Mammals are characterised by their ability to provision offspring with milk, but lactation is normally restricted to females. Why do most males not share this trait? The morphological and physiological modifications necessary for male lactation are possible and, although restrictive, the ecological factors that could select for male lactation seem common enough. Until quite recently, there was no evidence for male lactation in wild mammals, but it has now been reported in two species of Old World fruit bats. Here we review mechanisms that can cause milk production in males and also consider the possibility that male lactation in these bats is a unique adaptive phenomenon.