Empirical and theoretical constraints on the evolution of lactation.

For 829 mammalian species, data on age at weaning or age at first solid food were analyzed with respect to body mass, phylogeny, habitat, diet, length of gestation, basal metabolism, and neonatal development. The primary influence on lactation length is female mass, but phylogenetic constraints are important. Thus, lactation can be characterized as short (earless seals and baleen whales), long (marsupials, bats, and primates), or average (remaining eutherians). Among average mammals, lagomorphs have short lactations. Lactation may have different functions, evolutionary constraints, and physiological control depending on whether young first eat solid food near weaning or well before it. First solid food eaten near weaning occurs in polytocous species with altricial young; in this case, lactation has a clear energetic role. In contrast, first solid food well before weaning is common for mammals with single, precocial offspring. For these species, the energetic and nutritional constraints on lactation may be less important than the benefits of maintaining contact between mother and young, such as reduced juvenile mortality and increased opportunities for learning social or foraging patterns. Thus, the age at first solid food relative to the age at weaning may indicate the function of lactation within the reproductive biology of a given mammal. Delayed development and implantation alter the timing of energetic investment during gestation, so too, the age at first solid food may alter or reflect the rate of energetic investment during lactation. Thus, the age at first solid food relative to the age at weaning may indicate the function of lactation within the reproductive biology of a given mammal. Testing these hypotheses will require data from diverse species on the nutritional and energetic value of milk before and after first solid food as well as on the mechanics and consequences of nursing or suckling during the course of lactation.