Intrinsic Rates of Natural Increase of Altricial and Precocial Eutherian Mammals: The Potential Price of Precociality

Abstract

Although the production of altricial young has often been assumed to be an adaptation to increase reproductive output, no study has demonstrated that altricial species have higher intrinsic rates of natural increase (rm) than precocial forms of the same body mass. This study compares the intrinsic rates of natural increase of these two types of mammals. Altricial species have significantly higher intrinsic rates of natural increase, but only at small body masses. This advantage enjoyed by altricials over precocials at small body masses is due to higher numbers of young produced per year, and to younger ages of first reproduction in spite of similar growth rates. Small precocial species may have to forego reproduction until they reach a relatively large body mass in order to carry the heavier fetal mass associated with precociality. These findings explain why there are so few small precocial mammals. Conversely, at large body masses the differences between altrical and precocial species with respect to rm are insignificant, and precocial species often have the evolutionary advantage.