Sexual Dimorphism and Allometric Size Variation in a Population of Grass Snakes (Natrix natrix) in Southern England

Sexual dimorphism is widespread in animals, either because of sexual selection or ecological divergence between the sexes. In natricine snakes, for example, females are usually larger than males and have larger heads and shorter tails relative to body length. Grass Snakes (Natrix natrix) near Canterbury, U.K., generally proved to be typical natricines in these respects. Most differences between the sexes were apparent at hatching, rather than being confined to adults, but there was no evidence that the sexes differed in allometry. Despite high r2-values, allometry of relationships of body mass, head width, and tail length with body length were not satisfactorily explained by linear regressions (using log-transformed data). Rather, both polynomial and piecewise regressions provided better fits, suggesting that allometric coefficients were not constant but varied with body size. Nonetheless, all analyses were in agreement that small snakes had relatively heavier bodies and wider heads than large snakes. How general such patterns are, and their ecological significance, remain to be determined.