand if the authors prefer something that makes a bigger splash (although in the long term, I am not sure this is the best strategy).

2. I would recommend that the abstract be rewritten to state clearly and explicitly that humans understand (some) ape gestures, but that this could be for 2 reasons: 1. an innate communication system has been retained. 2. general intelligence allows humans to guess the meaning of some ape gestures.

Again, this might weaken the short-term impact of the paper, but it's a far more accurate assessment of the finding and will help with the long-term credibility of this (wonderful) line of research.

3. Still in the abstract, I think the authors could say in so many words that the 10 gestures studied were the most common ones. This strengthens the authors' point, as this is a clear criterion, which might defuse the reader's worry that the choice of items was biased.

4. The final discussion could be clarified. The following is a great improvement:

"It remains unknown whether the great ape repertoire itself is biologically inherited (Byrne et al. 2017), or whether apes - now including humans - share an underlying ability to produce and interpret naturally meaningful signals that are mutually understandable because of general intelligence and shared body plans and social goals, or the resemblance of gestures to the actions that they aim to elicit"

But I think this is still insufficiently accurate. This reads as if there are 2 salient theoretical possibilities.

Possibility 1: In apes and humans alike, the ape repertoire is (genetically) inherited.

Possibility 2: In apes and humans alike, the ape repertoire is understandable because of general intelligence etc.

But this incorrectly suggests that the most salient theories are ones in which apes and humans share an ability in this connection. This is not correct, as a very salient possibility is a third one, namely:

Possibility 3: In apes, the ape repertoire is (genetically) inherited. In humans, the ape repertoire is understood through general intelligence etc.

Possibility 3 is a deflationary account of the authors' finding. It's useful to mention it very clearly because it might trigger further research to prove or disprove the point.

5. The term 'meaning'

I appreciate the authors' response to my earlier suggestion, copied at the bottom of this report. But I think it would be easier if they stated clearly and explicitly at the beginning of the paper something along the following lines:

In this article, we use the term 'meaning' to refer to a reaction type, specifically to what has been called an 'apparently satisfactory outcome' (ASO) in the earlier literature.

with appropriate references to the literature, and examples. As I wrote earlier, without such a definition, linguists will be baffled by this use of the term 'meaning'. And it just takes a few clear words to provide a clear definition.

6. Human playback experiment

The description copied below (a version of which appears in the discussion) has the advantage of coolness ('performing ape-like playback experiments with humans, what a neat idea!'). But I think it doesn't do justice to the experimental situation. First, the authors employ a standard methodology of psychology/psycholinguistics: the subjects are just asked to guess the meaning of various things in perception. Second, no playback experiment with animals would look anything like this, as the human subjects are asked to do something entirely unnatural (they need to read complex instructions and provide guesses about meanings/functions). I would recommend restating all this far more simply. The coolness factor might decrease, but credibility would increase.

"We employ a method regularly used in studies of non-human primate communication, a 'play-back' experiment, in which recipient behaviour is analysed following exposure to a signal (Fischer et al. 2013, Radick 2005). This type of comprehension study has historically been employed to test nonhuman species on comprehension of human language (Herman et al. 1984, Sevcik & Savage-Rumbaugh 1994), but here we flip the paradigm to test humans on nonhuman communication. While language-competent humans seem to no longer typically produce"

As before, I think this is a great experiment and a super interesting paper!

EARLIER SUGGESTION ABOUT 'meaning'

Meaning: The term 'meaning' is used in the paper for 'Apparently Satisfactory Outcomes' (ASOs). This should be clarified in the paper. To anyone familiar with linguistics, it seems entirely implausible that ASOs are meanings, i.e. the cognitive representations of the informational content of gestures. Rather, ASOs are convenient ways to operationalize the various uses of gestures, but it seems clear that the core meanings of these gestures haven't been discovered yet. This is not a problem for the present study, but some readers will be misled if the authors do not state clearly what they understand by 'meaning' (namely ASOs).

Thank you, that's an important distinction that we have now clarified on Line 106 "We selected the 10 most common gesture types for which we were previously able to confirm "meaning" in both chimpanzees and bonobos, determined by recipient responses that consistently satisfy the signaller (Graham et al. 2018)"

Reviewer #3:

The authors have done an excellent job in responding to the reviewers' concerns. I recommend acceptance.

Decision Letter 3

Roland G Roberts

30 Nov 2022

Dear Dr Graham,

Thank you for the submission of your revised Short Report "Towards a great ape dictionary: inexperienced humans understand common non-human ape gestures" for publication in PLOS Biology. On behalf of my colleagues and the Academic Editor, Frans de Waal, I'm pleased to say that we can in principle accept your manuscript for publication, provided you address any remaining formatting and reporting issues. These will be detailed in an email you should receive within 2-3 business days from our colleagues in the journal operations team; no action is required from you until then. Please note that we will not be able to formally accept your manuscript and schedule it for publication until you have completed any requested changes.

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Roland G Roberts, PhD, PhD

Senior Editor

PLOS Biology

rroberts@plos.org

Associated Data

    This section collects any data citations, data availability statements, or supplementary materials included in this article.

    Supplementary Materials

    S1 Table. Gesture types used, with the meanings for which they are used in both bonobos and chimpanzees.

    (DOCX)

    Click here for additional data file. (17.6KB, docx)
    S1 Fig. Probability of success, separately for each combination of Context and Ambiguity.

    (DOCX)

    Click here for additional data file. (204.5KB, docx)
    Attachment

    Submitted filename: Graham and Hobaiter-Great Ape Dictionary-PLOS ONE.pdf

    Click here for additional data file. (1.3MB, pdf)
    Attachment

    Submitted filename: Response to Reviewers.docx

    Click here for additional data file. (36.8KB, docx)
    Attachment

    Submitted filename: PLOSbio_ResponseToReviewers.docx

    Click here for additional data file. (16.6KB, docx)

    Data Availability Statement

    Data and code are available in an open access repository at: https://doi.org/10.5281/zenodo.7347608. The experiment was presented in Gorilla.sc (www.gorilla.sc); a full preview and all importable sheets are available through Gorilla Open Materials (https://app.gorilla.sc/openmaterials/344409); and video data files are available at the Great Ape Dictionary on Youtube (https://tinyurl.com/greatapedictionary).

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