Isolation at Work: Body Size Divergence between Reptiles of Nevada's Pyramid Lake and Anaho Island

Abstract

This Island Rule is a long-held tenet of biogeography which states that insular populations trend towards increased mean body size in small species and decreased mean body size in large species—a trend that has been inconsistent and even contradictory with reptile taxa. This study examines insular and mainland reptile populations of Aspidoscelis tigris tigris¸ Crotalus oreganus lutosus, Callisaurus draconoides myurus, Sceloporus uniformis, and Sceloporus occidentalis longipes to determine whether the Island Rule conforms with the observed size trends on Anaho Island in Pyramid Lake, Nevada. The selective influences of predation and resource abundance on body size are evaluated by comparing (1) the frequency of caudal autotomy to determine the influence of predation pressure and (2) head shape as a trait affected by the availability of prey resources. Differences in head shape reveal patterns consistent with a shift to smaller prey in A. t. tigris as well as decreased head height for the C. o. lutosus, A. t. tigris, and C. d. myurus. Differences in tail-regeneration frequencies are consistent with an altered pattern of predator-prey interaction for A. t. tigris and S. uniformis. Body size results on Anaho Island contradict the Island Rule, with C. o. lutosus, A. t. tigris, and C. d. myurus males exhibiting smaller body sizes on the island while S. o. longipes and S. uniformis exhibit no size trend, perhaps as the result of a small sample size. Divergence in body size occurs on the island, in a direction that is consistent with the primary literature. This study supports the conclusion that Anaho Island harbors a community of reptiles that is distinct from the mainland in morphology and possibly in ecology and life-history evolution.