Quantification of cranial and tomiodont dimorphism in Testudines using the Midland Painted Turtle, Chrysemys picta marginata

摘要

Sexually dimorphic characters are common among vertebrates and are particularly well represented among emydid turtles. For 190 years, tomiodonts have been used as a descriptor in testudine anatomy, phylogenetics, and natural history; however, no quantitative evaluation of their function and potential dimorphism has ever been completed. Using morphometric analysis of data from a long-term study in Algonquin Provincial Park (Ontario, Canada), we show that the cranial morphology of male and female Chrysemys picta differs significantly, especially with respect to tomiodonts. Male C. picta have a relatively longer head and rostrum (among other traits), giving their head a more angular appearance in lateral profile. The tomiodont morphology of males reflects an optimized biomechanical arrangement, ideal for maximizing bite force and maintaining an effective biting grip. To explain patterns of cranial and tomiodont dimorphism, ecological (dietary partitioning, reproductive role) and sexual selection hypotheses (mate recognition, mating strategies) are discussed. Preliminary behavioral evidence from the long-term study in Algonquin Provincial Park supports the sexual selection hypothesis whereby tomiodonts of male C. picta function to coerce females into mating. This paper is the first quantitative study of tomiodont structure and among few studies to test cranial dimorphism in a testudine species, despite regular anecdotal reports of dimorphism in this group.