Search

Many animals engage in sociosexual behaviour, including that between same-sex pairs. Bonobos (Pan paniscus) are famous for their sociosexual behaviour, but chimpanzees (Pan troglodytes) apparently do not engage in sociosexual behaviour frequently. However, sociosexual behaviour in chimpanzees may have been overlooked. We observed 584 instances of sociosexual behaviour in chimpanzees at Ngogo, Kibale National Park, Uganda during three years of study. All ages and sexes engaged in sociosexual behaviour, which included mounting, touching of genitals, and pressing genitals together. Most sociosexual behaviour was between adult males. Sociosexual behaviour was often during tense contexts, such as subgroup reunions and during territorial behaviour. Among males, grooming and dominance rank relationships do not explain patterns of sociosexual behaviour. Although sociosexual behaviour may be less frequent in chimpanzees than in bonobos, and bonobos remain distinct in their genito-genital rubbing, our findings suggest that sociosexual behaviour is a regular part of chimpanzee behaviour. [ABSTRACT FROM AUTHOR]

Our closest living relatives, bonobos and chimpanzees, along with small-scale human societies figure prominently in debates about human nature. Here we emphasize and explain behavioural variation within and among these three species. In the logic of behavioural ecology, individuals have been selected to adjust their behaviour along evolved reaction norms that maximize fitness given current socio-ecological conditions. We discuss variation in three behavioural contexts: relationships between the sexes, hierarchy and inequality, and intergroup interactions. In each context, behavioural variation can be related to two broad socio-ecological conditions: (i) the defensibility of contested resources, and (ii) differences in bargaining power. When defensibility of resources and differences in bargaining power are great, interactions are rife with conflict; when they are minimal, interactions are more harmonious. These socio-ecological conditions therefore constitute key catalysts and obstacles of cooperation. We conclude that human nature should be seen as consisting of evolved reaction norms. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 populationlevel 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. [ABSTRACT FROM AUTHOR]

One of the most conspicuous behavioural differences among great apes is the paucity of tool use among wild bonobos (Pan paniscus) in comparison to chimpanzees (Pan troglodytes) who are one of the most prolific and skilled tool users in the animal kingdom. This is in spite of the fact that bonobo tool use repertories are as large and diverse as chimpanzees' in captive settings. In this study, we compared tool using behaviours and potential drivers of these behaviours in the Wamba bonobo population located in central Democratic Republic of Congo with the Goualougo chimpanzee population of northern Republic of Congo. The tool use repertoire of wild bonobos was comprised of only 13 behaviours, compared to 42 for chimpanzees. However, the number of tool behaviours observed in each study site was similar between bonobos and chimpanzees, and many types of tool use for social, self-grooming/stimulation, and comfort/protection functions were commonly used by both species. A marked difference is that 25 of 42 tool behaviours exhibited by chimpanzees are performed for feeding, in contrast to a single report of bonobos using a leaf sponge to drink water. We examined whether the differences in tool use repertoires can be explained by the necessity, opportunity, relative profitability, or invention hypotheses. We found that habitat composition and fluctuation of fruit production at these two sites were similar, particularly when compared with variation observed between sites within each species. Thus it was unlikely that the necessity hypothesis explains the lack of tool use for feeding in bonobos. Though further study at Wamba is needed, we did not identify any obvious differences in prey availability that would indicate differences in tool using opportunities between the sites. This study could not test the relative profitability hypothesis, and further research is needed on whether tool use is the most efficient means of calorie or protein intake for wild apes. Bonobos at Wamba formed much larger and stable parties than chimpanzees at Goualougo, which was contrary to the prediction by the invention hypothesis. Another explanation is that differences in tool use behaviour between bonobos and chimpanzees might not be explained by the current ecological or social conditions, but rather by circumstances during the Pleistocene Epoch. The observed species differences might also reflect divergent behavioural predispositions, rather than actual differences in cognitive abilities. [ABSTRACT FROM AUTHOR]

