In several species of primates, males often discern when to mate with a female based on cyclical changes in the size and firmness of her sexual swelling -- a visual signal of a female's probability to conceive. In a study of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, primatologist Pamela Heidi Douglas and colleagues investigated for the first time the relationship between ovarian hormones and sexual swellings in wild female bonobos. The likelihood that a female bonobo ovulates during her maximum swelling phase is much lower than in the closely related chimpanzees. Swellings are thus no reliable fertility signal for males and allow females to follow their own agenda when choosing a mate.
If swellings are reliable signals of ovulation males allocate their mating efforts in accordance with these signals in an attempt to maximise their reproductive success. This often results in males mate guarding females during brief periods of high fecundity. However, if sexual signals are less reliable then it becomes difficult for males to optimally time when to mate and mate guard females. Female bonobos (Pan paniscus) are renowned for their enlarged sexual swellings that often remain tumescent for unusually lengthy durations. As such, bonobos are an interesting species in which to study signal accuracy in tandem with reproductive endocrinology.
By monitoring females' sexual swelling patterns, collecting daily urine samples, and measuring levels of oestrogen and progesterone metabolites in the urine, the researchers investigated the temporal relationship between visual changes in swelling patterns on one hand and changes of hormones on the other, to assess whether sexual swellings are reliable signals of ovulation and fecundity in bonobos. "We present a unique data set that combines sexual swelling scores and ovarian hormone measurements from the first long-term study on the physiology and reproductive endocrinology of wild female bonobos in their natural environment," said study coauthor, Gottfried Hohmann.
Bonobo swellings are unreliable signals
The study found immense variability in the duration of the maximum swelling phase (MSP), ranging from just one day to thirty-one days. Additionally, they found high variability in the timing of ovulation relative to the onset of the MSP. Ovulation occurred during the MSP in only half of the analysed swelling cycles. "This resulted in a much lower day-specific probability of ovulation and fecundity for bonobos than comparable findings for chimpanzees," said coauthor Tobias Deschner. Ovulation also occurred before or after the MSP, and some females displayed swelling cycles without ovulating. "Bonobo sexual swellings appear to send mixed messages to males, as they do not always signal fecundity or imminent ovulation," opined lead author Douglas.
The low reliability of sexual swellings in signalling ovulation in bonobos makes it much harder for males to successfully compete over females. Mate guarding therefore might not be a cost-effective mating strategy for male bonobos. These findings are important for understanding the evolution of signals of female reproductive state, how they influence male and female mating strategies, and how decoupling visual signals of fecundity from ovulation may affect intersexual conflict. By prolonging the period during which males would need to mate guard females to ascertain paternity, the temporal variability of this signal may curb mate-guarding efforts by male bonobos and thereby enable females to express mate choice without being constrained by males. The results offer an interesting example of the potential of females to use sexual signals to manipulate the behaviour of males. Future research will investigate the extent to which male bonobos attend to female sexual swellings and use these signals to time their mating efforts and strategies.
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