Evolution in action: Soil hardness influences morphology in a subterranean rodent (Rodentia: Ctenomyidae)

Abstract

A major interest of evolutionary biologists is to understand which environmental features are associated with morphological and behavioural characteristics of species. Intraspecific studies addressing this question provide the best evidence for ecology-driven evolution over short time scales. Here, we evaluated whether two adjacent habitats differ in soil hardness and whether skull and forelimb morphology and estimated bite force differ between populations of a single species from sand fields and sand dunes. We used a total of 39 humeri and 88 skulls and mandibles of Ctenomys minutus from both habitats to estimate the bite force and generate morphometric data. Our results provide strong evidence that parapatric populations, occupying adjacent habitats, can respond differently in particular circumstances. This indicates that C. minutus uses different strategies (i.e. scratch-digging and tooth-digging) in the excavation of tunnels, and both of them respond to changes in soil hardness, establishing that the strategies are not evolutionarily exclusive. This difference is probably a consequence of the harder soils found in the sand fields, which are more difficult to excavate. Our results suggest the presence of divergent selection or strong phenotypic plasticity in the excavation-related morphology of populations occupying different habitats.