Brittany Fallon is a PhD candidate at the Université de Neuchâtel who works on the Sonso chimpanzee community of the Budongo Forest Reserve in Uganda. Here she shares some of her insights into their sexual displays.
In today’s focal party, the main characters are Nambi, the Alpha female who engages in regular sexual relations with young males; Nick, the former Alpha male, replaced by Nambi’s son Musa; and Zefa, Nick’s former Beta male, who is forming new alliances to overthrow Musa. It’s hard not to pretend I’m witnessing the real world version of Game of Thrones – except it’s not humans I’m observing, but chimpanzees.
There’s no Iron Throne involved in this power struggle – just the race for reproductive success. Dominant individuals enjoy a number of benefits that improve their chance of passing on genes, including richer access to sexual partners. But how does this influence their tactics for attracting mates? Perhaps no effort is needed, the chimpanzee equivalent to the philandering Robert, with potential sex partners who line up in hopes of birthing the next possible heir. Or is it that, like promiscuous Cersei, they’ve learned a number of coy and successful pick up lines (“Tears aren’t a woman’s only weapon.”)?
Elaborate metaphors aside, this is exactly the sort of question that I attempt to answer for my PhD on sexual displays in the Sonso chimpanzee community of the Budongo Forest Reserve in Uganda. Chimpanzees, like a variety of animals, produce ‘courtship displays’ to attract mates. Displays are largely comprised of gestures, which can be broadly defined as distinct bodily movements that do not physically manipulate the receiver toward the goal of the signaller. Both males and females, ranging in age from 2 to 52 years old in my community, can produce these solicitations. Displays can be elaborate, many signals strung together, or they can be simple, a single shaking of a branch followed immediately by copulation. What’s particularly amazing about chimpanzee solicitations is that they seem to be intentionally communicative: following a display, signallers will visibly wait for a response from their target by gaze-checking, and, if met with failure, will persist in gesturing.
One result that might be surprising is the homogeny among sexual displays; all adult individuals in my study used the same gestures, though in varied combinations, in their sexual displays. A few individually unique displays have been described in other studies in captivity or elsewhere in Africa, but most are also found in the Sonso community, making it more likely that the displays previously thought to be unique were in fact a result of a small sample size. As chimpanzees are known to have quite a large repertoire of gestures, how is it that all adult males use the same subset of gestures in their sexual displays?
This result might be explained by juvenile displays, which seem to differ from adults’ in their variety. For example, two infant males (ages 2 and 3) have been observed using the gesture ‘pirouette’ in a sexual display, which is behaviour more normally associated with play, and which was never observed in an adult courtship display. So far in my research, observations like this one suggest a ‘learning curve’ for sexual solicitations that begins before individuals reach sexual maturity (around age 8). One hypothesis to explain changes in sexual displays is that infants must experiment to determine which gestures are most effective. As they approach adulthood, they may narrow their sexual displays to the gestures most associated with sex. Another hypothesis is that male-female dyads may establish mutually selected signals from the complete repertoire, a process that they begin to develop from infancy.
With that in mind, is there a basic formula for a sexual display to be successful? Unfortunately, answering that question is about as easy as winning the Iron Throne. Apart from courtship displays, many other factors– such as female fecundity, or male aggression of females – influence mate choice. Communication is only one part of the story in the search for reproductive success, but for me it’s one of the most intriguing.
The topics discussed in this blog post are based on findings from Brittany Fallon’s PhD pilot study, conducted with Professor Klaus Zuberbühler at the University of Neuchâtel and Professor Richard Byrne from the University of St Andrews, as well as her Master’s thesis, which can be found in the University of St Andrews library. Watch this space to hear more about what Brittany discovers about chimpanzee sexual displays.
Photo Credit: Brittany Fallon