Tenacious nipple attachment in rodents: the sibling competition hypothesis

Tenacious attachment to the mother's nipples by suckling pups has been observed in many rodent species, but apeears to be absent in others. The natural history of this behaviour is reviewed, along with the possible adaptive value. Four hypotheses are extracted from the literature: the antipredator, anti-falling, anti-drowning, and infant-nipple contact hypotheses. These hypotheses are found to be ad hoc and supported by only anecdotal evidence. An alternative hypothesis is presented, that tenacious nipple attachment is an adaptation to sibling competition for maternal resources (nipples and milk). Morphological specialization, developmental time course, and patterns of pup mortality support the new model. The behaviour has been noted only in the Muridae. Quantitative interspecific comparisons within the Muridae demonstrate that tenacious nipple attachment is associated with reproductive tactics that promote sibling competition: breeding by means of litter overlap, low mammary number, and more offspring per mammary.