She's better at making her bed

Iulia Bădescu

Credit: Courtesy

When do you make your bed? In the morning when you get up? Well, if you were a chimpanzee – our closest genetic relative, with about 99 per cent of our DNA – you'd more likely make it at dusk, just before you go to sleep for the night. Primatologists call it "nest building."

In trees, chimps will twist, break and weave branches into a circle, and line the nest with twigs and leaves. Then, when the sun sets, they'll climb in over the rim, nestle down, shut their eyes and sleep till dawn. They'll often build nests during the day, too: places where they can rest, play, groom and eat.

It's all part of being a chimp. Like other great ape species, they've been doing it for millions of years, making sure they have a secure place to loll away the hours. The more they rest and the better they sleep, the more energy they have, and the better their brains remember and process the tasks ahead.

But here's the thing: as with a great many humans, when it comes to making one's bed, there appears to a gap with chimps between the sexes: females, it seems, do it more often than males, and from a younger age. And that shows they're more independent, earlier in life.

That's one of the findings of a new study co-led by Canadian primatologist Iulia Bădescu, an associate professor of anthropology at Université de Montréal who has spent a big part of her career getting to know the chimpanzees of Uganda's Kibale National Park, a top global habitat for the species.

Published in the American Journal of Primatology, the study was written with anthropologists from UdeM, the University of Toronto and Yale University. First author Tara Khayer was an UdeM undergraduate in biology at the time and is now pursuing a master’s at UdeM in biochemistry.

72 chimps studied

Nest building requires learning in early infancy and a threshold of physical development to manipulate tree branches.

Credit: Iulia Bădescu

Together, the researchers examined the daytime nest building of 72 wild immature chimpanzees in 2013-2014 and again in 2018 at a site in Kibale called Ngogo.

Bădescu and her team evaluated the effects of age, sex, and maternal parity (how often a chimp mother has raised offspring) on the likelihood of an infant to build a nest. They also looked at the rates (frequency per hour) and durations (time from start to finish of each nest) at which infants built nests.

Compared to the youngest infants, older infants were more likely to build nests, and built them at higher rates, which confirms what primatologists have observed: that nest building requires learning in early infancy and a threshold of physical development to manipulate tree branches.

Significantly, the researchers found that female infants were more productive in building nests than male infants, corroborating other developmental markers suggesting females in some chimpanzee communities attain independence from the mother at younger ages than males.

Varies with age and sex

Nests fulfill a functional, sleeping role for older youngsters, but may be used more for playing and ecological exploration when chimps are little.

Credit: Iulia Bădescu

"The likelihood and overall rates of daytime nest building in wild infant chimpanzees varied with age and sex," said Khayer. "Older and female infants were more likely to build nests, and build them more often, than younger and male infants. "

Among infants who were seen to build nests at least once, rates and durations were similar, regardless of age, sex, or maternal parity, which indicated that there was little inter-individual variation in the development of nest-building skills once infants started practicing.

The new study confirms previous studies have shown that infants older than 4 years build nests more for resting than younger infants, which means that nests fulfill a functional, sleeping role for older youngsters, but may be used more for playing and ecological exploration when chimps are little.

"Nest building also represents a type of tool use, since the chimps have to manipulate branches and other materials around them to construct their nest," said Khayer. "Just like cracking nuts with stones or fishing for ants with sticks, nest building involves putting their environment to good use."

What does all this mean for us?

"Humans, too, sleep in beds," said Bădescu, "so there are interesting evolutionary implications. Studying how chimps learn to build nests from a very young age helps us better understand how all hominids, including us, develop and learn new skills."