Heterosexual bonding and partner preferences among captive chimpanzees.

Abstract

This study tested hypotheses about the dynamics and function of male/female affiliative relationships (friendships) in captive chimpanzees by collecting data relevant to (1) the circumstances surrounding the formation of friendships; (2) the types of anestrous/estrous interaction between males and females once a friendship has formed; (3) the basis for particular partner preferences; and (4) the potential benefits of friendships. The data were collected over a 15-month period on a semi-free ranging colony of eleven chimpanzees at the Detroit Zoo in Michigan. This study was the first to investigate the formation and special characteristics of friendships in chimpanzees. Friends were defined on the basis of time spent in spatial proximity and grooming. Based on these criteria, three male/female dyads clearly formed friendships. The circumstances surrounding friendship formation and the special characteristics of friendly relationships, however, were less clear. The data indicated that (1) the frequency and degree of heterosexual interaction during the female's first three estrous cycles only weakly predicted later friendships; (2) among Friends, one individual was responsible for most grooming interaction, but the sex responsible for grooming was inconsistent among those dyads defined as Friends; (3) among Friends, the same individual was responsible for both grooming and maintaining proximity; and (4) females avoided proximity with their male Friends when in estrus. The benefits of forming friendships were difficult to determine. Some evidence suggested that one male received the benefit of sexual access to his Friend while she was in estrus. For both males and females, support during aggressive interaction with other group members was not a benefit of friendship formation. Female partner preference was probably based on male dominance rank or male temperament; male partner preference was probably based on a combination of female age/dominance rank, and potential agonistic support. Although the results are not conclusive, this study provides a detailed description of the introduction of unfamiliar males and females and the subsequent development of male/female affiliative relationships among captive chimpanzees, which will help to further our understanding of both chimpanzee social relationships and the evolution of hominid pair bonding.