The context‑dependence of social interactions in a family‑living skink : implications for social evolution

Understanding the origins, maintenance and diversification of sociality is a major focus in evolutionary biology. To date, much of the work on this topic has focussed on obligately social taxa where social behaviour exhibits considerable complexity, such as birds, mammals and eusocial insects. While such systems provide fantastic insights into the factors that have maintained social behaviour and allowed for its elaboration, they tell us less about how social behaviour might have initially come about. My thesis focuses on the early evolution of sociality by investigating how environmental and organismal factors mediate the extent and nature of interactions between family members in a facultatively social species; the lizard Liopholis whitii. I focussed specifically on interactions between parents and offspring, and between siblings. I used large-scale population-level experiments to examine how (i) the thermal environment and (ii) prenatal stress mediate the opportunities for parents and offspring to interact via their effect on individual activity and social network structure. I found that the thermal environment influenced offspring emergence and that a cool thermal environment promoted stronger associations between family members. I found that prenatal stress had strong effects on the strength of offspring social behaviour, mediating the strength of social interactions between offspring and their parents. I then investigated how individual and litter characteristics mediate the extent of sibling conflict in these simple family groups. I focussed specifically on offspring sex, size and birth order. To achieve this, I developed sex-specific genetic markers (as offspring sex is not morphologically distinguishable at birth). These markers provided evidence for a XX/XY sex determination system in this species and considerable amplification across some but not all of the 13 species of the Egernia group tested (group from which L. whitii is a member), suggesting that sex-specific genomic regions are partially conserved across the Egernia group. I then used these sex-specific markers to explore whether offspring sex mediated levels of aggression within and between litters. I found great variability in aggression levels both with and between litters that were not explained by the litter characteristics examined. Sibling conflict within aggressive litters was driven primarily by an individual’s birth order, with no evidence that an individual’s sex played a role. Aggressive offspring tended to associate more with their mother and monopolise access to their mother by keeping away their siblings, potentially providing indirect benefits in terms of survival.
Overall, results from my thesis emphasise how different aspects of the environment and individual characteristics can shape the opportunity and nature of social interactions during the early stages of social evolution. By focusing on species that are facultatively social, we can start to understand how simple social systems emerge and stabilise. Crucially, such simple social associations provide the foundation from which more complex forms of social behaviour and social organisation can emerge. I utilise my results to explain how such elaboration may play out and the implications that it has for understanding the evolution of social behaviour in other systems. Altogether, each chapter provides important insights on the factors that influence the emergence and maintenance of family life in the context of the early evolution of sociality but also provides new genetic tools from which other research areas can benefit.