Bonobos are the only ape species, other than humans, that have demonstrated prosocial behaviors toward groupmates and strangers. However, bonobos have not been tested in the most frequently used test of prosociality in animals. The current study tested the other-regarding preferences of bonobos in two experiments using the prosocial choice task. In the first experiment subjects preferred a food option that would benefit both themselves and another bonobo. This preference was likely the result of a location bias developed in the pretest since they showed the same preference in the non-social control condition within test sessions. A second experiment was designed to help subjects overcome this bias that might interfere with their social choices. Bonobos again did not prefer to choose the prosocial option. However, results suggest constraints of this paradigm in revealing social preferences. In discussing our results we consider why bonobos show robust prosocial preferences in other paradigms but not here. While others have suggested that such contradictory results might suggest interesting motivational or cognitive differences between humans and non-humans, we propose that the current 'standard' paradigm has failed validation due to three methodological constraints. Across the dozens of studies completed few have demonstrated that non-human subjects understand the causal properties of the apparatus, non-social biases quickly develop in inadequately counterbalanced pretests that typically explain subjects' choices in the test, and even human children found this choice task too cognitively demanding to consistently show prosocial preferences. We suggest it is time to consider switching to a variety of more powerful and valid measures. [ABSTRACT FROM AUTHOR]

We use Principal Component Analyses (PCA) to describe components of social relationship quality in bonobos. We find a three component structure, with the first two components, labelled Value and Compatibility, closely matching the theoretical constructs as well as components reported for chimpanzees and other primates. The third component differed but was abandoned based on Parallel Analysis. Among bonobos, female-female dyads have higher Value and Compatibility. Relationships between males are characterised by low Value and Compatibility. Dyads that had been housed together for a longer time and maternally related ones also have more valuable relationships, while individuals close in rank have low compatibility. The results confirm the strong bonds among female bonobos, but for the first time can describe how they differ qualitatively from close bonds reported for captive chimpanzee females. We suggest future studies should also include Parallel Analysis to more accurately describe the number of components in relationship quality. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Perineal sexual skin swelling in relation to menstrual cycle occurs in a variety of primate taxa. However, sexual swelling with exaggerated size and colour is found only in some Old World monkeys and the two Pan species. Although several hypotheses have been proposed (e.g., reliable indicator hypothesis and graded signal hypothesis), it seems unlikely that a single explanation can account for the significance of the sexual swelling in all of these species. Bonobos (Pan paniscus) provide an excellent opportunity for studying sexual swelling since they have the most prolonged maximal swelling periods among primates. In this study we propose a new hypothesis that sexual swelling in female bonobos increases their attractiveness to other females and thereby facilitates affiliative social interaction with them. We found that free-ranging female bonobos with maximal sexual swelling engaged in affiliative social interactions with other females, including genitogenital rubbing, staying in close proximity and grooming, more frequently than females without maximal swelling. These tendencies suggest that females with maximal swelling were attractive to other females. The results also suggest that the benefits of maximal swelling might vary among females depending on their life-history stage. In particular, young females may get more benefits from prolonged maximal swelling through increased grooming reciprocity and staying in close proximity to other females. Thus our study supported the hypothesis that one function of prolonged maximal swelling in bonobos is to increase attractiveness to other females, thereby enhancing affiliative relationships between females in a male-philopatric social system. [ABSTRACT FROM AUTHOR]

Sexual contacts are thought to play an important role in regulating social tension in bonobos (Pan paniscus), and are especially common following aggressive conflicts, either between former opponents or involving bystanders. Nevertheless, research on the factors determining post-conflict sexual contacts, their effectiveness in reducing social tension and the nature of post-conflict sexual behaviour is scarce. Here, we collected data on post-conflict affiliative contacts in bonobos occurring between former opponents (reconciliation) and offered by bystanders towards victims (consolation) to investigate the role of sexual contacts in the regulation of aggressive conflicts compared to non-sexual affiliation behaviours. We tested whether post-conflict sexual contacts: (1) alleviate stress, (2) confer reproductive benefits, (3) mediate food-related conflict and (4) repair valuable social bonds. Thirty-six semi-free bonobos of all ages were observed at the Lola ya Bonobo Sanctuary, DR Congo, using standardized Post-Conflict/Matched Control methods. Consolation and reconciliation were both marked by significant increases in the occurrence of sexual behaviours. Reconciliation was almost exclusively characterized by sexual contacts, although consolation was also characterized by increases in non-sexual behaviours, such as embrace. Adults were more likely to engage in post-conflict sexual contacts than younger bonobos. Consistent with the stress-alleviation hypothesis, victims receiving sexual consolatory contact showed significantly lower rates of self-scratching, a marker of stress in primates, compared to receiving non-sexual contact. Post-conflict sexual contacts were not targeted towards valuable social partners and they did not confer obvious reproductive benefits; nor were they used to mediate food-related conflicts. Overall, results highlight the role of sex in regulating tension and social conflicts in bonobos. [ABSTRACT FROM AUTHOR]

