Adaptive responses to seasonality in four species of Microchiroptera in coastal Kenya
A field study on the ecology of a multi-species association of insectivorous cave-bats was conducted on the south coast of Kenya from December 1976 to November 1979, to examine the seasonality of the climate and its effect, through the insect food supply and the body condition of selected bat species, on the timing of the reproductive cycles of these bats (Hipposideros commersoni, Hipposideros gigas, Taphozous hildegardeae and Miniopterus minor) and to investigate their social organisation. The environmental seasonality of the study area, induced principally by the rainfall regime, led to relatively high numbers of insects becoming available as food for bats in both the long (April-June) and the short (November-December) rains, with a pronounced drop in insect numbers occurring in the long dry season (July-October) and less markedly in the short dry season (January-March). Correlated with these fluctuations in their food supply, all the studied bat species showed very marked body weight cycles largely due to the deposition of fat during the rains, and low body weights were found in the dry seasons. The more resource demanding reproductive activities (regeneration of the reproductive tract and competitive mating in males, late pregnancy and early lactation in females) and the growth of young, were in all species coincident with the periods of higher food availability. The long cold dry season, when body condition was at a nadir, was avoided for all but the least demanding activities, such as early pregnancy among females. Reproductive adaptations of these monoestrus species to this seasonality are described. H.commersoni, H.gigas and M.minor seasonally migrated from the study area, apparently in response to the earlier onset of rain inland and improvement in food supply. These species have seasonally polygynous mating systems that involve competition between males for mating sites and females. Although T.hildegardeae also have a polygynous mating system, females did not migrate and suitable roost sites were limiting, resulting in the annual retention of territories and associated groups of females by competing males. Behaviours, displays and dispersion patterns associated with the social organisation of these species are described.