Sexual behaviour of group-living adolescent chimpanzees.

This paper reports a study of the heterosexual behavior of three male and four female captive adolescent chimpanzees living a semi-natural life style in a large field enclosure. Observations made with binoculars from an overhead deck were balanced over the daylight hours and the seven weekdays. We recorded 213 copulations in 741.25 hours of observation between February 14 and July 21, 1972. Analysis of the observations suggests that group-living chimpanzees in a large field enclosure behave more like free-living chimpanzees than like other captive chimpanzees paired in small cages. In this group, copulations were non-randomly distributed throughout the day, a finding in agreement with results on wild-living chimpanzees but not previously reported for captive chimpanzees. In this group, moreover, copulations were highly concentrated within each female's period of maximum tumescence, as are those of free-living chimpanzees; and individual differences in sexual attractiveness among females were apparently based on a maturational threshold of swelling size, a phenomenon that has also been reported for wild chimpanzees. This paper reports a study of the heterosexual behaviour of captive adolescent chimpanzees living a semi-natural life style. The most complete previous reports of chimpanzee sexual behaviour were based on captive animals housed in pairs in small cages (e.g., Yerkes and Elder, '36) or, more recently, on wild populations (van La wick-Goodall, '68). Previous studies of chimpanzee sex in group situations in captivity are largely non-quantitative and generally descriptive (e.g., Kollar et al., '68). By studying in detail group-living chimpanzees in a large field enclosure, we hoped to achieve an ideal compromise between the types of studies cited. We hoped to remove the artificial and sometimes distorting limitations of the bare cell and limited companionship situation. while at the same time retaining the advantages of uninterrupted, unimpeded observation not available in the wild. Results of a detailed ethological study of the elements of sexual interaction and their frequencies and sequences will be presented elsewhere. AM. J. PHYS. ANTHROP .• 38: 195-200. MATERIAL AND METHOD Three male and four female wild-born chimpanzees (Pan troglodytes) at the Delta Regional Primate Research Center, Covington, Louisiana, participated in the study. Five individuals formed the basis for the findings presented here: the male Shadow (age 9), and the four females Gigi (10), Polly (9), Belle (8) and Bido ( 8). A female infant was born to Gigi in the enclosure on May 1, 1972, and is being reared in the group. The chimpanzees arrived at Delta when between one to four years of age and have lived in a group or as subgroups since then. From September, 1969, to September, 1972, the chimpanzees were housed together (except for temporary absences) in a 30 x 120 m outdoor enclosure. It was open overhead, had walls 5 m high of sheet metal above and chainlink fence below, and was surrounded by pine forest. The enclosure was carpeted with natural vegetation and contained three elevated platforms, numerous upright and fallen poles and tree trunks, and a plentiful sup-