Lactation is almost exclusively associated with pregnancy and giving birth. Although lactation can be induced without a preceding pregnancy in some species, this requires exogenous hormones, artificially intense or extended suckling or both. Spontaneous lactation, lactation by females that have neither been pregnant nor experimentally manipulated, is extremely unusual among eutherians. Among nondomesticated animals, spontaneous lactation has been observed repeatedly only in the dwarf mongoose Helogale parvula. We report here spontaneous lactation by free-living dwarf mongooses using data on urinary oestrogen conjugate concentrations (n = 560, 65 females) and body weight (n = 3,096, 25 females) from a population in Serengeti National Park, Tanzania. We use demographic data from this population to demonstrate that spontaneous lactation, and thus the endocrine phenomena that induce it, increase the evolutionary fitness of lactating females.