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
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.
(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