Comparative Reproductive Strategies of Altricial and Precocial Eutherian Mammals

Mammals produce neonates of varying degrees of development described by the terms altricial and precocial. I rank eutherian mammals by a comprehensive measure of development in which neonates are scored in four areas of independence: thermoregulatory, sensory, locomotory and nutritional. The association between neonatal development and nine life-history traits is examined in 239 species. The distribution of neonatal developmental rankings is associated with adult mass and is related to taxonomic level, with precocial neonates more common among orders, and altricial neonates more common among genera and species. When the effect of adult mass is controlled, precocial mammals produce heavier neonates and heavier litters, possess longer gestation periods, attain adult mass more slowly, and have smaller litter sizes. Mass at weaning, length of the total reproductive interval, and age at first parturition do not differ among developmental categories. Although the continuous nature of development is intuitively appealing, mammalian development patterns do not map onto predictable, continuous changes in life-history patterns. Length of the lactation period is extended in precocial primates but shortened in precocial rodents. Neonate mass varies continuously with stage of neonatal development, while litter size varies dichotomously, and age at first parturition and litter mass differ uniquely in only one developmental category. Thus, neonatal developmental patterns do not appear to result from one continuous axis of selection.