Great ape gestural communication is known to be intentional, elaborate and flexible; yet there is controversy over the best interpretation of the system and how gestures are acquired, perhaps because most studies have been made in restricted, captive settings. Here, we report the first systematic analysis of gesture in a population of wild chimpanzees. Over 266 days of observation, we recorded 4,397 cases of intentional gesture use in the Sonso community, Budongo, Uganda. We describe 66 distinct gesture types: this estimate appears close to asymptote, and the Sonso repertoire includes most gestures described informally at other sites. Differences in repertoire were noted between individuals and age classes, but in both cases, the measured repertoire size was predicted by the time subjects were observed gesturing. No idiosyncratic usages were found, i.e. no gesture type was used only by one individual. No support was found for the idea that gestures are acquired by 'ontogenetic ritualization' from originally effective actions; moreover, in detailed analyses of two gestures, action elements composing the gestures did not closely match those of the presumed original actions. Rather, chimpanzee gestures are species-typical; indeed, many are 'family-typical', because gesture types recorded in gorillas, orangutans and chimpanzee overlap extensively, with 24 gestures recorded in all three genera. Nevertheless, chimpanzee gestures are used flexibly across a range of contexts and show clear adjustment to audience (e.g. silent gestures for attentive targets, contact gestures for inattentive ones). Such highly intentional use of a species-typical repertoire raises intriguing questions for the evolution of advanced communication.

increasing their access to key resources, such as food or mates.1-5 Alternatively, it has 5 been argued to be a non-adaptive result of human impacts, such as habitat destruction 6 or provisioning of food. [6][7][8][9] To discriminate between these hypotheses we compiled long-7 term information from 18 chimpanzee communities and 4 bonobo communities. Our 8 data include 152 killings (N=58 observed, 41 inferred, and 53 suspected killings) by 9 chimpanzees in 15 communities and one suspected killing by bonobos. We found that 10 males had the greatest involvement as attackers (92% of participants) and victims 11 (73%); most killings (66%) involved intercommunity attacks; and attackers greatly 12 outnumbered their victims (median 8:1 ratio). Variation in rates of killing among 13communities depended on demographic variables but was unrelated to measures of 14 human impacts. These results from all major study populations over the last five 15 decades are consistent with previously proposed adaptive explanations for killing by 16 chimpanzees but not with the human impact hypothesis. 17 18Conspecific killing has been documented at multiple chimpanzee study sites, 2-5,10-12 but rates 19 vary greatly among sites. The human impact hypothesis and the adaptive strategies 20 hypothesis yield contrasting predictions, which we test here (Tables 1, 2). The human impact 21 hypothesis states that killing occurs mainly as an incidental outcome of aggression, 22 exacerbated by human activities such as providing a concentrated food resource, 23 deforestation-induced crowding, anthropogenic diseases or hunting. Accordingly, lethal 24 aggression should be high where human disturbance is high. In contrast, the adaptive strategies hypothesis views aggression as an evolved strategic 27 response by which aggressors tend to increase their fitness through increased access to 28 territory, food, mates or other benefits. [1][2][3][4][5][10][11][12][13][14][15][16][17] 45Intracommunity infanticide by females may result from intense competition among females 46 for the best feeding areas.17 Population differences in rates of killing are accordingly 47 expected to result from socioecological factors such as differences in grouping patterns 2,11 48 and/or demography.14 Lethal aggression thus occurs within a diverse set of circumstances, 49 but is expected to be most commonly committed by males; directed towards males; directed 50 6 towards non-kin, particularly members of other groups; and committed when overwhelming 51 numerical superiority reduces the costs of killing. 52 53Previous studies have developed and tested these specific hypotheses 2,5,[11][12][13][14][15][16][17] ; the present study 54 represents the first effort to test multiple hypotheses simultaneously with a comprehensive 55 dataset. To do so, we assembled data from 18 chimpanzee communities from both eastern 56 (N=12) and western (N=6) clades 24 of chimpanzees studied over 426 years (median = 21 57 years; range: 4-53) and from 4 bonobo communities studied for 92 years (media...

Social groups of gorillas were observed in three captive facilities and one African field site. Cases of potential gesture use, totalling 9,540, were filtered by strict criteria for intentionality, giving a corpus of 5,250 instances of intentional gesture use. This indicated a repertoire of 102 gesture types. Most repertoire differences between individuals and sites were explicable as a consequence of environmental affordances and sampling effects: overall gesture frequency was a good predictor of universality of occurrence. Only one gesture was idiosyncratic to a single individual, and was given only to humans. Indications of cultural learning were few, though not absent. Six gestures appeared to be traditions within single social groups, but overall concordance in repertoires was almost as high between as within social groups. No support was found for the ontogenetic ritualization hypothesis as the chief means of acquisition of gestures. Many gestures whose form ruled out such an origin, i.e. gestures derived from species-typical displays, were used as intentionally and almost as flexibly as gestures whose form was consistent with learning by ritualization. When using both classes of gesture, gorillas paid specific attention to the attentional state of their audience. Thus, it would be unwarranted to divide ape gestural repertoires into 'innate, species-typical, inflexible reactions' and 'individually learned, intentional, flexible communication'. We conclude that gorilla gestural communication is based on a speciestypical repertoire, like those of most other mammalian species but very much larger. Gorilla gestures are not, however, inflexible signals but are employed for intentional communication to specific individuals.

