Keynote Series: Zarin Machanda — The hidden cost of motherhood for chimpanzees

By Alexander Albury

Play might be something that we as humans take for granted. We see it as a common activity; something that’s almost automatic. But our beliefs about play may be skewed towards the human experience. For many species, play among juveniles serves as an important catalyst for development, but play among or with adults is less common and parents do not always serve the role of play partner as they do in humans. Considering how uncommon adult play can be in the animal kingdom, it’s no surprise that chimpanzees – our closest genetic relatives – are a species whose play behaviour closely mirrors our own. But while play behaviour has adaptive benefits, it comes at a cost in the wild, which means that there’s important variation in when and why chimpanzees engage in play.

A group of researchers led by Drs. Zarin Machanda & Kris Sabbi set out to understand the factors that influence how chimpanzees play by collaborating with the Kibale Chimpanzee Project, a wildlife conservation and research initiative in Uganda. Over 10 years they observed how adult and juvenile chimpanzees interacted with each other. In particular, they were interested in cross-cohort interactions, that is, play between chimpanzees of different ages. They found that chimps of all ages engage in play at least some of the time; adults with juveniles, juveniles with juveniles, and even adults with other adults.

But the rate at which chimps play differs depending on context. When food resources are lower, chimpanzees are less likely to engage in play in an attempt to conserve energy. For example, when food resources are high, adult chimpanzees engaged in play with each other and with juvenile chimps that weren’t their own, however, as diet quality worsened, they stopped playing almost entirely. However, mother chimpanzees continued to play with their children even when food resources were lower, indicating that this type of play behaviour may have specific evolutionary benefits

The authors note that play between a mother chimpanzee and her child can be important in several ways. While social play can have benefits for all chimpanzees, it’s particularly important for developing chimps. And the social make-up of wild chimpanzee groups can make it difficult for juveniles to find adequate opportunities to play. Whereas most primates live in social groups that group newborns together in ‘nursery groups’, firstborn chimps often only have their mother to play with until their first sibling is born up to 5-7 years later. Mothers fulfill this need even when environmental conditions make it harmful to them.

Chimpanzee mothers acting as their childrens’ first play partners highlights their similarities to humans and further emphasizes the importance of social play in development. While the function of playing can vary for adults, the importance of play in development is so great that mothers will fulfill this need even at a disadvantage to themselves, revealing a hidden cost of motherhood for chimpanzee mothers.

Original Research: https://doi.org/10.1016/j.cub.2024.02.025 

If you’d like to know more about Dr. Machanda’s research, check out our interview with her here.