The social system of an endemic lava lizard, Microlophus atacamensis: the interplay between social structure and social organization

Abstract

Animal social systems can be described through 4 main components: social structure, social organization, mating system, and care system. Social structure describes the relationships between individuals in a population, while social organization describes the group composition, size and spatiotemporal variation of a population. I use these two frameworks to study the social system of Microlophus atacamensis, an endemic lizard found along the Chilean coast that inhabits the rocky intertidal zone between the Pacific Ocean and the Atacama Desert. Lizards in the genus Microlophus generally exhibit territorial behaviors, but their exact social system is largely undescribed. The area M. atacamensis inhabits poses specific challenges stemming from their use of two distinct habitat types: they forage in the cooler intertidal zone and use large rocks in more inland areas for basking. I characterized the social system of M. atacamensis using social structure and social organization as a framework, hypothesizing that social structure would be independent of habitat type, while social organization based on space use would differ with habitat type. My assessment focused on 2 separate populations: one to characterize social structure by means of focal observations and social network analysis, and a second to assess social organization by examining space use through home range and core area analyses. Social network analysis revealed that in the social structure of M. atacamensis larger individuals were more central to the social network and body size influenced the outcomes of aggressive interactions. Interactions were more common in the intertidal zone where lizards foraged, where males had more associates and more repeated interactions with those associates than females. Spatial analyses revealed that the social organization of M. atacamensis is characterized by high home range overlap, specifically in the intertidal zone where foraging occurs, but also by relatively exclusive core areas dispersed across both habitat types. I suggest that M. atacamensis has a shifting social system in which small exclusive territories are maintained on large rocks in inland areas, while a system of dominance relationships promotes access food in the intertidal zone.