Social dynamics modify behavioural development in captive white-cheeked (Nomascus leucogenys) and silvery (Hylobates moloch) gibbons.

Behavioural development was quantified in one family group of silvery gibbons (Hylobates moloch) and one of white-cheeked gibbons (Nomascus leucogenys) over 11 months during 2005 and 2008 at the Perth Zoo. Levels of locomotion, solo play and play solicitation peaked by 5 years of age but continued solo and social play in older immatures suggested that social development continued until at least 7 years of age. Mature offspring responded to play solicitations from younger siblings. The transition to sub-adulthood was marked by the presence of spatial peripheralisation from the parents, and coincided with aggression from the father to a sub-adult male. After the birth of a new infant, the male sub-adult stayed closer to his mother (and the infant) but not to his father; his juvenile brother was closer to both parents. Within-family observations of behaviour that is difficult to observe in the wild but can be observed in captivity contributes to our understanding of family dynamics in gibbons. Observations of these captive groups suggest that sub-adult peripheralisation may be influenced by family social dynamics as well as by local ecology, and that older offspring are responsive to the development of younger siblings.