Social status and reproductive cycle determine the effects of acute, low doses of alcohol on the social behavior of squirrel monkeys. Alcohol produces biphasic effects on the behavior of dominant but not subordinate monkeys, and only during the mating season. The change in alcohol sensitivity measured in dominant monkeys coincides with changes in plasma testosterone levels. In order to directly study the interaction between alcohol, testosterone and aggressive behavior, testosterone propionate (TP, 25 mg/kg/ day, SC) was administered to either dominant or subordinate male monkeys belonging to four separate groups, resulting in significantly elevated plasma levels of testosterone (i.e., 905__+43 ng/ml in subordinates; 171+19 ng/ml in dominants). Two to three weeks after the beginning of testosterone treatment, the monkeys were administered doses of alcohol (0.1-1.0 g/kg). The behavior of subordinate monkeys was unaffected by TP treatment (even after the dominant monkey from each colony was removed and housed separately for 6 weeks). Testosterone treatment altered the sensitivity of subordinate monkeys to alcohol. Low doses of alcohol increased the frequency of threats, grasps, and displacements exhibited by subordinate monkeys with exogenously elevated testosterone. Daily administration of TP to dominant monkeys during the non-mating season did not affect the behavior of the treated animals in the group, although the body weight of TP-treated monkeys was similar to that measured during the mating season. Low doses of alcohol increased the frequency of threats, grasps, and displacements in dominant monkeys maintained on TP.
We also tested the role of social context in maintaining high levels of plasma testosterone, and alcohol sensitivity in dominant monkeys. While living in their social groups, dominants exhibited a similar pattern of alcohol enhancement of aggressive behavior in dyadic confrontations compared to effects measured during the mating season in an undisturbed group. After 2 weeks of individual housing, both plasma testosterone values and sensitivity to alcohol's effects were significantly reduced.
These findings strengthen previous correlative studies indicating a relationship between plasma testosterone levels and the effects of alcohol on the aggressive behavior of squirrel monkeys. High levels of testosterone may activate alcohol-sensitive brain substrates of aggressive behavior.