Demographics of injuries indicate sexual coercion in a population of Painted Turtles (Chrysemyspicta)

Abstract

Sexually coercive reproductive tactics are widespread among animals. Males may employ specialized structures to harass, intimidate, or physically harm females to force copulation, and injuries to the head and neck are reported in taxa with sexually coercive mating systems. The mating tactics of Painted Turtles (Chrysemys picta (Schneider, 1783)) are typically described as involving male courtship and female choice. In contrast, female Painted Turtles in our study population display injuries on the head and neck indicative of bite wounds inflicted by sexually dimorphic tomiodonts and weaponized shell morphology of males during reproductive interactions. Using a 24-year data set, we demonstrate population-level trends in soft tissue wounds inflicted by conspecifics. Adult females experienced more wounding than adult males or juveniles, and larger females had a greater probability of wounding than smaller females. Wounding was concentrated on the dorsal head and neck of females, consistent with expectation of sexual coercion. Furthermore, elevated rates of fresh wounding occurred during late summer, concurrent with the breeding period. By assessing wound demographics, we provide indirect evidence that the tomiodonts and shell of male Painted Turtles inflict injury and function as sexual weapons. These findings shed new light on our understanding of mating system complexity in an often-overlooked and difficult-to-observe taxonomic group.