title: Life history strategy of western gorillas (Gorilla gorilla gorilla) : an investigation of juvenile and adolescent social development
Creators: name: Elizabeth Hutchinson, Johanna
Publisher: name: University of Liverpool Repository
Year: 2023

description: Life history theory seeks to explain how comparative developmental and maturational parameters vary between species and how such differences affect survival and reproductive success. In primates, the immature period is found to be longer than in other relatively sized mammals, delaying sexual maturation but being essential for brain growth and social development. Compared to adulthood, the primate immature period remains understudied, limiting our understanding of how delayed maturation contributes towards species fitness. This thesis investigates social development of western gorillas (Gorilla gorilla gorilla), who have a relatively prolonged period of immaturity, and focuses on the pre-reproductive period from weaning until sexual maturity. Behavioural data were collected from 13 male (3-11 years) and 9 female (3-8 years) non-reproductive individual (NRI) western gorillas, housed in 5 family groups in European zoos. Data from a further 9 male and 7 female NRIs housed in 3 peer groups in sanctuaries were collected for comparison. A combination of continuous focal sampling, as well as scan, all occurrence and ad libitum sampling was used; 1300 hours of data were collected over 180 days. Within the thesis, a quantitative method to categorise gorillas into a life-stage was designed, which demonstrated distinct behavioural disparities between immature life-stages and between the sexes, highlighting the problems of life stage categories that are based only on dtatic age classes. A study of social relationship development followed, finding that spatially, NRIs became increasingly peripheralised from the group core with age. Socially, younger NRIs invested in prominent maternal and conspecific relationships, whereas older NRls did not. Relatedness affected sociality, with full siblings being more interactive than paternal half siblings are. An investigation of alloparenting showed that both male and female NRIs displayed this behaviour, with females continuing infant interactions until adulthood. It was postulated that gorilla alloparental behaviour is driven by the NRI and supports the 'learning to parent' hypothesis. Social play behaviour was also examined and found to be most common between similar-sized peers, with males generally being the preferred play partner. Disparate-sized play partners were more likely to be full siblings than paternal half siblings were. Younger NRIs were found to engage in more predictable play sequences whilst older NRIs engaged in play that was more sporadic. Support for the 'neural' hypothesis of social play was found, with social play having delayed benefits for the individual, although the immediate benefits of play were not dismissed. Finally, family-raised NRI behaviour was compared to peer-raised NRI behaviour. Behavioural trajectories and spatial orientation were comparable between rearing groups. Peer-groups thus enabled the development of species-specific behaviour, although atypical behaviours also developed. In conclusion, the success of novel methods to understand behaviour in the pre-reproductive period and its function in gorilla life history has been demonstrated.