We compare the nursing behavior of two species, African lions(Panfhera Leo) and spotted hyenas(Crocuta Craig Packer crocuta), and show that non-offspring nursing is much less common in hyenas than lions. Hyenas spend less time with their cubs, are more alert during the suckling attempts of cubs, and more frequently resist the attempts of non-offspring. Vigilance against milk theft may therefore influence the distribution of non-offspring nursing across species. Our detailed study of non-offspring nursing in lions shows that females preferentially nurse their own offspring and that cubs are more surreptitious when attempting to suckle from other females. Non-offspring nursing in lions is most common when the costs are lowest. First, non-offspring nursing is more common among close kin. Second, females with small litters, and presumably more milk to spare, give a higher proportion of their nursing to non-offspring. Third, females give a higher proportion of their nursing to non-offspring as their own cubs grow older and need less milk. Cubs reared in créches do not appear to gain more milk that cubs raised alone, and females do not show any evidence of reciprocity in nursing one another's offspring. We suggest that non-offspring nursing in lions occurs as a by-product of the females' communal defense of their cubs against infanticide.