Feeding biomechanics reveals niche differentiation related to insular gigantism

Insular gigantism is an evolutionary phenomenon whereby small animals become bigger on islands compared to their mainland relatives. The abundance of insular giant taxa in the fossil record suggests the presence of a universal "giant niche" present on islands, with resource limitation as a potential driver for this process. However, insular habitats are ecologically diverse, suggesting that island taxa adopt different survival strategies, including adaptations for foraging behaviors. Here, we used finite element analysis to evaluate insular feeding niche adaptations in some of the most extreme examples of insular gigantism: Mediterranean giant dormice. We calculated stress, strain, and mechanical advantage during incisor and molar biting for 3 extinct insular giant species (Leithia melitensis, Hypnomys morpheus, and H. onicensis), an extant giant (Eliomys quercinus ophiusae), and their extant non-giant mainland relative, the generalist-feeder Eliomys quercinus. Our results show that dietary adaptations vary between giant taxa on different islands, and can occur relatively rapidly. Furthermore, the functional mandibular morphology in some insular taxa indicate adaptations moving away from a generalist feeding strategy toward greater trophic specialization. We show that the "insular giant niche" varies between islands and across time periods, arguing against a universal ecological driver for insular gigantism in small mammals.