Geographic variation in the skulls of the horseshoe bats, Rhinolophus
simulator and R. cf. simulator: determining the relative contributions
of adaptation and drift using geometric morphometrics.
The relative contributions of adaptation and drift to morphological
diversification of the crania of echolocating mammals was investigated
using two horseshoe bat species, Rhinolophus simulator and R. cf.
simulator 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. Shape variation was predominantly attributed to
selective forces; the between population variance (B) was not
proportional to the within population variance (W). Modularity was
evident in the crania of R. simulator but absent in the crania of R. cf.
simulator 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. Selection acted mainly on
the nasal dome region of R. cf. simulator whereas selection acted more
on the cranium and mandibles than on the nasal domes of R. simulator.
Probably the relatively higher echolocation frequencies used by R. cf.
simulator, the shape of the nasal dome, which acts as a frequency
dependent acoustic horn, is more crucial than in R. simulator, allowing
maximization of the intensity of the emitted calls and resulting in
comparable detection distances. In contrast, selection pressure is
probably more pronounced on the mandibles and cranium of R. simulator 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.