Cranial shape and diet variation in Myotis species (Chiroptera: Vespertilionidae): testing the relationship between form and function. ACTA CHIROPTEROLOGICA 18(1): 163-180.
Abstract: The relationship between cranial morphology and diet has long been investigated in bats. Bats of the genus Myotis include insectivorous, facultatively piscivorous, and piscivorous species. We tested the hypothesis that facultatively piscivorous (five Myotis
species) and piscivorous species (M. vivesi) present cranial morphological and functional changes with respect to insectivorous
taxa (16 Myotis species). Cranial shapes in skull and mandible modules were described with four geometric landmark configurations
in these dietary groups. Gape capacity was measured with the stretch factors for temporal and masseter muscles. Geometric
configurations from two skull and two mandible shapes were analyzed to detect differences in cranial morphology in relation to diet.
Differences in cranial morphology were found between piscivorous and insectivorous species involving the mandibular process
where masticatory muscles are attached. Linear regression analysis of Procrustes distances and gape capacity showed that the shape
of the mandibular process region was highly correlated with the stretch factor of the masseter muscle in piscivorous and facultatively
piscivorous species. These results suggest differences in cranial morphology and performance among diets but the hypothesis of
gradual changes in cranial shape among diets was only accepted for the mandible and not for the skull. Myotis vivesi appears to
improve mechanical advantage of masticatory muscles at lower gapes, presumably allowing more efficient chewing of slippery prey.
Key words: Myotis, geometric morphometrics, piscivory, stretch factor, masticatory muscles.