Behavioural Changes At Adolescence in Chimpanzees

Abstract

Male chimpanzees experience puberty (marked testicular growth and ejaculation) when they are about 9 years old and just over half adult weight. They reach adult weight at 14-15 years. Females start small sexual swellings at about 8 years, mate with adult males when they are about 10.5 years and have their first infants when they are about 12 years. Males reduce their constant association with their mothers when they reach puberty and associate more with adult males and cycling females. Some males form strong, long-lasting associations with particular adult males. Females remain with their mothers until first oestrus,and then associate with adult males either in their natal community or a new community during oestrus. The decline in association with the mother is more closely correlated with pubertal stage than with chronological age in both sexes. Immatures of all ages spend 3-13% of their time in social grooming, but grooming partners change during adolescence. Juveniles of both sexes groom their mothers and siblings most, but males groom individuals outside their maternal unit more than do females. Juveniles receive over 90% of their grooming from mothers and siblings. Adolescent males groom less with their mothers and more with cycling females and adult males. While early adolescent males start to be groomed by females only late adolescent males receive any grooming from adult males. Adolescent females without mothers groom little except with adult males when they are in oestrus. Social play is most frequent with individuals aged 2-9 years and individuals play most with others of their age. Time spent playing by both sexes declines markedly during adolescence. Juveniles and early adolescents of both sexes have friendly interactions with infants, but late adolescents rarely interact with infants. Males show frequent sexual behaviour with oestrous females from an early age. Males spend more time with oestrous females as they get older but frequency of copulation declines gradually through adolescence. Early adolescent males try to take females on consort, but only late adolescent males succeed. One female rejected the sexual advances of a male once he reached puberty. Rates of interference in copulations by males decline during adolescence and males become more cautious of copulating in the presence of adult males. Females show little sexual behaviour until they start getting large oestrus swellings. Males receive increased aggression from adult males during adolescence. They become tense in the presence of adult males and even late adolescents are peripheral to grooming clusters of adult males. Males start to challenge adult females in adolescence but are often attacked, and are not able to dominate them all until late adolescence. Male juveniles perform elements of charging displays without hair erection more frequently than females. The freqency of this declines in adolescence while the frequency of charging displays with hair erection by males increases in adolescence and displays become increasingly more adult-like in length and form. Males show the shoulder hunch for the first time in early adolescence. Males pant hoot more than females and rates of pant-hooting increase with age in both sexes.