Baboons have tactical sex cycle
A baboon mother with her baby.
Image: iStockphoto

Female baboons at Wellington Zoo might be modifying their reproductive cycle to deceive their mates.

Groups of female baboons—known as Hamadryas—typically have just one male baboon between them for mating.

But research conducted by Victoria University Master of Science student Ray Tobler shows that they might change their reproductive cycle in order to increase their chances of mating with someone other than the male in their group. 

Mr Tobler looked at two groups of baboons, known as harems, at the Wellington Zoo colony. One harem contained three females and the other had six.

He found that females in the smaller harem had, on average, fertile phases that were three days longer than the females in the larger harem.

The fertile phase in female baboons is marked by large, cumbersome swellings on their hindquarters, which are frequently wounded.

“Longer fertile phases lead to more fertile females in a harem at the same time, which is known to increase competition—and aggression—between females for access to the one male in their harem. It also lowers their chances of successfully conceiving. So, it appears that female baboons would benefit from shortening rather than lengthening the fertile phase.”

Mr Tobler says a benefit of having a longer fertile phase is that it increases a female’s chances to mate with a male outside of her harem. “Males should find it harder to monopolise sexual access to all the females in his group when several females are in the fertile phase of their cycle at the same time.” 

He says that females in the larger harem should be able to obtain the same number of mates outside their group in a shorter time than those from the smaller harem. “The longer fertile phases in the smaller harem are a product of having less fertile females together in the same group.”

So how does this promiscuity benefit female baboons?

“There are several possibilities. One that stands out is that by mating with other males outside of their harem, females might decrease the chance of these males committing infanticide in the future—as mating provides these males with some chance of paternity.”

Also, by mating with other males, females may increase the likelihood that these males will attempt to take this female from her harem leader at a later date. “This may be especially beneficial to females in harems who have a poor relationship with the other females or their harem leader. Moving to another harem where these relationships are better might significantly improve their fitness in the long term.” 

Editor's Note: Original news release can be found here.