Insular dwarfism in female Eastern Hog-nosed Snakes (Heterodon platirhinos; Dipsadidae) on a barrier island

The island rule postulates that the special ecological conditions on islands, such as limited resource availability, can cause populations of large-bodied animals to evolve smaller sizes and small-bodied populations to evolve larger sizes. Although support for the island rule is well documented (with notable exceptions and debate) in mammals and birds, similar trends are poorly explored in ectothermic vertebrates. As part of a larger study investigating the ecology of Eastern Hog-nosed Snakes (Heterodon platirhinos Latreille, 1801), we compared the mean and maximum sizes of a population from a barrier island (similar to 4 000 ha) to snakes on an adjacent larger island (similar to 363 000 ha) and two mainland sites (450 total snakes across all study sites). We did not observe a difference between the small and the large islands, but we did find differences between the smallest island and the mainland. Female snakes on the barrier island were 8% smaller than those on the mainland, and the female from the largest barrier island was 35% smaller than the largest documented H. platirhinos. In addition, we found that males did not exhibit dwarfism. We hypothesize that the observed dwarfism is a result of limited availability of large prey items and recommend that future studies distinguish between sexes in their analyses.