Network-based diffusion analysis demonstrates that a novel tool-use behavior, “moss-sponging”, spread via social learning in a wild East-African chimpanzee community.

Chimpanzees' use of gesture was described in the first detailed field study [1, 2], and natural use of specific gestures has been analyzed [3-5]. However, it was systematic work with captive groups that revealed compelling evidence that chimpanzees use gestures to communicate in a flexible, goal-oriented, and intentional fashion [6-8], replicated across all great ape species in captivity [9-17] and chimpanzees in the wild [18, 19]. All of these aspects overlap with human language but are apparently missing in most animal communication systems, including great ape vocalization, where extensive study has produced meager evidence for intentional use ([20], but see [21, 22]). Findings about great ape gestures spurred interest in a potential common ancestral origin with components of human language [23-25]. Of particular interest, given the relevance to language origins, is the question of what chimpanzees intend their gestures to mean; surprisingly, the matter of what the intentional signals are used to achieve has been largely neglected. Here we present the first systematic study of meaning in chimpanzee gestural communication. Individual gestures have specific meanings, independently of signaler identity, and we provide a partial "lexicon"; flexibility is predominantly in the use of multiple gestures for a specific meaning. We distinguish a range of meanings, from simple requests associated with just a few gestures to broader social negotiation associated with a wider range of gesture types. Access to a range of alternatives may increase communicative subtlety during important social negotiations.

Chimpanzees at Budongo, Uganda, regularly gesture in series, including 'bouts' of gesturing that include response waiting and 'sequences' of rapid-fire gesturing without pauses. We examined the distribution and correlates of 723 sequences and 504 bouts for clues to the function of multigesture series. Gesturing by older chimpanzees was more likely to be successful, but the success rate of any particular gesture did not vary with signaller age. Rather, older individuals were more likely to choose successful gestures, and these highly successful gestures were more often used singly. These patterns explain why bouts were recorded most in younger animals, whereas older chimpanzees relied more on single gestures: bouts are best interpreted as a consequence of persistence in the face of failure. When at least one gesture of a successful type occurred in a sequence, that sequence was more likely to be successful; overall, however, sequences were less successful than single gestures. We suggest that young chimpanzees use sequences as a 'fail-safe' strategy: because they have the innate potential to produce a large and redundant repertoire of gestures but lack knowledge of which of them would be most efficient. Using sequences increases the chance of giving one effective gesture and also allows users to learn the most effective types. As they do so, they need to use sequences less; sequences may remain important for subtle interpersonal adjustment, especially in play. This 'Repertoire Tuning' hypothesis explains a number of results previously reported from chimpanzee gesturing.

Great apes give gestures deliberately and voluntarily, in order to influence particular target audiences, whose direction of attention they take into account when choosing which type of gesture to use. These facts make the study of ape gesture directly relevant to understanding the evolutionary precursors of human language; here we present an assessment of ape gesture from that perspective, focusing on the work of the “St Andrews Group” of researchers. Intended meanings of ape gestures are relatively few and simple. As with human words, ape gestures often have several distinct meanings, which are effectively disambiguated by behavioural context. Compared to the signalling of most other animals, great ape gestural repertoires are large. Because of this, and the relatively small number of intended meanings they achieve, ape gestures are redundant, with extensive overlaps in meaning. The great majority of gestures are innate, in the sense that the species’ biological inheritance includes the potential to develop each gestural form and use it for a specific range of purposes. Moreover, the phylogenetic origin of many gestures is relatively old, since gestures are extensively shared between different genera in the great ape family. Acquisition of an adult repertoire is a process of first exploring the innate species potential for many gestures and then gradual restriction to a final (active) repertoire that is much smaller. No evidence of syntactic structure has yet been detected.

Gaze following has been argued to be uniquely human, facilitated by our depigmented, white sclera [M. Tomasello, B. Hare, H. Lehmann, J. Call, J. Hum. Evol. 52, 314–320 (2007)]—the pale area around the colored iris—and to underpin human-specific behaviors such as language. Today, we know that great apes show diverse patterns of scleral coloration [J. A. Mayhew, J. C. Gómez, Am. J. Primatol. 77, 869–877 (2015); J. O. Perea García, T. Grenzner, G. Hešková, P. Mitkidis, Commun. Integr. Biol. 10, e1264545 (2016)]. We compare scleral coloration and its relative contrast with the iris in bonobos, chimpanzees, and humans. Like humans, bonobos’ sclerae are lighter relative to the color of their irises; chimpanzee sclerae are darker than their irises. The relative contrast between the sclera and iris in all 3 species is comparable, suggesting a perceptual mechanism to explain recent evidence that nonhuman great apes also rely on gaze as a social cue.