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During spring-fall 1985-95, 5 groups of orphaned brown bear cubs (Ursus arctos) were captive-raised in the Central-Forest Biosphere Reserve in Russia. The groups of 2-12 individuals were raised to be released in the wild. We documented the onset of aggressive behavior between cubs. When cubs were 3 to 40-45 days old they were not aggressive toward one another. Low intensity aggressive behavior developed between 45-55 days. After 60 days of age, aggressive reactions were observed near food, including milk bottles, food cups, or natural foods. At age 5-7 months some cubs stayed in the wild surroundings of the station, did not return to their artificial den, and fed on ant (Formica spp.) brood pupae and larvae. Individual success in getting ants was inversely proportional to the level of social activity and aggression in the group. Individual success was highest in small groups (2-3 animals) where the number of aggressive contacts and the degree of dominant behavior was lower than in groups of 6-12 individuals. Subordinate members of large groups were very irritable and stayed immobile for long periods. They also revealed the pathological habit of paw sucking and their growth rate was retarded. After the age of 6 months when bears cubs stayed in the woods, these large groups spontaneously separated into smaller groups of 2-3 cubs. We suggest that aggressive behavior in cubs regulates group size, and the age of onset and level of aggression are adaptive features for brown bear survival. Aggression begins when ants are most abundant (Jun-Jul) and when lactation declines in free-ranging female bears. The more aggressive individual would likely be the most fit under these circumstances. We provide evidence that size of the brown bear family is an optimal solution of maintaining the population within the constraints of (1) the resource base, (2) the level of aggression among family members, and (3) the physiological traits of nursing by females. Based on these factors, we consider 3 cubs to be the optimal litter size. Larger litters would probably have difficulty in getting enough protein-rich food, while smaller litters could lead to population decline.