How to be social: understanding the mechanisms underpinning complex sociality

<p dir="ltr">Social living is a fundamental component of many organisms’ lives. As a result, a great deal of work has tried to identify the proximate mechanisms mediating social behaviour, from genes to neural circuits to hormones. This has largely been conducted on species with obligate forms of social living, such as mammals, birds, and social insects. Whether these same mechanisms underpin social behaviour in species where social living is more rudimentary remains unknown. The Australian skink clade known as the Egernia group provides an excellent model system in which to test this. The Egernia display an extraordinary range of social systems from completely solitary species to species living in large groups comprising overlapping generations. Facultatively group-living species are particularly valuable as they may represent a transitionary stage in the shift from solitary to group living and thereby may offer insights into the mechanisms mediating the evolution of group living. Parental care, in the form of simple offspring tolerance, and social recognition, are two of the key features of sociality in these skinks. In this thesis, I explore the neural, genetic and hormonal mechanisms that are linked to and mediate these vital behaviours.</p><p dir="ltr">I first compared the neural distribution of oxytocin, vasotocin, serotonin, and dopamine in the brains of two skink species, the facultatively family-living White’s skink (<i>Liopholis whitii</i>) and the solitary water skink (<i>Eulamprus quoyii</i>). These neurotransmitters have been linked to social behaviour in a wide range of organisms. While both species have these neurotransmitters, I show that vasotocin, serotonin, and dopamine are distributed across a wider variety of brain regions in the social species, supporting the suggestion that these transmitters play a role in moderating social behaviour. Oxytocin was only examined in <i>L. whitii</i>, and while its distribution was broadly consistent with the literature, I describe some novel cell populations.</p><p dir="ltr">I then tested the behavioural effects of one of these hormones, arginine vasotocin (AVT), by pharmacologically manipulating it via injectable agonists and antagonists. I show that AVT supplementation increased parent-offspring association in White’s skinks, whereas blocking AVT disrupted these associations. Further, these effects persisted long after the initial experimental manipulation, suggesting that small changes in AVT at a critical period can shape future social interactions. I also found that AVT did not mediate aggression in this species, despite evidence to the contrary from both group-living and solitary species. Additionally, I investigated if AVT plays a role in social recognition in a nongroup-living species, the bluetongue skink (<i>Tiliqua scincoides</i>). While the recognition abilities of these skinks appeared limited, AVT did increase aggression (bites and chases) and I found some evidence of an effect on tongue-flicking, a chemosensory behaviour. The results of these studies show that small changes in AVT can have profound effects on social behaviour in these skinks.</p><p dir="ltr">Finally, I tested whether gene expression differences in the brain underly parental care behaviours in White’s skinks. By housing mothers with or without their offspring, I simulated naturally variable parental care. I found that mothers housed alone behaved more aggressively when re-introduced to their offspring compared to a control object and also exhibited an upregulation of appetite stimulating genes. In contrast, mothers housed with their offspring upregulated an appetite suppressing gene. I suggest that upon encountering a juvenile, an adult lizard’s first response is to cannibalise it unless a secondary mechanism suppresses this.</p><p dir="ltr">Taken together, this thesis provides a valuable contribution to the understanding of the proximate mechanisms that underpin the formation of simple social associations and offers insight into the mechanisms that may be key in the transition from solitary to family and group-living.</p>