Road-crossing in chimpanzees: A risky business

The research is reported by Kimberley Hockings and James Anderson of the University of Stirling and Tetsuro Matsuzawa at the University of Tokyo and appears in the September 5th issue of Current Biology, published by Cell Press.

Prior research has shown that adult male monkeys reduce the risks of predatory attacks through adaptive spatial patterning, moving toward the front of the group when traveling toward potentially unsafe areas, such as waterholes, and bringing up the rear when retreating, but comparable data on great-ape progression orders have been lacking. Crossing of man-made roads, sometimes necessitated by intersection with longstanding chimpanzee travel routes, presents a new situation that calls for flexibility of responses by chimpanzees to variations in perceived risk. Understanding how chimpanzees cross roads as a group would help shape our hypotheses about the emergence of hominoid social organization.

Progression order–the order in which individuals travel within a group–was studied in the small community of chimpanzees (Pan troglodytes verus) at Bossou, Guinea, West Africa as they crossed two roads, one large and busy with traffic, the other smaller and frequented mostly by pedestrians. Adult males, less fearful and more physically imposing than other group members, were found to take up forward and rearward positions, with adult females and young occupying the more protected middle positions. The positioning of dominant and bolder individuals, in particular the alpha male, was found to change depending on both the degree of risk and number of adult males present, suggesting that dominant individuals act cooperatively, and with a high level of flexibility, to maximize group protection.