In Tanzania three different generation types can be recognized in Mastomys natalensis rats. The α-generation is born in the main breeding season and breeds in the next main breeding season. The β-generation is born in the main breeding season but reproduces already after abundant off-season rainfall. Its offspring, the γ-generation, is born in the off-season breeding period and breeds in the same year. Life histories of these three generation types are different. Generation a has a long growth stop, poor survival until breeding but up to six consecutive litters; generation β has a short growth stop, a better survival until reproduction but breeds only once or twice. The γ-generation, however, grows very fast and reproduces at young age. Through their daughters, the β-generation thus produces more than seven times as much young as the α-generation before the end of the following year