SEXUAL BARGAINING AMONG ADULT MALE CHIMPANZEES

OTTO M.J. ADANG, JAN A.R.A.M. VAN HOOFF, FRANS B. M. DE WAAL & JOOST C. VAN DEN ROOVAART

Paper presented at the scientific meeting of  the Netherlands Society for Behavioural Biology, December 1987, Dalfsen, the Netherlands

ABSTRACT

Data are presented relating to sexual competition between the three top ranking males in the Arnhem Zoo chimpanzee colony in the course of three successive years. Matings were distributed unequally over the males, and changes in the relationships between the males resulted in a changed distribution of matings.

In the year when the dominance relationships between the males were least stable, a subdominant male, who had formerly been the coalition partner of the alpha male, habitually groomed the alpha male just prior to sexually inviting an estrous female. In this period the subdominant male clearly had the highest mating frequency. When subsequently the subdominant male did not support the alpha male against his rival, the subdominant's pre-invitation grooming became less frequent and his mating frequency dropped dramatically.

The data are considered to provide evidence for the existence of a "sexual bargaining" process (part of "chimpanzee politics"), in which one of two males may gain undisturbed matings in return for favorable male-male agonistic support. Via pre-invitation grooming the willingness of both males to keep their side of the "bargain" is assessed.

Two adult males grooming next to oestrus female