Liran Samuni, Kevin Langergraber, and Martin Surbeck. 2022. “
Characterization of Pan social systems reveals in-group/out-group distinction and out-group tolerance in bonobos.” PNAS.
AbstractHuman between-group interactions are highly variable, ranging from violent to tolerant and affiliative. Tolerance between groups is linked to our unique capacity for large-scale cooperation and cumulative culture, but its evolutionary origins are understudied. In chimpanzees, one of our closest living relatives, predominantly hostile between-group interactions impede cooperation and information flow across groups. In contrast, in our other closest living relative, the bonobo, tolerant between-group associations are observed. However, as these associations can be frequent and prolonged and involve social interactions that mirror those within groups, it is unclear whether these bonobos really do belong to separate groups. Alternatively, the bonobo grouping patterns may be homologous to observations from the large Ngogo chimpanzee community, where individuals form within-group neighborhoods despite sharing the same membership in the larger group. To characterize bonobo grouping patterns, we compare the social structure of the Kokolopori bonobos with the chimpanzee group of Ngogo. Using cluster analysis, we find temporally stable clusters only in bonobos. Despite the large spatial overlap and frequent interactions between the bonobo clusters, we identified significant association preference within but not between clusters and a unique space use of each cluster. Although bonobo associations are flexible (i.e., fission–fusion dynamics), cluster membership predicted the bonobo fission compositions and the spatial cohesion of individuals during encounters. These findings suggest the presence of a social system that combines clear in-group/out-group distinction and out-group tolerance in bonobos, offering a unique referential model for the evolution of tolerant between-group interactions in humans.
Gal Badihi, Kelsey Bodden, Klaus Zuberbühler, Liran Samuni, and Catherine Hobaiter. 2022. “
Flexibility in the social structure of male chimpanzees (Pan troglodytes schweinfurthii) in the Budongo Forest, Uganda.” Royal Society Open Science.
Publisher's VersionAbstractIndividuals of social species experience competitive costs and social benefits of group living. Substantial flexibility in humans' social structure and the combination of different types of social structure with fission–fusion dynamics allow us to live in extremely large groups—overcoming some of the costs of group living while capitalizing on the benefits. Non-human species also show a range of social strategies to deal with this trade-off. Chimpanzees are an archetypical fission–fusion species, using dynamic changes in day-to-day association to moderate the costs of within-group competition. Using 4 years of association data from two neighbouring communities of East African chimpanzees (Pan troglodytes schweinfurthii), we describe an unexplored level of flexibility in chimpanzee social structure. We show that males from the larger Waibira community (N = 24–31) exhibited additional structural levels of semi-stable core–periphery society, while males from the smaller Sonso community (N = 10–13) did not. This novel core–periphery pattern adds to previous results describing alternative modular social structure in other large communities of chimpanzees. Our data support the hypothesis that chimpanzees can incorporate a range of strategies in addition to fission–fusion to overcome costs of social living, and that their social structures may be closer to that of modern humans than previously described.
Leveda Cheng, Liran Samuni, Stefano Lucchesi, Tobias Deschner, and Martin Surbeck. 2022. “
Love thy neighbour: behavioural and endocrine correlates of male strategies during intergroup encounters in bonobos.” Animal Behaviour.
Publisher's VersionAbstractAcross group-living taxa, males act aggressively towards outgroup males because they represent a threat to reproduction with ingroup females. This intergroup aggression can entail agonistic coalition formation between group members, is typically associated with higher
testosterone levels and prevents extended associations between members of different groups. However, in a few species, including humans, groups can interact peacefully, allowing the exchange of resources and information. To better understand how social relationships are manifested between groups and the involvement of males in these relationships, we investigated behavioural and endocrine correlates of male strategies during intergroup encounters in bonobos,
Pan paniscus. We found that, despite overt intergroup aggression, males rarely engaged in coalitionary attacks against outgroup individuals and their testosterone levels did not rise during intergroup encounters. Also, males sacrificed the time available to affiliate with ingroup individuals and actively affiliated with outgroup individuals, especially outgroup males. The paucity of cooperative group defence and high levels of intergroup affiliation suggest that male bonobos play a role in maintaining tolerant intergroup relationships. We discuss how our findings relate to the establishment of male–male social relationships beyond the group level in other species.
Aisha Bründl, Cédric Girard-Buttoz, Tatiana Bortolato, Liran Samuni, Mathilde Grampp, Therese Löhrich, Patrick Tkaczynski, Roman Wittig, and Catherine Crockford. 2022. “
Maternal effects on the development of vocal communication in wild chimpanzees.” iScience.
