Why do chimps nest on the ground?

All great apes spend each night in a freshly constructed nest. While gorillas often nest on the ground, the other great apes (chimpanzees, bonobos and orangutans) mostly nest in trees. Nevertheless, chimps have been observed to make the occasional ground nest. Why the move down from the trees? Kathelijne Koops and her colleagues from the University of Cambridge plus William McGrew from Kyoto University tested one possible reason.

The researchers hypothesized that males were putting their nests below the arboreal nests of sexually receptive females. In other words, the males were camping out below potential mates to guard them from the attentions of other males. To test this idea, the researchers went to the Seringbara region of the Nimba Mountains in the Republic of Guinea, a local where chimps are known to nest on the ground much more frequently than in other regions.

Because nests are abandoned every day, the researchers were able to approach old nests and extract hairs for genetic sequencing. DNA tests showed that at least twelve individuals out of a total community of 36 adults nested on the ground. Thus, about a third of the chimps in the area regularly nested on the ground. Of those ground nests, 65% had definitely been occupied by males and only 9% by females (the rest were indeterminate—you can’t always get good DNA samples from hair). So far so good for the Juliet’s balcony theory. Unfortunately, the closest tree nest above these ground nests also tended to be occupied by males. So much for the guarding sexual partners hypothesis.

By the time our hominin ancestors started sleeping on the ground, they may have had the protection of fire to keep away predators. This clearly is not the case for ground-nesting chimps. Likewise, the decision of where to spend the night was not influenced by either weather or scarcity of proper sleeping trees. In fact, many of the chimps built nests both in trees and on the ground, sometimes on the same night. If it’s not weather, resources, predators or sex, on what basis do chimps decide to sleep on the ground? This study was not able to answer this question, which is too bad because it might have shed some light on why our ancestors made the same decision a few million years ago.


Update: Since I wrote this, two new studies appeared on this subject, this time linking the choice of nest site to wind and humidity respectively. These studies suggest that chimps may simply be nesting wherever they feel most comfortable. I'm not convinced that the answer is that simple, especially since weather did not seem to be a determining factor in the original study.