April 23, 2012
Chimpanzees know how to make a bed. Every night they climb up trees and curl up in nests they build out of branches and leaves. They sleep in the treetops to avoid nighttime predators such as leopards. Many anthropologists think early hominids did the same thing when it was time to catch some zzz’s. But at least one population of chimpanzees enjoys sleeping on the forest floor, new research shows. This may mean that some early hominids did, too.
The ground-sleeping chimps live in the Nimba Mountains of southern Guinea. Kathelijne Koops of the University of Cambridge in England and colleagues analyzed 634 chimp nests there from 2006 to 2008. About 14 percent of these beds were on the ground. In most chimpanzee populations, less than 5 percent of nests are on the ground, the team reports in the American Journal of Physical Anthropology.
What’s different about these chimps? They do not lack appropriate trees, the team notes. But the area does seem to be missing the dangerous nocturnal predators that force other chimps to rest in trees. Without this pressure, Koops and her colleagues hypothesized, male chimps might be camping out beneath the trees of females they wanted to mate with, to keep other males away.
To test that idea, the researchers turned to DNA. They collected hair in 46 ground nests to analyze the owners’ genetic material to determine their sex. Thirty of the nest-makers were male, four were female, and the sex couldn’t be determined for 12, the researchers found. Since ground-nesting is largely a male behavior, it lends support to the idea that this could be a male mate-guarding strategy—but the males don’t appear to be slumbering beneath the beds of females. In analyses of tree nests above the ground nests, it turns out most males were resting below other male relatives. So the mate-guarding hypothesis doesn’t seem to hold up. For now, it’s unclear what motivates chimps in this community to sleep on the forest floor.
So what does this have to do with our ancestors? Anthropologists have looked to chimpanzee sleeping habits as a proxy for early hominids because early hominid beds aren’t preserved in the fossil record. (The earliest known hominid bed dates to 77,000 years ago.) Even though our earliest ancestors probably spent most of their time walking upright on the ground, their skeletal features reveal that they still retained some climbing capabilities. And since they were vulnerable to predators, anthropologists reason, they were probably safer in the trees, just as most chimps are today. Researchers speculate it wasn’t until Homo erectus, which had a modern body plan, that hominids started sleeping on the ground.
But the new research suggests that perhaps under certain circumstances, some earlier hominids snoozed on the ground as well. But more than that, I think the study reminds us that individuals within a species are variable, and not all populations behave the same. Chimpanzees in different parts of Africa, for example, eat different foods, use different tools and apparently build nests in different parts of the forest. It’s likely that in any given species of early hominid, individuals behaved differently both within and between different communities. Studying this variation in our early ancestors is an almost impossible task because most species have been found in only a handful of locations. And as with sleeping, some behavior doesn’t leave a mark on the fossil record. So analyzing the circumstances under which particular behaviors of modern animals vary—and looking for ways to correlate that to evidence that gets preserved in the fossil record—is a crucial aspect of human evolution studies.
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