AbstractEarly-life experiences, such as maternal care received, influence adult social integration and survival. We examine what changes to social behavior through
ontogeny lead to these lifelong effects, particularly whether early-life maternal environment impacts the development of social communication. Chimpanzees experience prolonged social communication development. Focusing on a central communicative trait, the “pant-hoot” contact call used to solicit social engagement, we collected cross-sectional data on wild chimpanzees (52 immatures and 36 mothers). We assessed early-life socioecological impacts on pant-hoot rates across development, specifically: mothers’ gregariousness, age, pant-hoot rates and dominance rank, maternal loss, and food availability, controlling for current maternal effects. We found that early-life maternal gregariousness correlated positively with offspring pant-hoot rates, while maternal loss led to reduced pant-hoot rates across development. Males had steeper developmental trajectories in pant-hoot rates than females. We demonstrate the impact of maternal effects on developmental trajectories of a rarely investigated social trait, vocal production.
Sven Grawunder, Natalie Uomini, Liran Samuni, Roman Wittig, and Catherine Crockford. 2022. “
Chimpanzee modification of vowel-like sounds and voice quality suggest expansion through the hominoid lineage.” Philosophical Transactions of the Royal Society.
AbstractThe origins of human speech are obscure; it is still unclear what aspects are unique to our species or shared with our evolutionary cousins, in part due to a lack of a common framework for comparison. We asked what chimpanzee and human vocal production acoustics have in common. We examined visible supra-laryngeal articulators of four major chimpanzee vocalizations (hoos, grunts, barks, screams) and their associated acoustic structures, using techniques from human phonetic and animal communication analysis. Data were collected from wild adult chimpanzees, Taï National Park, Ivory Coast. Both discriminant and principal component classification procedures revealed classification of call types. Discriminating acoustic features include voice quality and formant structure, mirroring phonetic features in human speech. Chimpanzee lip and jaw articulation variables also offered similar discrimination of call types. Formant maps distinguished call types with different vowel-like sounds. Comparing our results with published primate data, humans show less F1–F2 correlation and further expansion of the vowel space, particularly for [i] sounds. Unlike recent studies suggesting monkeys achieve human vowel space, we conclude from our results that supra-laryngeal articulatory capacities show moderate evolutionary change, with vowel space expansion continuing through hominoid evolution. Studies on more primate species will be required to substantiate this.
Sylvain Lemoine, Liran Samuni, Catherine Crockford, and Roman Wittig. 2022. “
Parochial cooperation in wild chimpanzees: a model to explain the evolution of parochial altruism.” Philosophical Transactions of the Royal Society.
Publisher's VersionAbstractParochial altruism, taking individual costs to benefit the in-group and harm the out-group, has been proposed as one of the mechanisms underlying the human ability of large-scale cooperation. How parochial altruism has evolved remains unclear. In this review paper, we formulate a parochial cooperation model in small-scale groups and examine the model in wild chimpanzees. As suggested for human parochial altruism, we review evidence that the oxytocinergic system and in-group cooperation and cohesion during out-group threat are integral parts of chimpanzee collective action during intergroup competition. We expand this model by suggesting that chimpanzee parochial cooperation is supported by the social structure of chimpanzee groups which enables repeated interaction history and established social ties between co-operators. We discuss in detail the role of the oxytocinergic system in supporting parochial cooperation, a pathway that appears integral already in chimpanzees. The reviewed evidence suggests that prerequisites of human parochial altruism were probably present in the last common ancestor between Pan and Homo.
Liran Samuni, David Lemieux, Alicia Lamb, Daiane Galdino, and Martin Surbeck. 2022. “
Tool use behavior in three wild bonobo communities at Kokolopori.” American Journal of Primatology, e23342.
Publisher's VersionAbstractComparative studies on tool technologies in extant primates, especially in our closest living relatives, offer a window into the evolutionary foundations of tool use in hominins. Whereas chimpanzee tool technology is well studied across populations, the scarcity of described tool technology in wild populations of our other closest living relative, the bonobo, is a mystery. Here we provide a first report of the tool use repertoire of the Kokolopori bonobos and describe in detail the use of leaf-umbrellas during rainfall, with the aim to improve our knowledge of bonobo tool use capacity in the wild. The tool use repertoire of the Kokolopori bonobos was most similar to that of the nearby population of Wamba and comprised eight behaviors, none in a foraging context. Further, over a 6-month period we documented 44 instances of leaf-umbrella use by 22 individuals from three communities, suggesting that this behavior is habitual. Most leaf-umbrella tool users were adult females, and we observed a nonadult using a leaf-umbrella on only a single occasion. While the study and theory of tool technologies is often based on the use of tools in foraging tasks, tool use in bonobos typically occurs in nonforaging contexts across populations. Therefore, incorporating both foraging and nonforaging contexts into our theoretical framework is essential if we wish to advance our understanding of the evolutionary trajectories of tool technology in humans.