Acoustic divergence in two cryptic Hipposideros species: a role for socialselection?

We present evidence that a relatively widespread and common bat from South East Asia comprises two morphologically cryptic but acoustically divergent species. A population of the bicoloured leaf-nosed bat (Hipposideros bicolo r) from Peninsular Malaysia exhibits a bimodal distribution of echolocation call frequencies, with peaks in the frequency of maximum energy at ca. 131 and 142 kHz. The two phonic types are genetically distinct, with a cytochr ome b sequence divergence of just under 7%. We consider the mechanisms by w hich acoustic divergence in these species might arise. Differences in call frequency are not likely to effect resource partitioning by detectable prey size or functional range. However, ecological segregation may be achieved by differences in microhabitat use; the 131 kHz H. bicolor is characterized by significantly longer forearms, lower wing loading, a lower aspect ratio n and more rounded wingtip, features that are associated with greater manoe uvrability in flight that may enable it to forage in more cluttered environ ments relative to the 142 khz phonic type. We suggest that acoustic diverge nce in these species is a consequence of social selection for a clear commu nication channel, which is mediated by the close link between the acoustic signal and receptor systems imposed by the highly specialized nature of the hipposiderid and rhinolophid echolocation system.