Chimpanzees prefer firm, stable beds

Chimpanzees use tree branches to build beds or nests in trees. They select certain tree species to sleep in more frequently than others, but the reason for selecting a particular tree is unclear. To determine whether the physical properties of trees influenced nesting site selection, scientists measured the physical characteristics of wood from common tree species at the Toro-Semliki Wildlife Reserve, Uganda. They measured the stiffness and bending strength of 326 branches from the seven tree species most commonly used by the chimps. Additionally, they measured leaf surface area and determined the structure or architecture of each of the seven species.

Of 1,844 nests sampled, chimpanzees selected Ugandan Ironwood for 73.6% of the nests, even though it represented only 9.6% of all trees in the sample area. Ugandan ironwood was the stiffest and had the greatest bending strength of all the trees tested, had the smallest distance between leaves on the branches, and had the smallest leaf surface area. The authors suggest that chimpanzees select trees, like the Ugandan ironwood, due to these properties, as they may provide protection from predators and pathogens, as well as provide temperature regulation and comfort.

Dr. Samson added, "Chimpanzees, like humans, are highly selective when it comes to where they sleep. This suggests that for apes there is something inherently attractive about a comfortable bed--down to what kind of wood you use to make it."