Altmann, S. A. , Altmann, J. 1970. Baboon Ecology - African Field Research. The University of Chicago Press, Chicago, IL.

Excellent overview of the species-typical behavior of wild baboons. At night, the baboons that we observed in Amboseli slept only in trees. In the Serengeti plains, on the other hand, baboons were observed on a kopje (rocky outcropping). ... The use of trees as sleeping sites is virtually universal among other monkeys - probably a reflection of their basically arboreal habits.