ABSTRACT
Some of the most fascinating opportunities for the study of behavioral adaptations in nature occur where two species w i t h common ancestry but w i t h different organizations come into contact. When we learned, at the beginning of our sec ond field study in 1968, that the newly founded Awash Na tional Park in Ethiopia was inhabited by both hamadryas and anubis baboons, we immediately decided to work there. I n the following days, the discussions in our team focused on the possible nature of the species border between the familyforming hamadryas and their family-less relatives. W h y was the border at Awash instead of somewhere else? Would an analysis of the border zone give us some hints on the ecologi cal factors that favor the specialized organization of the hamadryas? I f there were an ecological advantage in the one-male group system, the hamadryas would probably ex pand just as far westward as the habitat conditions support ing i t . I n the absence of physical barriers, anubis baboons would have to take over exactly where the changing environ mental factors began to favor a non-family organization w i t h large, closed groups of the anubis type. Before we took up the project, we developed a simple model in which two op-
posed gradients of success met in a zone of equal survival value for both species.
