Social structure of the Spiny-tailed Iguana, Ctenosaura similis

Nash, Ann-Elizabeth

aenash@gmail.com

Mackessy, Stephen P.

University of Northern Colorado

Greeley, Colorado USA

Social environments of most lizard groups are poorly understood, yet social interactions may influence mate choice, foraging, nesting sites, predator avoidance, and disease transmission. To address this knowledge gap, a study using a group of free-ranging Spiny-tailed Iguanas (Ctenosaura similis) investigated genetic relatedness, personality, and social network attributes. These non-cooperative animals form stable aggregations (>10 years), establishing relationships through repeated, non-random interactions. During four field seasons, ~28 lizards wore physical proximity tags, recording associations when lizards were ≤ 6 meters apart. Networks were analyzed for individual and group measures. To evaluate personality, lizards were assayed via flight initiation distance, arena tests, and conspecific agonistic encounters. There is strong support for distinct and stable personalities, and bold-aggression behavioral syndrome. Social network positions fluctuate by season; female associations remain constant during but differ between seasons, perhaps indicating a fluctuating need for proximity to a preferred mate. Network analysis demonstrates high group density with redundant social links, and the tolerance of social gaps when animals disappeared, suggesting a buffering through multiple social connections. Modularity, or sub-communities within the larger group, was also observed, signifying potentially biologically relevant differences among smaller sets of connected lizards. Notably, during the pandemic-induced Anthropause, food provisioning and predator release were found to have an insignificant impact, as C. similis individuals did not abandon individual retreats or the overall group. This is the first large-scale study to look at personality and social network structure in subfamily Iguaninae, an oviparous, Neotropical lizard with facultative group formation. Results demonstrate a level of complexity hypothesized as an important step in the evolution of more multifaceted sociality and expands the phylogenetic and geographic span of research on lizard social structure. As an IUCN “species of least concern”, this proxy may reveal social structure characteristics needed to support self-perpetuating populations of endangered iguanas.