Giant growth in insular fossil dormice

Drastic morphological changes often occur within mammalian species during prolonged isolation on islands. The depauperate nature of insular ecosystems is often associated with phenomena known as insular dwarfism and gigantism, with such changes in size particularly well-known in the fossil taxa of Mediterranean islands. Insular gigantism has been noted in rodents, lagomorphs and insectivores. Within Rodentia, dormice appear to increase body size more frequently during isolation than other rodents. At least eight different Mediterranean islands have been occupied by giant dormice from the onset of the Miocene. We studied the cranial and mandibular morphology of three fossil giants: Hypnomys onicensis, H. morpheus and Leithia melitensis. All three species are believed to be related to the garden dormouse Eliomys quercinus, a species still present on some Mediterranean islands, including a giant population on Formentera. We evaluated to what extent the shape of giant dormice is explained by allometry, or by non-allometric processes. A predicted size over size (PSOS) model was created to assess the cranial and mandibular shape. It appears that gigantism in dormice is not simply an extrapolation of the allometric trajectory; instead, both cranium and mandible show individualistic evolutionary shape changes, with some morphologies suggesting adaptations to the feeding apparatus. Our results suggest that insularity can lead to context-dependent phenotypic modifications, resulting in highly idiosyncratic fauna characteristic of islands.