Information on the incidence of non-offspring nursing in 100 mammalian species was assembled from the literature and from a questionnaire survey. A comparative analysis of these data revealed several factors that influence the occurrence of non-offspring nursing across species. The incidence of nonoffspring nursing is increased by captivity. In field studies, it is more common in species that have larger litters and there are several important differences in the context of non-offspring nursing between monotocous taxa (where females typically give birth to a single young) and polytocous taxa (where females routinely give birth to multiple young). In monotocous species, non-offspring nursing is associated with high levels of 'milk theft' by parasitic infants; and is more common in species where females continue nursing after they have lost their own young. In polytocous species, non-offspring nursing is not associated with 'milk theft' and is most common in species that live in small groups. These results are discussed in terms of the costs to females of tolerating non-offspring nursing.
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We are extremely grateful to the many people who took the time and effort to fill in our questionnaire: this paper simply could not have been written without the extraordinary cooperation of the large group listed in Appendix I. We also thank Alan Grafen, Paul Harvey and Mark Pagel for guiding us in the application of their programs; and T. H. Clutton-Brock and G. S. Wilkinson for comments on the manuscript. This paper was written while C.P. & A.E.P. were supported by J. S. Guggenheim fellowships and by NSF grant 8807702.