Maternal behavior in the rat varies inversely with the number of offspring in the litter. This pattern may have been selected-in to the heredity of these animals because of its possible survival value for the species. High maternal behavior toward small litters may promote survival by protecting against further loss of offspring in already small litters. Low maternal behavior toward large litters may promote survival by (1) protecting against excessive fatigue in the mother, upon whose health and strength the safety of the entire litter depends, and (2) by promoting survival of only the fittest offspring in large litters. Fatigue induced in the mothers of large litters may serve as a physiologic signal mechanism, activating a stepwise reduction (inhibition?) of maternal behavior as the fatigue level rises.