Socioecology of the Australian Tree Skink (Egernia striolata)

Julia L Riley,Martin J Whiting,Peri E Bolton,Daniel W A Noble,Siobhan Dennison,Richard W Byrne,Geoffrey M While,Adam J Stow

doi:10.3389/fevo.2021.722455

Abstract

There is great diversity in social behavior across the animal kingdom. Understanding the factors responsible for this diversity can help inform theory about how sociality evolves and is maintained. The Australian Tree Skink (Egernia striolata) exhibits inter- and intra-population variability in sociality and is therefore a good system for informing models of social evolution. Here, we conducted a multi-year study of a Tree Skink population to describe intra-population variation in the social organization and mating system of this species. Skinks aggregated in small groups of 2–5 individuals, and these aggregations were typically associated with shared shelter sites (crevices and hollows within rocks and trees). Aggregations were typically made up of one or more adult females and, often, one male and/or juvenile(s). Social network and spatial overlap analyses showed that social associations were strongly biased toward kin. Tree skinks also exhibited high site fidelity regardless of age or sex. There were high levels of genetic monogamy observed with most females (87%) and males (68%) only breeding with a single partner. Our results indicate that Tree Skinks reside in small family groups and are monogamous, which corresponds with existing research across populations. Similar to previous work, our study area consisted of discrete habitat patches (i.e., rock outcrops, trees, or both), which likely limits offspring dispersal and promotes social tolerance between parents and their offspring. Our study clearly demonstrates that there is intra-population variability in Tree Skink social behavior, but it also provides evidence that there is a high degree of inter-population consistency in sociality across their geographic range. We also highlight promising possible avenues for future research, specifically discussing the importance of studying the nature and extent of Tree Skink parental care and quantifying the fitness outcomes of kin-based sociality in this species, which are topics that will further our understanding of the mechanisms underlying variation in vertebrate social behavior.

Highlights

In a broad sense, animal sociality is the extent that conspecifics interact with one another and the complexity of these social interactions (Kappeler, 2019)
Tree Skink social associations and spatial overlap were highly correlated across years, which suggests these aggregations reflect consistent social groupings
We observed a diversity of social associations and intra-population variability in social behavior throughout our study, the spatial overlap and social associations of Tree Skinks were strongly biased toward kin

Summary

Introduction

Animal sociality is the extent that conspecifics interact with one another and the complexity of these social interactions (i.e., the number of conspecifics in a social group, the nature of interactions, as well as their mating and care systems) (Kappeler, 2019). The extent of sociality itself varies considerably within and between individuals (i.e., social plasticity or competence as per Taborsky and Oliveira, 2012), among populations, and across taxa (Ward and Webster, 2016). In turn, individuals residing in groups experience higher fitness, selection favors sociality (Ward and Webster, 2016). Limited resources may delay offspring dispersal from their natal social group, and association with their parents may offer protection, selecting for the formation of kin-based sociality and parental care (Halliwell et al, 2017). If we are to understand how the spectrum of animal sociality evolved, we need to connect the processes that underpin variation in social behavior at the level of the individual with patterns of social diversification we see at the population and species level. Understanding the factors that mediate this variation in sociality, as well as how this sets the stage for the evolutionary origins of sociality, are fundamental goals in animal behavior and evolutionary biology

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