Individual differences in response to a number of animate and inanimate moving and non-moving stimuli were studied in 18 wolf cubs from four litters. Social rank was closely correlated with reactivity, exploratory behavior and prey-killing ability. A wide range of variability in test scores was found in all litters. It is proposed that pack integration and coordination of activities is enhanced by intra-specific socialization, social facilitation, and leader-follower relationships between subordinates and the more exploratory alpha individual. The evolutionary advantages of selection for individual differences or behavioral polymorphism within litters are discussed and inferences drawn for significance in pack formation. This hypothesis is supported by contrasting evidence of greater behavioral homogeneity in less social canids, where intra-specific aggression and mutual proximity intolerance prevents pack formation and leads to dispersal of the litter.