Reproduction of the African murid genus Mastomys, multimammate rats, is seasonal, starting after the rains and extending well into the dry season. During a two-year study in Tanzania, we tested three hypotheses to investigate the proximal causes of this seasonality. Food availability was no limiting factor since food supply could not induce continuous breeding. Temperature was always high and thus not a restrictive climatic factor. Diet was probably always sufficiently varied and protein-rich to allow reproduction. This contradicts earlier hypotheses that consider Mastomys as an opportunistic breeder in which reproduction is seasonally limited by poor conditions; environmental predictors were believed not to be involved since the animals live in an unpredictably 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 sprouting grass has a stimulatory effect on reproduction, suggesting that breeding is triggered by new vegetation.
