Individual differences in the behavior of young and adult animals have been documented in diverse species. Possible sources of such variation are of interest to scientists representing many disciplines, including behavior, genetics, and population and evolutionary biology. Two variables that may be important in the ontogeny and maintenance of behavioral differences are (1) individual physical (aerobic and anaerobic) fitness and (2) possible genetic variations underlying individual abilities to engage in, and to benefit from, motor training early in life. The differential development of aerobic and anaerobic capacities may play a significant role in the ontogeny of individual differences in the performance of various motor skills. There also may be short- and long-term consequences of variations in physical fitness that influence individual abilities to perform energy demanding acts during aggressive encounters, interactions with prey or predators, and courtship and breeding. Genetic studies of a limited number of species indicate that specific genotypes are correlated with individual variations in motor performance, even among conspecifics. Multidisciplinary research concerning possible relationships among the ontogeny of physical fitness, genetics, and variations in behavior is needed. Recent work on the relationship between individual differences in physical fitness and variations in the behavior of adult cold-blooded vertebrates provides a good model for comparative research on warm-blooded species.