Insularity effects on the morphological space and sexual dimorphism of a tropical tree lizard in western Mexico

The Terrific Skink bite force suggests insularity as a likely driver to exceptional resource use

Natural history museum collections hold extremely rare, extinct species often described from a single known specimen. On occasions, rediscoveries open new opportunities to understand selective forces acting on phenotypic traits. Recent rediscovery of few individuals of Bocourt´s Terrific Skink Phoboscincus bocourti, from a small and remote islet in New Caledonia allowed to genetically identify a species of land crab in its diet. To explore this further, we CT- and MRI-scanned the head of the holotype, the only preserved specimen dated to about 1870, segmented the adductor muscles of the jaw and bones, and estimated bite force through biomechanical models. These data were compared with those gathered for 332 specimens belonging to 44 other skink species. Thereafter we recorded the maximum force needed to generate mechanical failure of the exoskeleton of a crab specimen. The bite force is greater than the prey hardness, suggesting that predation on hard-shelled crabs may be an important driver of performance. The high bite force seems crucial to overcome low or seasonal variations in resource availability in these extreme insular environments. Phoboscincus bocourti appears to be an apex predator in a remote and harsh environment and the only skink known to predate on hard-shelled land crabs.

Evaluating the island effect on phenotypic evolution in the Italian wall lizard, Podarcis siculus (Reptilia: Lacertidae)

Islands are compelling natural laboratories for studying evolutionary processes. Nevertheless, the existence of general rules underlying morphological evolution on islands remains an unresolved issue. In this study, we investigated the insular phenotypic variability of the Italian wall lizard (Podarcis siculus) on a large geographical scale, in order to assess the putative existence of an island effect on three morphological head traits: shape, size and degree of sexual dimorphism. A geometric morphometric analysis was performed on 30 island and 24 mainland populations, involving a total of 992 specimens, and we analysed differences in both mean trait values and variances (disparity). We found increased shape disparity in insular lizards with respect to mainland ones. On the other hand, both size disparity and mean head dimensions of males decreased on islands, leading to a reduction in sexual dimorphism. Our results provide evidence for a multidirectional morphological diversification on islands concerning head shape of both sexes, while directional and canalizing selection likely occurred for head size, but only in males. Our findings improve our knowledge on the effect of insularity in Podarcis siculus, and highlight the need for an exstensive sampling scheme and a multi-trait methodological approach.