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Edited by Brian Hare and Shinya Yamamoto

This volume includes twelve novel empirical papers focusing on the behaviour and cognition of both captive and wild bonobos ( Pan paniscus). As our species less known closest relative, the bonobo has gone from being little studied to increasingly popular as a species of focus over the past decade. Overall this volume demonstrates how anyone interested in understanding humans or chimpanzees must also know bonobos. Bonobos are not only equal to chimpanzees as our relatives, but they are also unique.

The majority of papers in this volume show that whether you are interested in the evolution of culture and tool use, social relationships and sharing or foraging ecology and cognition, bonobos have a major contribution to make. Four papers provide further evidence that the behaviour and psychology of bonobo females is radically different from that observed in chimpanzees. Foraging behaviour and cognition of bonobos is the focus of three papers that each show important ways that bonobos spatial cognition differs remarkably from chimpanzees. Two papers are relevant to solving the puzzle of why bonobos are expert extractive foragers in captivity but have never been seen using tools to obtain food in the wild.

The articles presented in this volume are previously published in a Special Issue of Behaviour, Volume 152, Parts 3-4 (March 2015).

Evan L. MacLean and Brian Hare


Previous research has shown that chimpanzees exploit the behavior of humans and conspecifics more readily in a competitive than a cooperative context. However, it is unknown whether bonobos, who outperform chimpanzees in some cooperative tasks, also show greater cognitive flexibility in competitive contexts. Here we tested the cooperative-competitive hypothesis further by comparing bonobos and chimpanzees in a series of tasks where a human gesture indicated the correct (cooperative) or incorrect (competitive) choice. A human either pointed cooperatively to the object a subject should choose, or competitively to the object subjects should avoid choosing. In contrast to previous research, subjects were most skilled at choosing the correct location when the communicator was cooperative and there were no major differences between bonobos and chimpanzees. Analysis of gaze direction revealed that in some cases subjects visually followed the direction of the experimenter’s gesture despite choosing incorrectly, dissociating gesture following from gesture comprehension. This supports the hypothesis that, unlike human children, nonhuman apes respond to the direction of social gestures more readily than they understand the communicative intentions underlying them. We evaluate these findings in regard to previous studies comparing the cooperative and communicative skills of bonobos and chimpanzees.

David Beaune, François Bretagnolle, Loïc Bollache, Gottfried Hohmann and Barbara Fruth


In an Afrotropical forest, we tested the hypothesis that fleshy-fruit plants with interspecific differences in fruit quality and quantity affect ranging behaviour of their seed dispersal vector. If fruiting plants could affect their dispersal vector, the plants also affect their seed dispersal distance and eventually their plant population biology. From 2007 to 2011, we measured seed transport by georeference daily bonobo group movements via GPS. Seed dispersal distance was estimated with mechanistic model, using 1200 georeferenced dispersal events and the average seed transit time through bonobo (24.00 h). We compared dissemination for eight plant species that deal with this trade-off: attracting dispersers by means of fruit quality/quantity versus retaining them in the patch because of the same quality/quantity value that attracted them. Because fruit traits of these eight species were different, we expected a difference in seed dispersal distance. Surprisingly, seed dispersal distances induced by bonobos were not affected by fruit traits. Although fruit nutrient contents, abundance and average patch feeding duration differed between plant species, patch feeding time was not related to subsequent dispersal distances. The apes’ dispersal distance survey gave an average dispersal distance estimated of 1332 ± 24 m from the parent plant (97.9% > 100 m). To conclude, feeding time invested in the patch, fruit quality and abundance had no apparent effect on bonobo seed dispersal distance. The possible effects in plant population biology are discussed.

William D. Hopkins, Jennifer Schaeffer, Jamie L. Russell, Stephanie L. Bogart, Adrien Meguerditchian and Olivier Coulon


The evolutionary origins of human right-handedness remain poorly understood. Some have hypothesized that tool use served as an important preadaptation for the eventual evolution of population-level right-handedness. In contrast, others have suggested that complex gestural and vocal communication served as prerequisite for the evolution of human right-handedness. In this study, we tested these competing hypotheses by comparing the handedness of bonobos and chimpanzees, two closely related species of Pan, on three different measures of hand use including simple reaching, manual gestures and coordinated bimanual actions. Chimpanzees are well known for their tool using abilities whereas bonobos rarely use tools in the wild. In contrast, many have suggested that bonobos have a more flexible gestural and vocal communication system than chimpanzees. The overall results showed that chimpanzees were significantly more right-handed than bonobos for all three measures suggesting that adaptations for tool use rather than communication may have led to the emergence of human right-handedness. We further show that species differences in handedness may be linked to variation in the size and asymmetry of the motor-hand area of the precentral gyrus. The results are discussed within the context of evolutionary theories of handedness, as well as some limitations in the approach to handedness measurement in nonhuman primates.

