The social development of immature mountain gorillas (Gorilla gorilla beringei).

Abstract

This thesis investigates the social development of immature, wild mountain gorillas
(Gorilla gorilla beringei) in the Virunga Volcanoes in Central Africa. Three maturational
phases in the behavioural development and socialisation of primates are discussed infancy,
juvenescence and adolescence. A literature review of behavioural studies on the mountain
gorilla forms a summary of the social structure within which the immature develops.
Twenty-five immature gorillas were observed for a total period of 1500 hours
between September 1990 and December 1992. Attainment of independence from the mother
was a gradual process with the infant playing the primary role in decreasing mutual contact
and association. The overall group activity pattern, namely feeding or resting, exerted a large
effect on the behaviour of the infant. There was some evidence of increased maternal
investment in male infants. Transport by the mother was investigated in detail. Transport
began with the infant ventrally supported by the mother, followed by ventral unsupported
riding, which was gradually replaced by dorsal riding. However, there was some variation
between individuals, with infants of multiparous mothers riding dorsally sooner than first
offspring. Weaning, measured by suckling frequency and mother-infant agonism, was
particularly severe in two infants while it appeared to be very gradual in others, with
increased conflict observed in individuals that were weaned at a younger age.
Total play increased through infancy, began to decrease in the juvenile period and
then increased again briefly during adolescence. Partner preference was examined and
largely supported the prediction that individuals play with like-sized partners of similar age.
The ontogeny of play types was examined. Social play replaced solitary play by the end of
infancy. Rough play emerged later than gentle play and the latter was present at relatively
higher frequencies throughout immaturity. Infant and juvenile males engaged in more
energetic play than similarly aged females, although this pattern was reversed in adolescence
when males more commonly restricted their play to that with young infants.
Immature-silverback association was examined in detail and was found to change
with the age of the immature. Having not exhibited a clear pattern in infancy, a strong
preference for individuals to associate with the dominant male emerged during juvenescence.
Patterns of association were similar in both groups. Adolescent-silverback association was
different between sexes with females preferring the proximity of the subdominant male
during feeding while males preferred to associate with the dominant male.
Grooming by immature gorillas was extremely low in infancy, increasing only
slightly in adolescence. A clear sex difference was evident, even as early as infancy, with
females grooming others more than males did. Self grooming tended to intensify when there
was little grooming received from others.
Sociosexual behaviour first emerged during play in infancy and occurred in both
dorso-ventral and ventro-ventral positions. The actor was usually the older of the dyad.
Later it was seen outwith the context of play, with juveniles initiating more bouts than any
other age class. Heterosexual pairs were most common with female-female dyads the least
frequent. By adolescence the behaviour pattern still bore several differences to adult
copulatory behaviour.
Agonism was rare in immatures as is the case in adults. Displacements were usually
initiated by an older individual and reflected the age-related dominance heirarchy already
reported for immature mountain gorillas

Date of Award	1994
Original language	English