Ecological release and allometry explain insular gigantism and shape variation in a widespread North American rodent
Date
2020
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Publisher
Austin Peay State University
Abstract
Island Rule is defined as a trend of gigantism in small-bodied species and dwarfism in large-bodied species inhabiting islands and island-like systems. Several hypotheses attempt to explain gigantism in small-bodied species including immigrant selection, thermoregulation and endurance, resource subsidy, and ecological release, but these are often incompletely tested and infrequently compared directly. In this study, we used geometric morphometrics to obtain indices of size and shape variation across 17 island populations of meadow voles (Microtus pennsylvanicus). We first test whether gigantism occurs in two geographically separated island systems inhabited by M. pennsylvanicus and then evaluate which of the above hypotheses best explains observed variation. Finally, we assess whether shape varies in accordance with size, geography, or other island-specific factors. Resource subsidy and immigrant selection are inapplicable to this system, so we focused on the two remaining hypotheses (thermoregulation and endurance, and ecological release) to evaluate significant signals of gigantism detected among Atlantic island populations. Despite linear models revealing that M. pennsylvanicus cranium size follows a weak pattern with temperature seasonality, results are inverse of Bergmannian expectations, contradicting the Thermoregulation and Endurance Hypothesis. In contrast, regression-based path analyses permitted us to detect significant associations between skull size and island-specific factors, including area and number of predators, each consistent with the Ecological Release Hypothesis. For this species, the primary source of shape variation in island populations is its variation with size due to static allometry. Random, residual (size and allometry-controlled) patterns of shape are likely explained by population-level differences that resulted from founder effects, genetic drift, or natural selection and thus are unlikely a product of Island Rule. Taken together, our findings indicate that Island Rule is a latent evolutionary process that depends on biogeographic context. Further, allometry may manifest in important functional consequences as size varies according to expectations of Island Rule, but without corresponding independent selection on cranium shape. Unfortunately, these important evolutionary perspectives and insular variants are threatened by the introduction of non-native predators and projected sea level rise.