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Genital-Contacts among Bonobo Females: Use and Proximate Function

Barbara Fruth 1&2 & Gottfried Hohmann 2

1 Max-Planck-Institut fur Verhaltensphysiologie, 82319 SEEWIESEN;
2 Max-Planck-Institut fur evolutionare Anthropologie, 04155 LEIPZIG


    Mounting between females is known from insects, birds and mammals. Female bonobos (Pan paniscus) show a mounting behaviour which physically differs from other primate species. They embrace each other ventro-ventrally and rub their genital swellings laterally against each other. This behaviour is known as genital contact (GC) or genito-genital rubbing. Although many functions were suggested, so far none has been object of detailed investigation.
    Here we test five hypotheses generated by information of female-female mounting from both primate and non-primate species. These consider mounting behaviour to proximately serve the (1) reconciliation of former opponents, (2) attraction of mates, (3) regulation of tension, (4) expression of social status, and/or (5) social bonding among individuals. Each hypothesis allows several predictions which are tested with data collected during six field seasons (1993-1998; 27 months) on members of one bonobo community in Lomako (DRC).
    A total of 466 genital contacts was sampled and related to factors such as agonistic conflict, individual affiliation, party size and composition, mating, female cycle, access to food, and rank.
    When we tested the (1) reconciliation hypothesis, we found that only 19% of all genital contacts, were preceded by agonism. However, the post-conflict rate was about four times higher than the pre-conflict rate. Although not every female engaged in either pre- or post conflict genital contacts, those who did were in 80% of the cases former opponents. Contrary to our expectation, the rate of genital contacts was not higher among closely associated individuals. Although only two of the four predictions were accepted, reconciliation clearly was able to partly explain the use of GCs.
    Considering the (2) mate attraction hypothesis we found that GCs were higher in mixed sex than in all female parties. However, when we investigated whether or not copulations (COP) were preceded by GCs, only 22% of all cases were so independent of the time which had elapsed between both events. On average this was 73-137 minutes. Looking closely to the intervals between the different sexual events, only the follow up of GC and GC was significant, that of GC and COP was not. When the state of tumescence of the females involved in GCs was taken into account, fully swollen females engaged more often in GCs than less tumescent ones. However, these females did not enhance matings more often than those who were less swollen. Because of the large independence of both events, GC and COP, and the lack of influence of potentially receptive females, this hypothesis was rejected.
    When we tested the (3) tension regulation hypothesis, we found that the GC-rate increased with party size, a result which vanished when we controlled for food patch size. However, comparison of tense situations such as feeding on clumped and monopolisable food patches (Treculia africana) with more relaxed situations such as feeding on vaster distributed items (Irvingia gabonensis), revealed a clearly higher rate of GCs in the tense situations. Nevertheless, GCs did not appear to be a substitute for aggression since in food sharing episodes, where aggression was highest among bystanders, the majority of GCs occurred between owners and bystanders.
    When we tested the (4) display of social status hypothesis, we found status dependant asymmetries in both the initiation of the event and the individual spatial position within the dyad. Low ranking females initiated GCs more often than high ranking ones (92%) and the majority of all females in the top position (66% mounters) were of high rank. Therefore status acknowledgement was considered to explain an important part of the GCs.
    Last we investigated was the (5) social bonding hypothesis. Here, neither genetical relatedness (mother-daughter dyads) nor close social ties (measured by spatial association) were able to explain GC-rates. We took social grooming as indicator for social affiliation. When we compared female dyads for the frequency of social grooming and GCs, we found that when grooming frequencies were high, frequencies of GCs were low and vice versa. Therefore, we rejected this hypothesis.
    To summarise the results given above: no single hypothesis can account for the variety of utilisation of GCs among bonobo females. Instead, GCs are largely multifunctional. No sufficient evidence was found for hypothesis (2) and (5). However, our data are in support of hypotheses (1), (3) and (4). The accepted hypotheses clearly show one thing in common: Social relationships among female bonobos depend to a large extent on the status of each individual involved. Both conflict and tension can be resolved by the acknowledgement of the position of the superior individual.
We therefore suggest that in future genital contacts can be used to investigate the quality and dynamics of social relationships among female bonobos, which appear to be hardly bonded by mutual affiliation but instead by the continuously emphasised respect of the individual position within the hierarchical system of bonobo social structure.


References:

(1) FRUTH, B. , HOHMANN, G. & W.C. MCGREW. 1999. The Pan Species, in: The Nonhuman Primates, eds. P. Dolhinow & A. Fuentes, pp. 64-72, Mayfield Publishing Company.
(2) GERLOFF, U., HARTUNG, B., FRUTH B., HOHMANN, G., & D. TAUTZ. 1999. Intra-community relationships, dispersal pattern and control of paternity in a wild living community of bonobos (Pan paniscus) determined from DNA analyses of faecal samples. Proceedings of the Royal Society of Britain, 266, 1189-1195.
(3) HOHMANN, G. , GERLOFF, U., TAUTZ, D. & B. FRUTH (in press), Social bonds and genetic ties: kinship, association and affiliation in a community of bonobos (Pan paniscus), Behaviour.



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