Evolution on a desert island: body size divergence between the reptiles of Nevada's Anaho Island and the mainland around Pyramid Lake

Abstract

AbstractPatterns of body size evolution on islands provide compelling cases of rapid and dramatic phenotypic evolution in terrestrial vertebrates, yet debate remains over the relative roles of predation and resource availability in driving such evolution. We compared the morphology of five reptile species (four lizards, one snake) from Anaho Island, a desert island in Pyramid Lake, Nevada, and the nearby mainland, using museum and live‐caught animals. We also examined head‐shape allometries to make inferences about dietary shifts and recorded tail‐regeneration frequencies (in lizards) to examine predation intensity. Compared with mainland samples, two phyrnosomatid lizard species are larger on Anaho (Callisaurus draconoides and Sceloporus occidentalis), whereas the largest (S. uniformis) is not different on the island. Conversely, the teiid lizard Aspidoscelis tigris is smaller in body and head size on the island, and the pitviper Crotalus oreganus is especially diminutive on the island, with males and females 25 and 15% smaller, respectively. Our results appear consistent with the hypothesis that body size is related to resource availability. The change in body size of the two smaller phrynosomatids may be due to interference competition. The reduction in body and head size in A. tigris suggests a dietary shift, and the dramatic difference in C. oreganus is likely due to a switch in diet from mammals to lizards. Future work is needed to determine whether body size differences reflect genetic evolution or environmental differences in growth rates or resource use. Regardless, Anaho Island, although remarkably young (early Holocene), appears to harbour a unique community of reptiles with distinct morphologies and possibly divergent life histories.