Discovery and description of a novel sexual weapon in the world’s most widely-studied freshwater turtle

Sexually coercive reproductive tactics are widespread among animals, where one sex employs specialized structures, called sexual weapons, to harass, intimidate, and/or physically force the other sex to mate. Painted turtles (Chrysemys picta) have been extremely well-studied over the last two centuries, and their mating system has been described as female choice based on male courtship display. The present study arises from observation that males seemingly have more protracted and serrated anterior marginal scutes than females. We hypothesized that the anterior carapace is sexually dimorphic, and that this morphology is a weapon used by males in coercive mating. We quantified anterior carapacial morphology using geometric morphometric analysis of digital photographs, drawing on samples of painted turtles from North American museum collections and our field site in Algonquin Provincial Park. We found that the anterior carapace of males had a significantly more serrated and projected shape compared to females, consistent with the sexual weapon hypothesis. Additionally, anterior carapacial shape was more strongly related to body size in males. Behavioural field observations strongly suggest that males use this morphology as a weapon to harm females during reproduction. The present study complements and strengthens the recent hypothesis that male painted turtles engage in coercion as an alternative reproductive tactic, questioning the long-understood paradigm of exclusive female choice in this well-studied species. Our study invites new avenues of research on the evolution of female harm in a system with extreme selection on female longevity and for which operational sex ratios vary among populations. Further, our work underlines how basic natural history observations can transform our understanding of well-studied systems.