The spiny tenrecs, an endemic subfamily of Malagasy insectivores (Tenrecinae), are wide ranging and fairly conspicuous, yet long-term studies on free-ranging populations remain sparse. Basal to most eutherian mammals, they share many ecological and morphological traits with proposed eutherian ancestors. Understanding of their unusual life histories is therefore important to the understanding of mammalian evolution. Here we present the results of a 3-year study on a population of Setifer setosus in the dry deciduous forest of Western Madagascar. The annual activity cycle of this species includes a 5–7-month hibernation period, during the dry season, and a dramatic increase in body mass during the active season. Females, observed giving birth to up to three litters in a single season, entered hibernation later than males, after weaning their last litter. Short intervals between parturition dates and simultaneous gestation and lactation provide evidence of post-partum oestrus, previously observed in only one other species of tenrec (Geogale aurita, subfamily:Geogalinae). High levels of mortality, primarily by snakes and ground predators, were also observed and likely contribute, along with the unpredictability of Madagascar's climate, to the unusually fast life history of these mammals.