At birth and for some time thereafter altricial mammals are, by definition, almost totally dependent on their parents or other caretakers for provision of many of the necessities of life. Although many mammals are born relatively helpless, none continue to depend on conspecifics for sustenance throughout their life cycles. Prior to reaching reproductive age each altricial mammal must become competent to acquire directly from the environment those goods and services which it previously acquired only indirectly as the result of interaction with conspecific caretakers. This transition from an infantile dependence on others for transduction of environmental resources to an adult mode of independent acquisition of necessities is in two senses a gradual process. First, independence with respect to any single need is rarely achieved suddenly; most often a series of graded transitional stages intervene between total dependence on caretakers for supply of a given resource and independent acquisition of that resource. The developing rat pup, for example, moves successively from in utero total dependence for nutrition on mother’s blood, to postparturient total dependence on mother’s milk, to a mixed diet of mother’s milk and solid food, to a diet of solid food transported to the home burrow by adult conspecifics, to feeding trips with adult conspecifics, to independent acquisition of solid food. Second, the juvenile may exhibit adultlike behavior in meeting each of its various needs at different ages. The rat pup, for example, initiates endogenous thermoregulatory behavior some time before it initiates independent feeding on solid foods.