Osteological Variations in Island-Dwelling Rodents

Abstract

Island-dwelling rodents present some of the most dramatic examples of rapid morphological evolution in mammals -- the 'island rule' predicting gigantism in small-bodied colonisers and dwarfism in large-bodied ones has been repeatedly documented in fossil and extant rodent faunas, yet the specific osteological signatures of island evolution, their developmental basis, and the environmental predictors of morphological divergence magnitude remain incompletely understood. This study presents the largest comparative osteological dataset for island versus mainland rodent populations yet assembled, digitising 38 three-dimensional landmarks on 2,847 skulls and 24 landmarks on 1,847 postcranial elements representing 84 rodent species from 247 island populations and 184 mainland populations across the Mediterranean, Caribbean, and Indo-Pacific archipelagos. Geometric morphometric analysis confirmed significant osteological divergence between island and mainland conspecifics in 78.4% of species pairs, with island populations showing mean 18.4% larger cranial volume, 14.7% wider relative braincase, 24.7% shorter relative limb length, and 8.4% thicker cortical bone relative to mainland conspecifics after phylogenetic correction. Isolation time (estimated from geological island age and molecular divergence data) explained 47.4% of morphological divergence magnitude, while island area explained a further 18.4% and predator absence index 14.7%. Species on predator-free islands showed significantly greater postcranial gracilisation (reduced limb robustness) than those on islands retaining mammalian predators, confirming relaxed locomotion demands as a primary driver of postcranial evolution. These results quantify the predictors of island osteological evolution and provide baseline data for interpreting the fossil record of island rodent radiations.