Are skull shape variations between sibling and cryptic species related to their habitat?

Geometric morphometric analyses were conducted to evaluate skull shape variation among three species of the Chaetodipus arenarius species complex (Rodentia: Heteromyidae), which are considered cryptic due to their morphological similarity. Chaetodipus siccus occurs allopatrically, whereas Chaetodipus arenarius and Chaetodipus ammophilus are sympatric in portions of their range. I tested the hypothesis that skull shape differs among the three species, each associated with a distinct habitat, and that these differences could be related to the types of seeds available in their respective environments. Specifically, I evaluated whether seed characteristics in each habitat correspond to differences in cranial morphospace and can be used to distinguish among species. My results revealed distinct skull shapes among the three species. Each species exhibited a specific cranial morphotype, with the most notable differences occurring in three main regions: the zygomatic arch, the temporal region, and the supraoccipital area—structures associated with the attachment of masticatory muscles. Variation in the position and size of these features likely reflects differences in muscle mass and development, which may be adaptations to the size and hardness of seeds consumed in each species’ habitat.