Geographic variation in the skulls of the horseshoe bats, and : Determining the relative contributions of adaptation and drift using geometric morphometrics.

The relative contributions of adaptation and genetic drift to morphological diversification of the skulls of echolocating mammals were investigated using two horseshoe bat species, and , as test cases. We used 3D geometric morphometrics to compare the shapes of skulls of the two lineages collected at various localities in southern Africa. Size and shape variation was predominantly attributed to selective forces; the between-population variance () was not proportional to the within-population variance (). Modularity was evident in the crania of but absent in the crania of and the mandibles of both species. The skulls of the two lineages thus appeared to be under different selection pressures, despite the overlap in their distributions. Difference in the crania of was centered largely on the nasal dome region of but on the cranium and mandibles of . It is likely that the size and shape of the nasal dome, which acts as a frequency-dependent acoustic horn, is more crucial in than in because of the higher echolocation frequencies used by . A larger nasal dome in would allow the emission of higher intensity pulses, resulting in comparable detection distances to that of . In contrast, selection pressure is probably more pronounced on the mandibles and cranium of to compensate for the loss in bite force because of its elongated rostrum. The predominance of selection probably reflects the stringent association between environment and the optimal functioning of phenotypic characters associated with echolocation and feeding in bats.