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Description
Islands have played a key role in our understanding of rapid evolution. There is a large body of literature examining morphological and behavioral changes in response to insularity, which has yielded useful generalizations about how animals can adapt to live in these habitats. Some of these changes fall under the “island rule”, a generalization describing morphological and behavioral changes a species undergoes after being isolated on an island. It can be difficult to examine longitudinal patterns of growth and development, as such studies require long term mark-recapture of a large sample of individuals. Although it is often known that typical adult body size differs drastically on island populations, it is rare to understand the ontogeny leading to those differences. Rattlesnakes provide a unique model to address this question, as rattle segment widths correlate closely with the body size of the snake at the time in which they are formed, and (to an extent) age. We used a large database of rattle segment sizes recorded from both island and mainland populations of Western Rattlesnakes (Crotalus oreganus) to compare body sizes at different ecdysis cycles, and patterns of sexual size dimorphism. Our results show that island rattlesnakes are born slightly smaller than mainland snakes but grow much more slowly while retaining the same degree of sexual dimorphism. This pattern may indicate that mechanisms underlying changes and body size and patterns of sexual dimorphism in rattlesnake are strongly affected by local prey resources. Compared to morphological studies, studies examining behavioral changes on island populations are rare. In addition to morphological changes, we also quantify and compare behaviors between these two populations. Previous studies suggest that in the absence of predation pressure, insular populations become more docile, bolder and more exploratory of their surroundings compared to mainland populations. Our results show that C. o. calignis is bolder and potentially more explorative than C. oreganus, but we did not find evidence for differences in docility. We conclude that C. o. calignis fits some of the patterns documented in the broader literature on insular evolution, but docility may be highly context dependent and warrants future study.
