Chimpanzees choose tree branches that give them the most firm and stable place to sleep, a new study says.
What's more, the research bolsters the theory that high-quality sleep may have led to the evolution of modern humans, said study leader David Samson, an anthropologist at the University of Nevada, Las Vegas.
Chimpanzees build their beds—called nests—in tree canopies using branches that they harvest from specific tree species. They spend about eight to nine hours a night on these platforms.
But until the recent study, published April 16 in the journal PLOS ONE, scientists didn't know how they selected the building materials.
Samson and his colleagues measured the stiffness and bending strength of seven tree species most commonly used by chimps in the Toro-Semliki Wildlife Reserve in western Uganda. The team also looked at the surface area of the leaves on the trees and the structure of each tree species.
What they found was remarkable: Of the 1,844 chimpanzee nests studied, 73.6 percent were made from a sturdy tree called Ugandan ironwood—even though that species made up only 9.6 percent of trees in a survey of the region.
"Despite the fact it's relatively rare, they're saying seven out of ten times, 'I want to sleep in this species,'" Samson said.
Chimps in the Toro-Semliki Wildlife Reserve sleep in trees in part to avoid nighttime predators such as leopards or lions.
For that reason, their beds have to be extremely durable, with no risk of the chimps falling out during the night. Ironwood was the strongest of the seven tree species tested in the study, so it's likely the best bed-building material, Samson said.
Ironwood branches also have a distance between shoots that makes them amenable to being woven together, which chimpanzees do to ensure that their bed is strong. (Watch a video of a chimp outsmarting humans.)
Ironwood allows for a "more sturdy locking system; the box spring to their mattress is a little more tight-knit," he said. "They are just as concerned about a comfortable night sleep as you or I."
The chimps know that ironwood is a "special tree species—they can recognize these properties," he said.
Aaron Sandel, a biological anthropologist at the University of Michigan in Ann Arbor who wasn't involved in the study, said that there aren't enough studies on ape sleep, and that the new research is a "cool idea."
He agreed with the results that "overall the chimps are choosing trees based on safety and ease of making a nest."
What about chimps in other parts of Africa? Study author Samson said there's evidence that other populations also choose wood offering the best night's sleep, depending on their environment and available tree species.
Sleep Made Us Smart?
Ultimately, Samson wants to know if sleeping played a role in human evolution.
For instance, great apes (including us) are unique in that we build sleeping platforms, or beds. Other primates sleep on branches.
Sometime in the Miocene period, 23 to 5 million years ago, ancient apes changed their sleeping locations from branches to platforms. That, in turn, led to a better night's sleep.
Studies in both humans and orangutans show that better quality sleep, with longer periods of rapid eye movement, improves cognition and memory. (See "Chimps, Orangutans Have Human-Like Memory.") Ancient apes' improved slumber may have led to improved next-day cognition after nights on comfortable beds.
But it's also possible that apes' big brains may have led to the need for more sleep, not the other way around, noted Sandel.
In any case, Samson said, an added boost in cognition certainly gave apes and humans an evolutionary edge.
"Big brains," he said, "need big pillows."