Lizards, snakes and amphisbaenians (worm lizards) form a monophyletic group (the squamate reptiles), which contains 9,712 species (Uetz & Hošek, 2015) in 61 families (Wiens, et al., 2012). New species are constantly being described, particularly with the advent of modern molecular systematics and improved access to remote regions. Consequently, this group is likely to be considerably larger in the future (Pyron, et al., 2013). Not only is this a taxonomically diverse group of terrestrial vertebrates, but species occupy a wide range of habitats and ecosystems, and occur on all continents except Antarctica. Furthermore, they span a wide range of body sizes and forms from miniature chameleons and geckos that perch comfortably on a matchstick, to reticulated pythons in excess of 6 m in length. While snakes have traditionally been viewed as a group separate from lizards (e.g. different suborders in traditional taxonomic terms), they are in fact embedded within lizards such that some lizards are more closely related to snakes than they are to other lizards (Wiens, et al., 2012; Pyron, et al., 2013).
Squamate reptiles display a wide array of life history strategies, reproductive tactics, and social behaviors that frequently bring males and females into conflict and which are invariably further influenced by high levels of male contest competition (Baird, 2013). As a consequence, most species are polygynous, polyandrous, or both (polygynandry), and many are territorial (Stamps, 1977). However, some species also occur in stable social aggregations (Gardner, et al., 2016). Of these, a small proportion live in family groups with a socially and mostly genetically monogamous parental unit. Interestingly, a single radiation of lizards in Australia (Egernia group of scincid lizards) (Figure 13.1), commonly forms long-term pair bonds and have a particularly high incidence of family living (Chapple, 2003; While, et al., 2015; Gardner, et al., 2016). The Egernia group is a Melanesian-Australian radiation (Gardner, et al., 2008) consisting of seven genera (Egernia, Liopholis, Lissolepis, Bellatorias, Cyclodomorphus, Tiliqua, and Corucia), and is so called because the former Egernia genus was paraphyletic and split into four monophyletic genera. This review will focus primarily upon the evolution of kin-based sociality in lizards with a heavy emphasis on the Egernia group. We draw upon data from snakes only when it helps inform the evolution of kin-based sociality in “typical” lizards. Our review is therefore admittedly biased towards lizards because our overall goal is to advance our understanding of the evolution of sociality in general, but kin-based sociality in particular.
Lizards provide a unique opportunity to understand the early evolution of vertebrate sociality because (1) social behavior is relatively simple and easily quantifiable, (2) social behavior is not obligate (e.g. most species exhibit facultative or temporary forms of group living), and (3) there is enough variation in social strategies, both within and between species, to allow for meaningful tests. In lizards, mating systems vary from monogamy to polygynandry, parental care if present is typically through parent-offspring association (i.e. presence of parent deters potential predators, particularly conspecific adults), with social bonds and interactions between parents and offspring typically less complex than in many avian and mammalian systems (While, et al., 2014a). As such, they offer great potential as models for understanding transitions to more complex forms of social structure and for uncovering the mechanisms that triggered the initial origins as well as the maintenance of family living and sociality in animals (Chapple, 2003; Doody, et al., 2013; While, et al., 2015). To understand the evolution of kin-based sociality in lizards, it is important to first consider the nature of social interactions in lizards more generally, since social selection (sensu Lyon & Montgomerie, 2012) acts on all aspects of social interactions that might lead to pair bonding and group formation (Kavaliers & Choleris, 2013).