What makes female bonobos so eager to have sexual interactions with each other?
Some people refer to bonobos as "the hippie apes."
Bonobos are a now endangered species of great ape. They live in the forests of the Democratic Republic of Congo.
The nickname of "hippie ape" refers to the remarkable social practices of these primates, which display tight cooperation.
This includes sharing food, the largely equal standing of females and males in bonobo communities, and same-sex sexual behavior among males and females alike.
Recently, researchers from various academic institutions — including the Leibniz Institute for Farm Animal Biology in Dummerstorf, Germany, Harvard University in Cambridge, MA, and the University of Zurich in Switzerland — have been looking into why female bonobos display same-sex sexual behaviors.
The researchers' interest in female bonobos in particular arose from the fact that in the wild, all adult females engage in genito-genital rubbing (rubbing the genitals together) on a frequent basis.
Although males also engage in same-sex sexual behavior, they do so with less frequency, making the females' behavior even more remarkable by contrast.
So far, the investigators explain, there have been various theories about why females have so much sex with each other. These include the idea that this behavior could help females reduce social tensions and form social bonds.
However, they add, previous studies have only provided indirect evidence in support of these hypothesis.
In the new study — the findings of which appear in the journal Hormones and Behavior — the researchers focused on a well-established community of bonobos in the wild: the Bompusa bonobo community at LuiKotale, in the Democratic Republic of Congo.
Same-sex sexual behavior and cooperation
The researchers followed the adult members of the bonobo community for 1 year. During this time, they recorded how many times they had sexual interactions, and with partners of which sex.
They also recorded which partners female bonobos preferred for various other activities, including offering support in a situation of conflict.
The researchers also collected urine samples from the females after each time they had sexual interactions, either with males or other females. They did this so that they could measure changes in levels of oxytocin. This is a hormone that plays a key role in social bonding.
They found that in competitive contexts, when they needed to ensure cooperation, female bonobos preferred to engage in sexual interactions with other females.
Also, females that had engaged in same-sex sexual behaviors tended to remain more closely bonded than females that had mated with a partner of the opposite sex, and most social coalitions occurred between female bonobos.
After sexual interactions with other females, female bonobos also displayed higher levels of oxytocin in the urine. The same, however, did not occur after they had mated with males.
Female bonobos, it seems, derive more pleasure from sexual engagement with other females. This may also allow them to establish themselves as equal to the males in the community — by sticking together.
"It may be that a greater motivation for cooperation among females, mediated physiologically by oxytocin, is the key to understanding how females attain high dominance ranks in bonobo society," says co-lead study author Martin Surbeck.
"While it is important to not equate human homosexuality with same-sex sexual behavior in animals, our study suggests that in both humans and a close phylogenetic relative [the bonobo], the evolution of same-sex sexual behavior may have provided new pathways to promote high levels of cooperation."
Co-lead author Liza R. Moscovice