Harassment of adults by immatures in bonobos (Pan paniscus): testing the Exploratory Aggression and Rank Improvement hypotheses

Spontaneous playful teasing in four great ape species

Joking draws on complex cognitive abilities: understanding social norms, theory of mind, anticipating others' responses and appreciating the violation of others' expectations. Playful teasing, which is present in preverbal infants, shares many of these cognitive features. There is some evidence that great apes can tease in structurally similar ways, but no systematic study exists. We developed a coding system to identify playful teasing and applied it to video of zoo-housed great apes. All four species engaged in intentionally provocative behaviour, frequently accompanied by characteristics of play. We found playful teasing to be characterized by attention-getting, one-sidedness, response looking, repetition and elaboration/escalation. It takes place mainly in relaxed contexts, has a wide variety of forms, and differs from play in several ways (e.g. asymmetry, low rates of play signals like the playface and absence of movement-final 'holds' characteristic of intentional gestures). As playful teasing is present in all extant great ape genera, it is likely that the cognitive prerequisites for joking evolved in the hominoid lineage at least 13 million years ago.

Female-directed aggression by adolescent male chimpanzees primarily constitutes dominance striving, not sexual coercion

OBJECTIVES

Chimpanzees (Pan troglodytes) are notable for exhibiting high levels of male-to-female aggression. Much of this aggression from adult males serves sexually coercive functions. Despite being smaller and lower-ranking than adult males, adolescent males also engage in regular aggression against adult females. Here, we test whether the primary function of this aggression is sexual coercion, as in adult males, or, alternatively, whether adolescent males use aggression to establish social dominance over females.

MATERIALS AND METHODS

We analyzed 1771 copulations and 1812 instances of male-initiated aggression between adolescent males (aged nine through 14 years) and adult females across 21 years of observation of the Kanyawara chimpanzee community in Kibale National Park, Uganda.

RESULTS

Our test of the sexual coercion hypothesis revealed that adolescent males did not selectively target cycling females for aggression, nor did aggression against cycling females predict rates of copulation with those females. Our test of the social dominance hypothesis showed that males succeeded in dominating all adult females before, or soon after, dominating their first adult male. Additionally, we found that adolescent males dominated females approximately in the order of the females' own ranks, from the bottom to the top of the female hierarchy.

DISCUSSION

Our data illustrate that the establishment of social dominance was more important than sexual coercion in explaining patterns of adolescent male aggression toward females. In comparison, evidence for sexual coercion was clear and compelling in adult males. These findings highlight that the primary function of male-to-female aggression differs between adolescent and adult males.

Look at me while having sex! Eye-to-eye contact affects homosexual behaviour in bonobo females

In humans, eye-to-eye contact (EEC) or mutual gazing is a reflexive predisposition occurring in intimate contexts. We investigated the role of EEC during bonobo socio-sexual contacts. Females engage in homosexual ventro-ventral, genito-genital rubbing (VVGGR) during which they embrace each other while rubbing part of their vulvae and, sometimes, clitoris. VVGGR facilitates conflict resolution, anxiety reduction and social bonding. We found that EEC was negatively affected by female bonding: the more the eye contact, the weaker the social relationship. This suggests that EEC promotes an intimate contact between the more unfamiliar subjects. Moreover, VVGGRs were successfully prolonged in presence of at least one event of EEC compared to VVGGRs during which none of the partners looked towards the other or only one looked at the other’s face. EEC has been probably favoured by natural selection to enhance the cohesion between bonobo females, who can gain social power through socio-sexual contacts.