Annual Reproductive Cycles in Selected African Mammals in Response to Environmental Cues

The assumption that natural selection is an Optlmlslllg process, means that reproductive seasonality in mammals evolved to maximise gene survival. Although the constraints usually imposed on optimality theory are applicable, it can be assumed that periodicity in reproduction represents an adaptation to fluctuation in the availability of potentially limiting resources. In areas where these fluctuations in primary production are the result of a predictable seasonal climatic cycle, such as seasonal rainfall, a seasonal reproductive cycle becomes a viable strate gem. Seasonal reproduction involves the temporal placing of mating, gestation, parturition, lactation and weaning in such a way to make maximum use of fluctuating resources (Skinner et at., 1984). In order to ensure that the young are born during favourable environmental conditions, the initiation of reproductive activity has to commence some time before the favourable season, or during it, depending on the duration of gestation. Larger mammals will tend to initiate reproductive activity earlier than small mammals, as gestation length usually increases with size (Rowlands & Weir, 1984). A number of environmental cues operate as proximate causes to initiate reproductive activity. A large variety of African mammal species are seasonal breeders (Fairall, 1968 ; Smithers, 1983). The thrust of this paper is not to review the occurrence of seasonal breeding, but rather to examine some factors that influence their seasonal reproductive cycles by referring to five interesting examples. The reproductive cycles of three ofthese is controlled by the photoperiod.