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 caffer and H. ruber: Chiroptera: Hipposideridae) in The Gambia during the wet season (July August). All three species emitted brief CF/FM echolocation calls. In A. tridens CF (constant frequency) frequencies between 108 and 122 kHz were recorded. This variation was caused mainly by sex and age difference in call frequencies: juveniles used lower frequencies than did adults, and males were lower in frequency than females. Among adults, CF frequency was related to forearm length in a polynomial manner. Asellia tridens is unusual for a microchiropteran in that males are larger than females: nevertheless this species follows the typical trend for bats using CF components in their calls in that males call at lower frequencies than do females. The spread in frequencies noted for A. tridens in group flight was not caused primarily by individuals shifting their frequencies when flying in a group compared with when flying alone. Three bats were flown separately and then together in a small room. The hypothesis that bats shift emitted frequencies in group flight to minimize confusing their own echoes with those from conspecifics was not supported. Rather, we suggest each bat has a personal CF frequency determined by sex, age and size, and that the variation created by these factors reduces confusion with other bats' echoes 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 support the classification of these bats into two sibling species. We compared the flight performance of A. tridens and H. ruber by flying bats through 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. tridens. Both species were feeding mainly on Coleoptera during the study period, and in H. ruber individuals with higher aspect ratios and longer wingspans tended to eat more moths than did bats with broader and shorter wings.