Biological market models explain variability in reciprocity and interchange between groups. In groups with a shallow dominance gradient, grooming will be mostly exchanged for itself (i.e. exchange will occur). In groups with steep dominance hierarchies, interchange is expected: individuals will groom higher ranking individuals to get access to limited resources or commodities such as support in conflicts, and grooming will be traded for these commodities. We examine patterns of reciprocity in grooming and support, and of interchange of grooming for support or for tolerance in six captive groups of bonobos. We test whether differences between groups in patterns of reciprocity and interchange can be attributed to differences in a measure of steepness of dominance hierarchies, which is based on dyadic agonistic interactions. We found that grooming was reciprocal in some, but not all groups. Support was highly reciprocal, but this was a side effect of dominance in most groups. Interchange between grooming and support was observed in some groups. Corroborating earlier findings, this was a side effect of individuals preferring high ranking individuals as grooming and support partners, possibly because these high-ranking individuals provide more efficient support in conflicts. There was no evidence for interchange of grooming for tolerance. Variability in grooming reciprocity was explained by differences in steepness of dominance hierarchies, as predicted by the biological market models. In groups with a shallow dominance hierarchy, grooming was more reciprocal. This was not true for reciprocity in support. There was some evidence that individuals groomed dominants more frequently in groups with a steep dominance hierarchy. The variation in interchange relations between grooming and support did not depend on the steepness of dominance hierarchies. We suggest that grooming in itself is a valuable commodity in bonobos, especially under captive conditions, which can be exchanged reciprocally. Bonobos may interchange grooming for another value equivalent, with food sharing as a very likely candidate. This interchange effects seem more dependent on potential to monopolise food than on steepness of dominance hierarchies per se. [ABSTRACT FROM AUTHOR]

Previous work on bonobos (Pan paniscus) has focused on the role of aggression in the context of conflict resolution in captive groups. This study investigates events of aggressive behaviour of wild bonobos to evaluate its significance in the context of mating. Temporal association (15 minutes) between aggression and mating was used to assume functional relationship. The proportion of events that were found to be related to mating activity was highest for femalefemale aggression followed by male-male, male-female, and female-male aggression. The results suggest that intrasexual aggression is used to manipulate the mating success of competitors: Males competed for access to oestrus females, aggression between males was high on mating days, and aggressors had higher mating rates than targets. Harassment by females disturbed mating attempts of targets, the rate of aggression increased with the number of oestrous females per party and following harassment, aggressors tended to mate more often with the male partner of the target female. Unlike intrasexual aggression, support for the predicted functions of inter-sexual aggression in the context of mating was weak. Aggression by males against females was rare and was almost never followed by mating between aggressor and target. Female aggression against males occurred frequently but appeared to be independent of mating behaviour. The results did not support the female-defence alliance hypothesis (Parish, 1996). However, when males and females engaged in close association, the rate of aggression tended to be lower and rates of mating were higher than during control periods. We suggest that male intersexual aggression is incompatible with intersexual bonding and propose that the potential benefits that males derive from affiliative long-term association with females prevent males from being aggressive against females. [ABSTRACT FROM AUTHOR]

Bonobo (Pan paniscus) social structure is characterized by partial female dominance, in contrast to the male dominated chimpanzee (Pan troglodytes) society. Furthermore, female bonobos exhibit more overt aggressiveness and a prolonged period of proceptivity during menstrual cycle compared to chimpanzees. Since dominance, aggressiveness and proceptivity are suggested to relate to high T levels, we expected T concentrations of bonobo females to be high. To test this, urinary T metabolite concentrations (measured by immunoreactive 5αandrostane-17α-ol-3-one) were determined, which reliably reflect T status in both species. We predicted a larger overlap between T metabolite concentrations of both sexes in bonobos compared to chimpanzees. Our results support this hypothesis. However, interspecific comparison for each sex revealed substantially lower T metabolite concentrations in bonobo males than in chimpanzee males, while female bonobos displayed levels about equal to those of chimpanzee females. It thus appears that low T metabolite levels in bonobo males are responsible for the large overlap instead of high T metabolite levels in bonobo females. In conclusion, this study suggests that quantitative sex differences in T levels may be predictive of social system. However, partial female dominance in bonobos is probably not mediated by high female T levels. Rather it appears that T levels of mainly male bonobos are adapted to social organization. We hope that these preliminary results ignite further research in this novel area. [ABSTRACT FROM AUTHOR]