Chimpanzees choose tree branches of a particular type, especially those that are firm and flexible for a good night's sleep, according to a University of Nevada and Indiana University study.

Chimpanzees use tree branches to build beds, called nests, in tree canopies using branches from specific tree species. The primates reportedly spend about eight to nine hours a night on these beds, National Geographic reports. To protect themselves from predators and to avoid night-time accidents (falling out), their beds need to be extremely durable.

To determine why the primates select certain tree species over others, the researchers analyzed physical properties of the branches. They measured characteristics (stiffness, bending strength, leaf surface area and structure of each tree species) of 326 branches from seven tree species, commonly used by chimps at the Toro-Semliki Wildlife Reserve, Uganda.

Out of the 1,844 nests, chimpanzees selected Ugandan Ironwood 73.6 percent of the time, even though the trees constitute a smaller portion (9.6 percent) of the sample area.

"Despite the fact it's relatively rare, they're saying seven out of ten times, 'I want to sleep in this species,'" study leader David Samson, an anthropologist at the University of Nevada, Las Vegas, said.

The researchers found Ugandan ironwood to be the stiffest among the seven tree species tested. It featured the greatest bending strength. Also, the distance between leaves on the branches was the shortest and their leaves had the smallest surface area.

The researchers said that chimpanzees select trees like the Ugandan ironwood because it protects against predators like leopards or lions. They also prevent pathogens from affecting the chimps, aid temperature regulation and provide comfort.

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," Samson said.

Aaron Sandel, a biological anthropologist at the University of Michigan in Ann Arbor, said that it is a significant finding as no study has focused on chimps' building materials yet. "Overall the chimps are choosing trees based on safety and ease of making a nest."

The finding is published in the journal PLOS ONE.