Spending the night in a chimpanzee nest taught me why ancient humans may have preferred to sleep on the ground, says Fiona Stewart
Why did you decide to spend the night in a chimp nest?
People have suggested many hypotheses as to why chimpanzees build a nest every night
, so I decided to climb up into nests and sleep in them to really understand how and why nests are built.
How do chimps go about building a nest?
They bend and break branches to form a platform. Starting with larger branches that form a support structure, they then pull in side branches and other twigs to form a mattress. The chimpanzees then line the surface with twigs and leaves, creating a nice bowl shape.
The first couple of times, I slept in a chimpanzee-made nest. In the end, I decided it would be more effective to sleep in a fresh nest, so for the remaining four trials I built my own. Chimpanzee nests are about 90 centimetres across, but mine were longer because I'm longer than a chimpanzee. I also slept out on the ground for five nights.
What was it like sleeping like a chimpanzee?
It was incredibly uncomfortable. I slept much less than on a normal night. But I was surprised to find I slept better in a chimpanzee nest than I did on the ground. It's the safety aspect, the security of curling up in a nest.
On the ground I was more disturbed. I would hear noises and wake up more often. Chimpanzees elsewhere, where there are more predators, build nests much higher up in the trees than the chimpanzees in Fongoli, Senegal, where I did my experiment. That might be to keep clear of predators.
What did you discover about the nests?
The nests helped to keep my temperature up. Chimpanzees will also build a warmer nest on a colder night, which supports the idea that they have a thermoregulatory function. I also got bitten less in the nests than on the ground, so they may help protect against diseases carried by insect bites such as malaria. Perhaps the chimpanzees are selecting trees that repel insects.
What can your experiments tell us about human evolution?
One of the interesting questions is when, in our evolution, did we come down to the ground, and why? When did we make the transition from sleeping in trees to sleeping on the ground?
To have the same quality of sleep on the ground, you need protection. Some people have suggested it wasn't until we had something else to protect ourselves, like fire, that we started sleeping on the ground. There may have been creatures that were bipedal but still had to return to sleep in trees. Even Homo habilis retained some traits that are linked to arboreality [living in trees]. Could it be they retained them because they were sleeping in the trees, even though they spent their days on the ground?
Are you going to repeat the chimp-nest sleeping experiment?
I learned a lot, and ideally I would have done it a few more times. But no, not in the near future.
Profile
Fiona Stewart is a biological anthropologist at the University of Cambridge. She studies chimpanzees to understand the evolution of humans and other primates and is currently conducting field studies of chimps in Tanzania
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