Alexandra G. Rosati


Primates must solve complex spatial problems when foraging, such as finding patchy resources and navigating between different locations. However, the nature of the cognitive representations supporting these types of behaviors is currently unclear. In humans, there has been great debate concerning the relative importance of egocentric representations (which are viewer-dependent) versus allocentric representations (which are based on aspects of the external environment). Comparative studies of nonhuman apes can illuminate which aspects of human spatial cognition are shared with other primates, versus which aspects are unique to our lineage. The current studies therefore examined spatial cognitive development in one of our closest living relatives, bonobos (Pan paniscus) across contexts. The first study assessed how younger bonobos encode locations in a place-response task in which apes first learn that one of two locations is consistently baited with a reward, and then must approach the two locations from a flipped perspective. The second study examined how a larger age sample of bonobos responded to a spatial relations task in which they first experience that one location is baited, and then can generalize this learning to a new set of targets. Results indicated that while bonobos exhibited a predominantly allocentric strategy in the first study, they consistently exhibited an egocentric strategy in the second. Together, these results show that bonobos can use both strategies to encode spatial information, and illuminate the complementary contributions to cognition made by egocentric and allocentric representations.

Kara Schroepfer-Walker, Victoria Wobber and Brian Hare


While natural observations show apes use grooming and play as social currency, no experimental manipulations have been carried out to measure the effects of these behaviours on relationship formation in apes. While previous experiments have demonstrated apes quickly learn the identity of individuals who will provide food in a variety of cooperative and non-cooperative situations, no experiment has ever examined how grooming and play might shape the preferences of apes for different individuals. We gave a group bonobos (N = 25) and chimpanzees (N = 30) a choice between an unfamiliar human who had recently groomed or played with them and one who had not. Both species showed a preference for the unfamiliar human that had interacted with them over the one who did not. The effect was largely driven by the males of both species while interacting with females showed little effect on their preferences for unfamiliar humans. Subjects showed this preference even though they only had social interactions with one of the unfamiliar humans for a few minutes before each trial and their choices were not rewarded with food differentially. Our results support the long held idea that grooming and play act as a form of social currency in great apes (and likely many other species) that can rapidly shape social relationships, particularly between unfamiliar individuals.

Hare Brian and Yamamoto Shinya

Victoria Wobber and Esther Herrmann


Levels of the steroid hormone testosterone have been found to impact diverse features of cognition from spatial memory to decision-making regarding risk, both in humans and other animals. However less is known about whether closely-related species differ in their testosterone-cognition relationships in line with pressures shaping each species’ cognitive evolution. We therefore examined relationships between testosterone and cognition in two-closely related species that differ markedly in their social behaviour, cognition, and patterns of testosterone production: bonobos (Pan paniscus) and chimpanzees (Pan troglodytes). We presented individuals of both species with a battery of 16 cognitive tasks and determined whether performance on these tasks correlated with average testosterone level. We found that among male chimpanzees, high levels of testosterone correlated with higher performance in numerous tasks, including tasks assessing spatial cognition and physical cognitive abilities more broadly. Meanwhile, in male bonobos we found no correlations between testosterone and performance on the cognitive tasks, and found no correlations in females of either species. Building on prior comparative research, these results suggest that bonobos and chimpanzees differ critically in the proximate mechanisms influencing their cognitive capacities, and that in particular the role of testosterone in shaping behaviour and cognition differs dramatically between the two species.

Brian Hare and Shinya Yamamoto


This Special Issue of Behaviour includes twelve novel empirical papers focusing on the behaviour and cognition of both captive and wild bonobos (Pan paniscus). As our species less known closest relative, the bonobo has gone from being little studied to increasingly popular as a species of focus over the past decade. We suggest that bonobos are ready to come off the scientific endangered list as a result. This Special Issue is exhibit A in showing that a renaissance in bonobo research is well underway. In this Editorial we review a number of traits in which bonobos and chimpanzees are more similar to humans than they are each other. We show how this means that bonobos provide an extremely powerful test of ideas about human uniqueness as well as being crucial to determining the evolutionary processes by which cognitive traits evolve in apes. This introduction places the twelve empirical contributions within the Special Issue in the larger evolutionary context to which they contribute. Overall this Special Issue demonstrates how anyone interested in understanding humans or chimpanzees must also know bonobos.

Shinya Yamamoto


Food sharing is considered to be a driving force in the evolution of cooperation in human societies. Previously postulated hypotheses for the mechanism and evolution of food sharing, e.g., reciprocity and sharing-under-pressure, were primarily proposed on the basis of meat sharing in chimpanzees. However, food sharing in bonobos has some remarkably different characteristics. Here I report details pertaining to fruit sharing in wild bonobos in Wamba based on 150 events of junglesop fruit sharing between independent individuals. The bonobos, primarily adult females, shared fruit that could be obtained individually without any cooperation or specialized skills. There was no evidence for reciprocal exchange, and their peaceful sharing seems to contradict the sharing-under-pressure explanation. Subordinate females begged for abundant fruit from dominants; this might indicate that they tested the dominants’ tolerance based on social bonds rather than simply begging for the food itself, suggesting existence of courtesy food sharing in bonobos.