Aggression and paternal behavior can be linked in a variety of ways, and the relationship between hormones and these social behaviors may be equally variable. We have illustrated how high levels of aggression can be compatible with high levels of paternal care. Under such conditions, T may be positively associated with paternal care and aggression, even in species in which a decrease in T occurs with the onset of paternal care. Similar to T, AVP also can be positively associated with both aggression and paternal care. Individual variation in paternal care and aggression may be mediated by variation in AVP and T levels and receptors. This physiological variation could in turn be important for survival of offspring and also for shaping variation in paternal behavior and aggression of those offspring. Behavioral and endocrine changes may be passed on to multiple generations. The degree of plasticity in these relationships remains to be elucidated, but our results suggest that variation in AVP and T may be important for altering paternal and aggressive behaviors in response to the social environment.