Island biogeography predicts skull gigantism and shape variation in meadow voles Microtus pennsylvanicus through ecological release and allometry

Abstract

Island rule describes the graded trend of gigantism in small-bodied species to dwarfism in large-bodied species inhabiting islands, but causal explanations remain unresolved. We used geometric morphometrics to quantify cranial morphology of 544 meadow vole Microtus pennsylvanicus samples across 11 island and 3 mainland populations from the Outer Lands of New England (Atlantic) and the Alexander Archipelago of Alaska (Pacific). We compared the thermoregulation and endurance (TRE) and ecological release (ER) hypotheses using all-subsets linear models employing residual randomization permutation procedures (rrpp), and Akaike information criterion (AIC) for model selection. We decoupled direct and indirect effects of island variables on size using path analysis. We evaluated shape with principal components analysis (PCA) and Procrustes ANOVA on Procrustes shape coordinates, then assessed the impact of static allometry and TRE and ER variables on shape. Six Atlantic island populations exhibit significant signals of gigantism with the largest voles occurring on the smallest islands lacking predators. ER explains 63% of cranial size differences. Island area has a significant total effect on size by influencing the number of mammalian predators, resulting in a 0.011 increase in unit centroid size for a 100 km 2 decrease in island area. This corresponds to a predicted 0.9% change in size for every 100 km 2 . Given static allometry, cranial shape does not respond to insularity independent of size. These results suggest that island rule is a latent evolutionary process whose manifestation depends on nuanced biogeographic and ecological contexts that have important conservation and taxonomic implications.

A practical, step-by-step, guide to taxonomic comparisons using Procrustes geometric morphometrics and user-friendly software (part A): introduction and preliminary analyses

“…Using adults, we suggested an adequate minimum sample size, on average, in the order of at least 25-40 specimens per homogeneous group (i.e., within taxon or, if SDM is large, within taxon and sex). This number is mostly congruent with previous estimates in morphometric studies of other terrestrial vertebrates (Cope & Lacy 1992;Cope 1993;Polly 2005;Cardini & Elton 2007;Cardini et al 2015;Brown & Vavrek 2015;Kryštufek et al 2016;Schlis-Elias 2020). I stress, here, something discussed better later: large samples are not just a matter of statistical power (i.e., being able to demonstrate differences), but also of accuracy in the description of patterns (means and their similarities or dissimilarities among species, the amount of variation in a species etc.).…”

Beyond CREA: Evolutionary patterns of non‐allometric shape variation and divergence in a highly allometric clade of murine rodents

“…Cardini & Polly, 2013 ). A conserved, allometric ‘one‐to‐many’ mapping of cranial function might also explain the clade's unique overall shape but slow morphological evolution through time (Goswami et al., 2022 ), and finds that rapid and extensive changes in shape, such as island gigantism, are possible within species along the allometric line (Schlis‐Elias & Malaney, 2022 ).…”