Female mammals sometimes nurse offspring that are not their own. The causes and function of allonursing for females remain puzzling given that lactation is energetically costly and augments the risk of pathogen transmission between mothers and alien offspring. To date five hypotheses have been put forward to explain why females nurse alien offspring. (1) Allonursing results from misguided parental behaviour. (2) Females reciprocate by nursing each other's offspring. (3) Females nurse related juveniles for inclusive fitness benefits. (4) Females nurse alien offspring to evacuate milk that their own offspring did not drink. (5) Inexperienced females that lactate spontaneously without reproducing themselves or that have lost their litter nurse alien offspring to improve their maternal skills. A review of empirical evidence showed that observations were consistent with predictions of the misdirected parental care, kin selection and milk evacuation hypotheses but not with predictions of the reciprocity and parenting hypotheses. However, probably because the study of mammals is not a trivial exercise, it is unclear to what extent each hypothesis can account for inter- and intraspecific variation in the female propensity to nurse alien offspring. Since the five hypotheses are not mutually exclusive, some observations said to support a given hypothesis may also be consistent with an alternative one. To tease apart the exact role of competitive hypotheses I propose some experimental designs. A detailed discussion of each hypothesis also reveals that the reason why females nurse alien offspring may be more intricate than was previously believed. (C) 2002 The Association for the Study of Animal Behaviour.