The Development of Sociosexual Behaviour in Free-Living Baboons, Papio Anubis

Abstract

The development of mounting behaviour and the communicatory functions of mounting were studied in a free-living troop of olive baboons in the Gombe Stream National Park, Tanzania, over a period of 16 months. 1. Early mounting attempts by baboon males were disoriented and incomplete. Components of mounting such as penile erection and pelvic thrusting at first occurred in isolation and in inappropriate contexts, but at 10-15 months of age, males became able, rather suddenly, regularly to orient themselves correctly to the female when mounting. 2. From about 10 months of age, thrusts became more rhythmical and increased in number per mount, and at about 12 months, intromission and the ejaculatory pause first occurred. At this age also, the frequency of male mounting reached a peak. Between 20 and 50 months of age, the characteristics of male-female mounting changed rather little, but after puberty, the proportion of mounts in which an ejaculatory pause occurred was twice as great as in younger age groups, and the number of thrusts required to reach ejaculation decreased. 3. Tumescent adult females were available for mounting by young males throughout the year, though young males could not gain access to females whilst they were maximally swollen and in consort with adult males. From about one year of age, males increasingly selected adult females as mounting partners. Male-male mounts, however, continued into adulthood. 4. The presence of cycling females in the troop with which young males could achieve intromission was thought to be particularly important for the development of the orientation and integration of male mounting patterns in baboons. This is supported by studies of captive peer-raised macaques, in which the course of development of male mounting behaviour tends to be delayed compared with that in naturalistic or wild groups of primates. 5. The nature and time scales of development of male mounting patterns in the genera Papio and Macaca appear to be very similar, though in olive baboons, unlike macaques, the ejaculatory pause occurred before puberty. In primates and in other mammals, important changes in mounting development seem to occur both during the early months of life and around puberty. 6. Mounts were observed in the contexts of 'dominance', play, appeasement/greeting, and enlisting as well as in a 'sexual context. Mounts between males were not always in the direction that agonistic status or play relationships would predict, and for any male-male pair, mounts could occur in either direction. The direction of mounting was dependent on the nature of the communication involved, and in one type of mounting (appeasement/greeting), it was generally the subordinate male that mounted. The general function of non-sexual mounting seemed to be to reduce the likelihood of aggression.