ECHOLOCATION, FLIGHT MORPHOLOGY AND FORAGING STRATEGIES OF SOME WEST-AFRICAN HIPPOSIDERID BATS

We studied echolocation call structure, flight morphology and feeding behaviour of three hipposiderid bats (Asellia tridens, Hipposideros ca ffer and H. ruber: Chiroptera: Hipposideridae) in The Gambia during th e wet season (July August). All three species emitted brief CF/FM echo location calls. In A. tridens CF (constant frequency) frequencies betw een 108 and 122 kHz were recorded. This variation was caused mainly by sex and age difference in call frequencies: juveniles used lower freq uencies than did adults, and males were lower in frequency than female s. Among adults, CF frequency was related to forearm length in a polyn omial manner. Asellia tridens is unusual for a microchiropteran in tha t males are larger than females: nevertheless this species follows the typical trend for bats using CF components in their calls in that mal es call at lower frequencies than do females. The spread in frequencie s noted for A. tridens in group flight was not caused primarily by ind ividuals shifting their frequencies when flying in a group compared wi th when flying alone. Three bats were flown separately and then togeth er in a small room. The hypothesis that bats shift emitted frequencies in group flight to minimize confusing their own echoes with those fro m conspecifics was not supported. Rather, we suggest each bat has a pe rsonal CF frequency determined by sex, age and size, and that the vari ation created by these factors reduces confusion with other bats' echo es during group flight. Bats of the genus Hipposideros were separated into two groups by discriminant analysis on flight morphology. The two groups showed little overlap of CF frequencies used in echolocation. Bats identified as H. ruber called with CF resting frequencies between 121 and 136 kHz, while H. caffer used 128-153 kHz. Our results suppor t the classification of these bats into two sibling species. We compar ed the flight performance of A. tridens and H. ruber by flying bats th rough obstacle courses: H. ruber had a lower wing loading and was able to negotiate more complex arrays of obstacles than could A. tridens. In the wild, H. ruber fed in more cluttered situations than did A. tri dens. Both species were feeding mainly on Coleoptera during the study period, and in H. ruber individuals with higher aspect ratios and long er wingspans tended to eat more moths than did bats with broader and s horter wings.