Forelimb morphology as an adaptation for burrowing in kangaroo rat species (genus Dipodomys) that inhabit different soil substrates

Abstract

Among burrowing rodents, forelimb morphology frequently shares an intricate relationship with soil substrates. Soils vary widely in texture and density, leading to differences in friability (e.g., the relative ability of particles to be broken apart), which often requires forelimb specializations in digging animals. Kangaroo rats (Dipodomys spp.) dig and occupy burrows, a trait that is essential to their survival. Some Dipodomys species are restricted to particular substrates that presumably require species-specific forelimb traits. Here we examined the forelimb morphology and soil substrates inhabited by range-restricted and widely ranging Dipodomys species to explore the variation in soil usage, forelimb specializations, and the relationship between these traits. We assessed size and shape traits of preserved specimen forelimb bones using traditional and geometric morphometric techniques and extracted soil data associated with the collecting locality of each specimen. We expected species that inhabit dense soil substrates to display specialized forelimb morphology typified by an elongated scapula and robust humerus, radius, and ulna. The species differed substantially in their soil associations and forelimb morphologies. In particular, forelimb traits that enhance mechanical digging ability were detected in D. elator, a range-restricted species that inhabits dense clay-rich soils. These findings suggest that Dipodomys species that inhabit dense substrates may require correspondingly specialized forelimb morphology and that these traits may limit the desirable geographic ranges inhabited by these species. This may provide important information when making conservation decisions given that unlike other habitat features, soil substrates cannot be easily modified to suit the needs of the organism.