Chimpanzees play with sticks as though they're dolls

In many cultures, girls play with dolls more often than boys do. Now chimps have been observed treating sticks like dolls. Is there a gender imbalance in the way chimps play with their dolls, too?

The differences in how boys and girls play is well-documented but controversial, which raises a tricky question. Is this tendency the result of purely social factors that create specific gender roles for boys and girls, or is there something biological going on here? Earlier research with captive monkeys has suggested male and female chimps prefer the human toys that their gender would stereotypically want to play with. But now, for the first time, researchers have observed animals in the wild playing with objects in different ways depending on whether they are male or female.

Harvard University's Richard Wrangham and Bates College's Sonya Kahlenberg have collected 14 years worth of observation from the Kanyawara chimpanzee community in Uganda's Kibale National Park. Specifically, they focused on how chimpanzees used sticks. One particular activity came to be known as "stick-carrying," in which certain chimps would carry a stick around and treat it like a child.

The researchers started to suspect female chimps were doing it more often than their male counterparts, and more detailed observations revealed this to be exactly the case. Wrangham explains:

"We thought that if the sticks are being treated like dolls, females would carry sticks more than males do and should stop carrying sticks when they have their own babies. We now know that both of these points are correct."

What's more, the stick-carrying females would sometimes play with their sticks in exactly the same way that chimpanzee mothers are seen treating their young. The fact that both humans and chimpanzees display this behavior suggests there really could be a biological component to why females are more likely than males to play with dolls.

Now, there's a very important caveat to this research. Wrangham and Kahlenberg acknowledge that stick-carrying may not be a widespread practice. No other chimp researchers have reported stick-carrying elsewhere, and it's rare even among the Kanyawara chimps that they studied. Now, chimpanzee play is generally not well understood because most chimp communities are small, with only a few youngsters around at any one time.

So, all things considered, we can't yet be certain that the chimps are playing with dolls because of a biological predilection or because a specific practice has popped up that's specific to the Kanyawara chimps. But, as Wrangham explains, that would actually be just as intriguing a result:

"This makes us suspect that stick-carrying is a social tradition that has sprung up in our community and not others. It will be the first case of a tradition maintained just among the young, like nursery rhymes and some games in human children."

[Current Biology; image via ARKive]