Female bonobos use ‘sexual swelling’ to trick protective males

5 Jul 2016

New research into the sexual behaviour of bonobos has found that ‘sexual swelling’ may be used by female bonobos to confuse loyal males and increase the number of sexual partners available to them.

For all animals, knowing when females are ovulating is key. It allows them to prioritise mating at particular times, enhancing the likelihood of breeding and, in turn, promoting the species.

For bonobos – primates linked heavily with highly sexual behaviour – the females give males strong clues about their ovulation cycle by swelling around their perineal tissue (around their sexual organs).

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Thus, an interested male swings by and stands guard, ensuring his partner is his and his alone, mating at the moment of peak swelling. However, the swelling can last for nearly three weeks, meaning the timing that males rely upon is not too accurate.

And a new study points at a potential reason why.

Smoke and mirrors

Whereas, in other primates, the maximum swelling phase is linked to ovulation, in bonobos it’s not, which Pamela Heidi Douglas, author of a new paper, thinks is a bit of a ploy.

If the swelling lasts too long, males cannot stand guard and dissuade other males from getting involved. So, this could well be an attempt by females to encourage varied partners, improving the potential gene pool available.

“It appears that bonobo sexual swellings send mixed messages to males, as they do not always signal fecundity or imminent ovulation,” said Douglas, when talking of her paper published in BMC Evolutionary Biology.

Sometimes, females showed signs of swelling despite no ovulation occurring whatsoever, while the maximum swelling phase bared little correlation to actual ovulating dates.

Male and female bonobos have differing mating interests, and use different strategies. Females, according to Douglas, use their bodies, essentially, to trick males.

“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.”

Putting mixed signals front and centre of the reproductive cycle.

A slow learning curve

Behavioural studies of wild animals take quite some time, with the drip, drip of information only natural considering the months spent out in the field.

In this instance, Douglas and colleagues collected urine samples in the wild over the course of three years, later testing the fluid to determine ovulation timings, correlated with swelling.

There has been a raft of primate findings in recent months. In May, research into the communication techniques of bonobos and chimpanzees revealed the different ways primates interact with each other.

Bonobos’ interactions are quicker, with their use of a “gaze” allowing them to anticipate incoming signals “before they have been fully articulated”, according to Marlen Froehlich, first author of the study.

This is closer to how humans communicate when compared with chimpanzees, who go through a slower process of signals, pauses and responses.

Last April, a new look at genetic mutations in primates found a novel technique that both defends against disease and speeds up the evolution of a species in the process. While scientists have discovered that the male-specific Y chromosome in humans has more in common with gorillas than chimpanzees in some ways.

Laughing bonobo image via Shutterstock

Gordon Hunt was a journalist with Silicon Republic