We investigated sexual size dimorphism and trophic morphology dimorphism in Eastern Garter Snakes (Thamnophis sirtalis) to test predictions derived from the reproductive role hypothesis. Our first objective was to examine whether structures involved in feeding (head and jaws) are more divergent than body size. Female T. sirtalis were larger than males and had proportionally larger heads. Our second objective was to look for an advantage of large head size by examining the relationship between body condition and head size. Body condition was positively correlated with relative head size in T. sirtalis for both sexes, in agreement with the reproductive role hypothesis
We captured 63 garter snakes during egress and ingress from a communal hibernaculum in the vicinity of the Queen’s University Biological Station, approximately 100 km south of Ottawa, Ontario, Canada. We took 5 head measurements with digital calipers (table 1). Due to the highly kinetic skulls of snakes, however, measurements with calipers may not be reliable. Therefore, we also took digital photographs against a ruler of the dorsal, ventral, and right lateral sides of the head of each snake, and took the same measurements with ImageJ software (National Institute of Health, USA). We measured snout-vent length (SVL) by juxtaposing the snake against a ruler and mass with a spring scale. Snakes were sexed with a clean probe inserted in the cloaca to check for presence of hemipenes. To avoid recapture, we marked all snakes with paint prior to release.
There were small differences between head dimensions measured with calipers and those measured from digital photographs (ca. 5%). Therefore, we used measurements from the digital images in the analyses. Head dimensions were highly correlated, so we performed a principal component analysis to summarize the variation. We calculated body condition by performing a regression of mass on SVL (both log transformed) for each sex. The residuals of these regressions were divided by the predicted mass for each sex (obtained from the regression equations) to yield an index of condition (% above or below the predicted mass) (Blouin-Demers et al., 2005). Because we only captured snakes just after exit or just before entry into hibernation, they had empty stomachs and our body condition index is thus unaffected by variation in stomach content. We conducted all analyses on JMP version 7 (Statistical Analysis Systems, Cary, North Carolina). All assumptions of normality and homogeneity of variance were met and verified by examining box-plots. We accepted significance of tests at α = 0.05.