- Linking behaviour and physiology of female bonobos (Pan paniscus) (2005)
- The present study investigates sexual behaviour and the importance of dominance in female bonobos. Detailed behavioural studies were carried out on four mixed-sex captive bonobo groups; morphological data and faecal samples were collected to allow non-invasive assessment of physiological parameters. First, I addressed the background for the variability in patterns of menstrual and swelling cycles in bonobos. The duration of menstrual intervals was influenced by reproductive history. Menstrual cycles and phases of maximum tumescence of the sexual swelling lasted longer in lactating females. Parity did not influence cycle patterns. Further, the variability of the duration of intermenstrual intervals was found to be mainly caused by variability in follicular phase length. The luteal phase, contrastingly, was much less variable and did not influence the length of intermenstrual intervals. This raises the question whether sexual swellings serve as a reliable indicator of ovulation in bonobos. Hormone analyses showed that the day of ovulation could not be predicted from the onset of maximum tumescence. Also detumescence of the swelling was no clear sign that ovulation had occurred. Nonetheless, sexual interactions were found to vary according to the degree of tumescence, being most frequent at maximum tumescence. The frequency of sexual interactions did not change in the peri-ovulatory phase and no difference between was seen between follicular and luteal phase. This indicates that sexual swellings are not a reliable signal of ovulation. Rather, they could be a graded signal that advertises the probability of ovulation which allows females to follow a mixed strategy of confusing and biasing paternity. Next, the context and function of same-sex sexual interactions among female bonobos were investigated. Genital rubbing took place more often between non-related than between related females. Females were able to get hold of and defend monopolisable food items without the help of other females in feeding experiments. No relationship between genital rubbing and food sharing was found and no female-female coalitions were formed during these experiments. Interventions in conflicts outside the feeding experiments were observed mainly in heterosexual conflicts but females supported each other irrespective of preferences for genital rubbing. The frequency of genital rubbing did not vary in dependence of the degree of genital swelling and the solicitation of genital rubbing was not asymmetric in dependence of the relative degree of swelling of the two partners. The degree of tumescence did not influence the frequency of received or overall aggression for a female. Although most female dyads were involved in same-sex conflicts, genital rubbing was observed in less than half of the dyads after the conflict. No increase of gg rubbing occurred 15 minutes after a conflict compared to 15 minutes before. High-ranking females took the male position during genital rubbing significantly more often than low ranking ones. The direction of initiation of genital rubbing was not influenced by social status. The data do no support the hypothesis that genial rubbing serves to form or maintain alliances, to reduce competition among females or to reconcile former opponents. However, genital rubbing may be a display of social status or may serve to reduce tension. Finally, the correlation of physiological parameters and social status in female bonobos was studied. I investigated whether females differ in their excretion pattern of the glucocorticoid 11-ketoetiocholanolone (DOA) in dependence of their social status. Based on displacements, females were categorised to either high or low social status. During the feeding experiments carried out on one group, high social status transferred in access to monopolisable food, the low ranking female was not successful in monopolising the food item. However, female social status was not reflected in the faecal excretion pattern of glucocorticoid metabolites in any of the three groups analysed. In two groups the high-ranking individuals had higher levels of iDOA than low-ranking individuals, in the other group it was vice versa. The fourth group did not allow this investigation as the high ranking individual was pregnant and pregnancy was found to result in elevated glucocorticoid levels. The results indicate that also in bonobo females, high social status translates into better access to monopolizable resources. Status dependent differences in cortisol excretion may not exist in bonobos or may become obvious only during periods of social instability. Alternatively, social status may not influence adrenocortical function but other physiological parameters of the so-called stress axes. Taken together, this study provides an interdisciplinary view on bonobo female behaviour and physiology helping to better understand the adaptive significance of a species’ social and mating system.