Bonobo societies are unique among the apes. Rather than living in a male dominated world – such as those of chimpanzees, gorillas, and humans – bonobos (Pan paniscus) exist in one where the females hold most of the power. In the communities, despite living in social structures similar to their close cousins the chimpanzees in which the males keep control through fighting and conflict, male bonobos instead derive their status from their mothers. How this has been achieved has long baffled researchers.
A new study suggests that the females may keep control by creating uncertainty as to when they she is ovulating, and thus who fathers her offspring. By maintaining what are known as “sexual swellings”, which usually indicate when a female is at her most receptive and therefore most likely to get pregnant, for a period as long as a month, it makes it almost impossible for a single male to guard her for the entire time. This means that the males have to come up with other tactics to get her to mate, namely spending time socially interacting with and grooming her.
The females were found to display their sexual swellings for up to 31 days, much longer than the period for which they are fertile. Pamela Heidi Douglas/LKBP
“The sexual swellings of female bonobos appear to send mixed messages to males, making it much harder for males to successfully time their mating effort,” explains Pamela Heidi Douglas, who led the research published in BMC Evolutionary Biology. “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.”
What was also interesting, Heidi Douglas told IFLScience, was that sometimes the female bonobos had long periods where their sexual swellings were at their maximum, and sometimes they were only swollen for a couple of days, something that was unexpected.While in most other species of primates that show sexual swelling the females tend to ovulate towards the end of the period of swelling, in the bonobos they found that the females might ovulate at any point. Not only that, but some females even displayed swellings when they weren’t even ovulating.
This incredibly deceptive signaling, it seems, may help put an end to male-male competition over females, one of the main drivers of conflict and fighting among other primates, meaning that the male bonobos need to seek out other strategies to coerce a female into becoming their mate. This could, the authors suggest, manifest in the males spending more time with the females and engaging in more peaceful behavior, such as grooming.
The males of the group, rather than fighting, instead spend their time grooming the females. Pamela Heidi Douglas/LKBP
“It makes it very challenging for males to monopolize females, and it could therefore enhance a female’s ability to exercise unrestrained mate choice,” says Heidi Douglas. In other words, it might help the females call the shots. "It is still too early to say anything definitive, but it is one plausible hypothesis [that this could lead to the bonobos’ peaceful societies].”
But the lack of certainty when it comes to paternity also has other consequences. It means that males can never be certain whether or not one single offspring in their group is his or not, which could help to explain the lack of aggression they display to infants and juveniles, after all, they could be his. It also means, however, that the females are left to do all the caring, because again with so much uncertainty, a male bonobo does not want to invest in a child that is not his.