Archives for February 2025

Next time you wander around the woods, take a moment to appreciate the fact that you are surrounded by liars.

The natural world is filled with examples of deception: Opossums that play dead when cornered. Butterflies that mimic a predator’s gaze.  Bugs that look like sticks. Flowers that look like bugs. Bugs that look like flowers. I swear, you can’t trust anyone out there.

Chimpanzees are not above such trickery and, given their social intelligence, may be among the most advanced practitioners of the art, at least when it comes to their behavior. In fact, there’s a considerable amount of evidence for chimpanzees’ abilities to consciously deceive other apes. But lately I’ve been obsessed with another form of deception that is almost certainly not under conscious control but which is nonetheless quite interesting and surprisingly understudied, and that is the female chimpanzee’s ability to falsely advertise her own ovulation.

It’s well known that female chimpanzees in the wild tend to time their emigration from their natal communities with the onset of their first sexual swellings. These swellings, in which the skin around the anogenital region becomes pink and engorged with fluid, are generally thought to be an honest signal of fertility. Peak swelling, or tumescence, tends to coincide with the maximum probability of ovulation, which occurs every 35 days or so. So a swelling is essentially a big billboard that says Mate with Me Now if You Want the Best Chance at Having Offspring. And the boys certainly take notice.

There are a number of theories to explain the benefits of such advertisements to the females’ own reproductive success. But one prominent theory holds that females use these swellings to their advantage in another way entirely—for their own protection. By delaying emigration until they are tumescent, the swellings can act as a “social passport” to facilitate their acceptance into the new group. It’s dangerous for chimpanzees to enter different communities—after all, they are largely unrelated to the locals but end up competing for precious resources, including food and mating opportunities. But the males just can’t say no to a young female with a big backside so they will personally welcome the newcomer and protect them from resident females that may feel less generous. How thoughtful of them!

The fact that female chimpanzees time their journeys so as to manipulate the boys in this way, consciously or not, is pretty cool in and of itself. But those of us who care for chimpanzees in captivity have noticed that social dynamics can actually cause changes in tumescence. Specifically, when there’s a sudden influx of unknown males and the risks of conflict increase, which is rare in the wild but happens regularly during social introductions in captivity, females have been seen becoming swollen spontaneously, regardless of where they are in their reproductive cycles. We’ve seen this in CSNW’s own females when new groups are formed during intros—the girls begin the day with flat butts and go to bed with big, bulbous backsides. Why would this happen? We don’t know for sure, and I can’t believe that no one has studied it in detail (ahem…incoming graduate students looking for a thesis topic!). But it’s not difficult to imagine these swellings serving a role similar to the social passport and building off the same two facts of chimpanzee life: First, males are easily manipulated. And second, at times of great peril, it pays to have a big butt—even if you have to lie.