A. Rummel

Synopsis Geometric morphometrics has had a profound impact on our understanding of morphological evolution. However, factors such as sample size and the views and elements selected for two-dimensional geometric morphometric (2DGM) analyses, which are often dictated by specimen availability and time rather than study design, may affect the outcomes of those analyses. Leveraging large intraspecific sample sizes (n > 70) for two bat species, Lasiurus borealis and Nycticeius humeralis, we evaluate the impact of sample size on calculations of mean shape, shape variance, and centroid size. Additionally, we assessed the concordance of multiple skull 2D views with one another and characterized morphological variation in skull shape in L. borealis and N. humeralis, as well as a closely related species, Lasiurus seminolus. Given that L. seminolus is a morphologically cryptic species with L. borealis, we assessed whether differences in skull shape and in 2DGM approach would allow species discrimination. We found that reducing sample size impacted mean shape and increased shape variance, that shape differences were not consistent across views or skull elements, and that trends shown by the views and elements were not all strongly associated with one another. Further, we found that L. borealis and L. seminolus were statistically different in shape using 2DGM in all views and elements. These results underscore the importance of selecting appropriate sample sizes, 2D views, and elements based on the hypothesis being tested. While there is likely not a generalizable sample size or 2D view that can be employed given the wide variety of research questions and systems evaluated using 2DGM, a generalizable solution to issues with 2DGM presented here is to run preliminary analyses using multiple views, elements, and sample sizes, thus ensuring robust conclusions.

The primary functions of the tetrapod neck are to maintain head stability and facilitate head mobility. Both stability and mobility should be especially important during foraging. Head stability facilitates the function of the vestibulocochlear, auditory, and visual organs while mobility allows for the motion of that visual field as well as the mouth for food capture and processing. Species that rely on different resources should be under different selective pressures with regard to range of motion of the head and neck and the musculoskeletal morphologies that sustain them. Bats are useful model species to investigate these pressures because they display a wide variety of foraging behaviors. This study tests the hypothesis that dietary regime influences maximum ranges of motion found in the head and neck. To test this hypothesis, a dietarily diverse group of bats were caught in the field and their active ranges of head–neck motion were measured via photographs. Diet information was taken from the literature. Additionally, gross neck dimensions (mediolateral neck width and craniocaudal neck length) were measured using calipers. Phylogenetic statistical methods support the hypothesis and demonstrate that frugivorous species have much smaller ranges of head and neck motion. The results indicate that frugivorous species may require stiffness in their cervical spine in order to carry heavy fruits and maintain head stability simultaneously. Future work should investigate the anatomical differences in the head and neck among bats that influence this stiffness as well as other dietary behaviors that could be shaping the form and function of the head and neck.