The Role of Social Play in the Development of Wild Mountain Gorillas (Gorilla beringei beringei) at Bwindi Impenetrable National Park, Uganda
Open Access DepositedSocial play is found in almost all mammals and many other animals, suggesting ithas a high benefit-to-cost ratio, yet there remains little consensus about its evolutionary function. Many different hypotheses have proposed costs and benefits of play, but these hypotheses can be difficult to test in a mutually exclusive framework. The 'motor training hypothesis’ posits that play is a platform for honing motor skills. The ‘social relationship hypothesis’, postulates that the benefit of play has two components; 1) it helps develop and hone skills to interact socially and 2) it is a framework to form social relationships. Other hypotheses have proposed that play behaviour is energetically costly, and its frequency of occurrence is constrained by energy availability. Most research has centred around describing the ontogeny of play which does not provide consistent support for any one hypothesis. This dissertation presents a new framework to better distinguish between competinghypotheses to further understand the function of social play behaviour, which is then implemented in the study of wild immature mountain gorillas (Gorilla beringei beringei) from Bwindi Impenetrable National Park, Uganda. Chapter 1 introduces the subject of social play, what it is and why gorillas are a good model for exploring functional hypothesis of play. Chapter 2 reviews previously published studies of social play across all mammals, examines the current evidence in support of different hypotheses and discusses the difficulties in studying play. Chapter 2 concludes by proposing a study framework to better delineate between the proposed functions of social play. This new framework is then employed in the subsequent three chapters, which outline novel research on social play in immature mountain gorillas of Bwindi Impenetrable National Park. Chapter 3 investigates the biological, ecological and social correlates of variation in rates of social play, with the aims of examining if patterns of immature play rates support predictions of motor and social development hypotheses based on adult behavioural patterns and to test if social play is energetically constrained. Chapter 4 explores play partnership preferences based on age and sex and examines if play partnerships are stable over time and correlated with immature and maternal partnerships in proximity, with the aim of distinguishing motor development, social skill development and social bond formation functions of play. Chapter 5 analyses the costs and benefits of social play, focusing on how time and energy spent in play trade-off with time spent in other daily activities and energy spent in growth. Chapter 5 also explores the potential benefits of play by examining correlates between play and developmental milestones. Chapter 6 provides a summary of the main findings of the dissertation and the conclusions that can be drawn from this study. Overall, this dissertation provides a comprehensive framework to study social playfrom multiple perspectives and presents novel findings for Bwindi mountain gorillas, with broader implications for the study of play behaviour across mammals. We show that sex differences in social play during immaturity may be related to sex differences in reproductive strategies and social behaviour in adult mountain gorillas. We find higher play rates in males than females, particularly during 2-6 years of age, supporting both a social and motor development function of play, as males were predicted to need these skills more for competitive interactions. Additionally, we show that play is likely energetically constrained as individuals played more before being weaned and during periods of higher fruit consumption. We also show variation in play partnerships patterns; immature gorillas prefer play partners similar to their own age, males prefer male partners, but females have no partner sex-preference. Play partnerships were stable from year to year suggesting that bond formation could be a function of play. Our sex specific patterns suggest that females and males are likely harnessing the behavioural flexibility of play to gain different benefits from it, with males likely gaining more motor and fighting skills beneficial for dominance and physical competition. Furthermore, we show possible trade-offs between play and other functions such as growth and feeding, suggesting that play is at times prioritised over these other activities. Additionally, we show that increased play rates correlate with earlier age of attainment of developmental milestones, specifically agonistic and proximity milestones. Therefore, play ontogenies likely are related to other measures of behavioural development. We found support for all four major hypotheses tested in this study: motor traininghypothesis, social skill development, social bond formation and energetic constraints hypothesis. Specifically, we demonstrate how studying play partnerships can aid in delineating support for the two different social hypotheses for the evolution of play. Furthermore, we show how ecological variables and trade-offs with body size can test the energetic constraints hypothesis. We suggest future research examining structural elements of play, including play types and how these connect with developmental benefits to better distinguish between motor and social hypotheses. Additionally, including more direct measures of energetics and growth will provide better information about energetic trade-offs, beyond what we have already documented.
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