Catherine Hobaiter

Gestural communication of the gorilla (Gorilla gorilla): repertoire, intentionality and possible origins

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 species-typical 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.

Lethal aggression in Pan is better explained by adaptive strategies than human impacts

Observations of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) provide valuable comparative data for understanding the significance of conspecific killing. Two kinds of hypothesis have been proposed. Lethal violence is sometimes concluded to be the result of adaptive strategies, such that killers ultimately gain fitness benefits by increasing their access to resources such as food or mates. Alternatively, it could be a non-adaptive result of human impacts, such as habitat change or food provisioning. To discriminate between these hypotheses we compiled information from 18 chimpanzee communities and 4 bonobo communities studied over five decades. Our data include 152 killings (n = 58 observed, 41 inferred, and 53 suspected killings) by chimpanzees in 15 communities and one suspected killing by bonobos. We found that males were the most frequent attackers (92% of participants) and victims (73%); most killings (66%) involved intercommunity attacks; and attackers greatly outnumbered their victims (median 8:1 ratio). Variation in killing rates was unrelated to measures of human impacts. Our results are compatible with previously proposed adaptive explanations for killing by chimpanzees, whereas the human impact hypothesis is not supported.

Social network analysis shows direct evidence for social transmission of tool use in wild chimpanzees.

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

Serial gesturing by wild chimpanzees: its nature and function for communication

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.

Deictic gesturing in wild chimpanzees (Pan troglodytes)? Some possible cases.

Referential pointing is important in the development of language comprehension in the child and is often considered a uniquely human capacity. Nonhuman great apes do point in captivity, usually for a human audience, but this has been interpreted as an interaction pattern learned from human caretakers, not indicative of natural deictic ability. In contrast, spontaneous pointing for other apes is almost unknown among wild ape populations, supporting doubts as to whether apes naturally have any capacity to point referentially. Here the authors describe and illustrate 4 cases of gestures by juvenile chimpanzees in the Sonso chimpanzee community in Budongo, Uganda, that, at some level, may appear to be deictic and referential. The authors discuss the possible reasons why chimpanzees, if they possess a capacity for referential pointing, do not use it more frequently.

Laterality in the gestural communication of wild chimpanzees

We examined hand preference in the intentional gestural communication of wild chimpanzees in the Budongo forest, Uganda. Individuals showed some tendency to be lateralized, although less than has been reported for begging and pointing gestures in captivity; on average, their absolute bias was around 0.25 (where 1.0 represents complete right‐ or left‐hand use and 0.0 represents no bias). Lateralization was incomplete even in individuals with major manual disabilities. Where individuals had a stronger preference, this was more often toward the right hand; moreover, as age increased, the direction (but not the extent) of hand preference shifted toward the right. While the gestural repertoire as a whole was largely employed ambilateraly, object‐manipulation gestures showed a strong right‐hand bias.

How old are chimpanzee communities? Time to the most recent common ancestor of the Y-chromosome in highly patrilocal societies

Many human societies are patrilineal, with males passing on their name or descent group affiliation to their offspring. Y-chromosomes are also passed on from father to son, leading to the simple expectation that males sharing the same surname or descent group membership should have similar Y-chromosome haplotypes. Although several studies in patrilineal human societies have examined the correspondence between Y-chromosome variation and surname or descent group membership, similar studies in non-human animals are lacking. Chimpanzees represent an excellent species for examining the relationship between descent group membership and Y-chromosome variation because they live in strongly male philopatric communities that arise by a group-fissioning process. Here we take advantage of recent analytical advances in the calculation of the time to the most recent common male ancestor and a large sample size of 273 Y-chromosome short tandem repeat haplotypes to inform our understanding of the potential ages of eight communities of chimpanzees. We find that the times to the most recent common male ancestor of chimpanzee communities are several hundred to as much as over two thousand years. These genetic estimates of the great time depths of chimpanzee communities accord well with behavioral observations suggesting that community fissions are a very rare event and are similar to genetic estimates of the time depth of patrilineal human groups.

Wild chimpanzees’ use of single and combined vocal and gestural signals

We describe the individual and combined use of vocalizations and gestures in wild chimpanzees. The rate of gesturing peaked in infancy and, with the exception of the alpha male, decreased again in older age groups, while vocal signals showed the opposite pattern. Although gesture-vocal combinations were relatively rare, they were consistently found in all age groups, especially during affiliative and agonistic interactions. Within behavioural contexts rank (excluding alpha-rank) had no effect on the rate of male chimpanzees’ use of vocal or gestural signals and only a small effect on their use of combination signals. The alpha male was an outlier, however, both as a prolific user of gestures and recipient of high levels of vocal and gesture-vocal signals. Persistence in signal use varied with signal type: chimpanzees persisted in use of gestures and gesture-vocal combinations after failure, but where their vocal signals failed they tended to add gestural signals to produce gesture-vocal combinations. Overall, chimpanzees employed signals with a sensitivity to the public/private nature of information, by adjusting their use of signal types according to social context and by taking into account potential out-of-sight audiences. We discuss these findings in relation to the various socio-ecological challenges that chimpanzees are exposed to in their natural forest habitats and the current discussion of multimodal communication in great apes.Significance statementAll animal communication combines different types of signals, including vocalizations, facial expressions, and gestures. However, the study of primate communication has typically focused on the use of signal types in isolation. As a result, we know little on how primates use the full repertoire of signals available to them. Here we present a systematic study on the individual and combined use of gestures and vocalizations in wild chimpanzees. We find that gesturing peaks in infancy and decreases in older age, while vocal signals show the opposite distribution, and patterns of persistence after failure suggest that gestural and vocal signals may encode different types of information. Overall, chimpanzees employed signals with a sensitivity to the public/private nature of information, by adjusting their use of signal types according to social context and by taking into account potential out-of-sight audiences.

Local traditions in gorilla manual skill: evidence for observational learning of behavioral organization

Elaborate manual skills of food processing are known in several species of great ape; but their manner of acquisition is controversial. Local, “cultural” traditions show the influence of social learning, but it is uncertain whether this includes the ability to imitate the organization of behavior. Dispute has centered on whether program-level imitation contributes to the acquisition of feeding techniques in gorillas. Here, we show that captive western gorillas at Port Lympne, Kent, have developed a group-wide habit of feeding on nettles, using two techniques. We compare their nettle processing behavior with that of wild mountain gorillas in Rwanda. Both populations are similar in their repertoires of action elements, and in developing multi-step techniques for food processing, with coordinated asymmetric actions of the hands and iteration of parts of a process as “subroutines”. Crucially, however, the two populations deal in different ways with the special challenges presented by nettle stings, with consistently different organizations of action elements. We conclude that, while an elaborate repertoire of manual actions and the ability to develop complex manual skills are natural characteristics of gorillas, the inter-site differences in nettle-eating technique are best explained as a consequence of social transmission. According to this explanation, gorillas can copy aspects of program organization from the behavior of others and they use this ability when learning how to eat nettles, resulting in consistent styles of processing by most individuals at each different site; like other great apes, gorillas have the precursor abilities for developing culture.

Great ape gestures: intentional communication with a rich set of innate signals

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.