“Promiscuous Monkey Sex: Why Everybody Benefits.” That title may have been the reason so many people came to Anatasha Plummer ‘10’s biology comps talk last week. The other title she considered using, “Facultative Cooperative Polyandry in Callitrichidae,” doesn’t have quite the same, well, draw.
The more scientific title does give a good outline of what Anatasha’s comps is about, though. Callitrichidae is a family of New World monkeys found in the Amazon rainforest, commonly known as the tamarins and the marmosets. They’re the same kind of primates that the Psych department keeps for studies on learning and behavior. Their favorite foods are fruit and insects – Anatasha served fruit with her talk, but she decided to leave out the insects.
The biological interest of researching Callitrichidae monkeys is that they have an extremely unusual mating system. They engage in polyandry, where multiple males mate with a single female. Usually they are found in a trio of two males and one female that stays together for several mating seasons. What makes their polyandry even stranger is that it’s cooperative: the two males don’t show any aggression towards each other.
Why do the monkeys do this? Is there an evolutionary explanation? The dominant idea among biologists who study Callitrichidae is that it is the female that mostly benefits because she gets extra help with childcare. Anatasha contends that polyandry benefits all the monkeys involved.
Tamarins and marmosets are very small monkeys that usually give birth to twins. As a result, caring for their young is unusually difficult for the females. She must carry the twins on her back everywhere until they’re old enough to climb on their own, and she needs to eat more so that she can lactate. If a male helps care for the young, they are more likely to survive. Raising monkey twins is so expensive that two males working together are needed for the offspring to have a good chance of survival. In a polyandrous arrangement, each male has a 50% chance that he’s caring for offspring that aren’t his. But a 50% chance of passing on his genes is still better than a 0% chance. And because groups tend to stay together for several seasons, in the long run, there is a good chance that both males will eventually reproduce.
That’s not to say that the males don’t compete with each other at all. The Callitrichidae probably have sperm competition – the more sperm a male produces, the more likely he is to be the father of the offspring. This leads to a testicular arms race where the males have unusually large testicles for their body size.
