THE BASIS OF REPRODUCTIVE SEASONALITY IN MASTOMYS RATS (RODENTIA, MURIDAE) IN TANZANIA

Reproduction of the African murid genus Mastomys, multimammate rats, i s seasonal, starting after the rains and extending well into the dry s eason. During a two-year study in Tanzania, we tested three hypotheses to investigate the proximal causes of this seasonality. Food availabi lity was no limiting factor since food supply could not induce continu ous breeding. Temperature was always high and thus not a restrictive c limatic factor. Diet was probably always sufficiently varied and prote in-rich to allow reproduction. This contradicts earlier hypotheses tha t consider Mastomys as an opportunistic breeder in which reproduction is seasonally limited by poor conditions; environmental predictors wer e believed not to be involved since the animals live in an unpredictab ly unstable environment. However, although the occurrence of rain may be unpredictable, it is highly predictable that heavy rainfall will be followed by good conditions. Laboratory experiments show that sprouti ng grass has a stimulatory effect on reproduction, suggesting that bre eding is triggered by new vegetation.