Female bonobo sexual swellings send mixed messages to males

BMC Evolutionary Biology recently published research investigating whether the sexual swellings of bonobos reliably signals ovulation to males. First author Pamela Heidi Douglas talks more about what they found and how this keeps the peace among the bonobo society.

Mating and mate guarding

In many species of primates, females signal that they are willing to mate and capable of conceiving by displaying certain behaviors or through sexual signaling. One particularly conspicuous signal is the sexual swelling–the skin surrounding a female’s perineum that shows cyclical changes in size, color, and firmness across a female’s menstrual cycle. The function and evolution of sexual swellings have fascinated biologists ever since Darwin.

Female sexual swellings are usually most tumescent around the day of ovulation, when females are fecund and likely to conceive. Studies have found that when sexual swellings reliably signal ovulation, male primates allocate their mating efforts in accordance with these signals in an attempt to maximize their reproductive success.

This often results in males mate guarding females for several days around ovulation. However, if sexual swellings are not very reliable signals, then it becomes difficult for males to optimally time when to mate with, and mate guard, females.

A challenging task


Bonobos (Pan paniscus) are renowned for their enlarged sexual swellings that often remain tumescent for an unusually lengthy period of time, e.g., several weeks.

As such, bonobos are an interesting species in which to study signal accuracy in tandem with reproductive hormones that allow us to detect the occurrence of ovulation.

One of the primary objectives of my Ph.D. research is to investigate how reliably the sexual swellings of female bonobos signal the timing of ovulation.

At the Luikotale research site, located in the depths of D. R. Congo’s rainforest, research assistants and I monitored females’ sexual swellings daily and collected urine samples over an intense three-year period.

The study would not have been possible without the hard work and dedication of my research assistants, who helped me follow the bonobos from before dawn until dusk, every day throughout various females’ menstrual cycles. This was often a monumental task given the challenging conditions and dense undergrowth in the bonobos’ habitat.

Back in Germany, I analyzed the collected urine samples in the endocrine laboratory of the Max Planck Institute for Evolutionary Anthropology, to measure levels of estrogen and progesterone metabolites to pinpoint the timing of ovulation.

What did we find?

We found immense variability in the duration of the maximum swelling phase (MSP), ranging from just one day to thirty-one days. Additionally, we found high variability in the timing of ovulation relative to the start of a female’s MSP.

Most surprisingly, although ovulation usually occurs near the end of a female’s MSP in other species, ovulation occurred during the MSP in only half of the cycles we analyzed. In other cycles, ovulation occurred before or after the MSP, and sometimes females displayed cycles of maximally tumescent swellings but did not ovulate. This resulted in a much lower day-specific probability of ovulation and fecundity for bonobos than comparable findings for chimpanzees.

P H Douglas/LKBP
P H Douglas/LKBP

In summary, it appears that bonobo sexual swellings send mixed messages to males, as they do not always signal fecundity or imminent ovulation. Sometimes the swellings falsely signal that females are fecund during months when they do not ovulate. At other times, females may ovulate when their swellings are not maximally tumescent, i.e., when males might not expect the females to be able to conceive.

The low reliability of sexual swellings in signaling ovulation makes it challenging for male bonobos to accurately time their mating efforts to coincide with ovulation. Additionally, the prolonged periods of time that swellings remain tumescent render it practically impossible for males to monopolize females during periods of high fecundity.

By prolonging the number of days during which males would need to monopolize females to sire offspring, 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.

Why is this important?

P H Douglas/LKBP
P H Douglas/LKBP

Our findings are important for understanding the function of sexual signals and how they influence mating strategies. Males and females have divergent reproductive interests and therefore use different mating strategies to maximize their reproductive success.

This results in an arms race between the sexes. If the partial decoupling of swelling tumescence from ovulation enhances female ability to express unconstrained mate choice, this could provide females with leverage in controlling their mating and reproductive success.

I am currently investigating how mating strategies of female bonobos vary according to their reproductive state, and to what extent sexual signals affect female mating behavior and mate choice.

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