In Bonobo Society, Females Keep the Peace by Sending Mixed Messages

July 4, 2016 | Erica Tennenhouse

Female bonobo monkey with sexual swelling
Photo credit: Courtesy of PH Douglas

Sexual swellings cause widespread confusion among males.

When female primates are ready to mate, they often advertise their fertile state by displaying sexual swellings — enlargement of the skin surrounding their genitalia. Appearance of these swellings can spark violent competition among males over a shot at mating with a female during her limited window of fertility.

Not so for bonobos. While females of this species do produce sexual swellings, which can remain enlarged for up to 31 days, new research published in the journal BMC Evolutionary Biology shows that these supposedly accurate signals actually provide males with unreliable information about when a female is ovulating.

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Researchers collected urine samples from 13 females of a bonobo community in the Democratic Republic of Congo repeatedly over a 3-years period. Hormones present in the urine allowed them to determine the timing of ovulation, which was compared with observations of sexual swellings.

"The sexual swellings of female bonobos appear to send mixed messages to males, making it much harder for males to successfully time their mating efforts,” lead author Pamela Heidi Douglas from the Max Planck Institute in Germany said in a press release.

“We found that sometimes females would advertise they were fertile when they were not ovulating and thus unlikely to conceive. During other cycles, females did not display that they were fertile even though they were ovulating."

Out of the 34 cycles that were analyzed, female fertility was accurately signalled by sexual swellings in only half of them.

With visual signals decoupled from fertility, males can never be quite sure when the time is right to compete over their prospective mates. Consequently, rather than fighting for reproductive privileges, a male that is able to get in a female’s good graces by grooming her or giving her first dibs on preferred foods has the best chance of being selected as a mate. It is in this way that deceptive signals of fertility are believed to ultimately contribute to the remarkably peaceful nature of bonobo society.

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