Amphetamines and Their Effects on Dominance Hierarchy in Primates


Humans are primates, as are monkeys and apes. Evolution tends to be very conservative and so the brains of humans are very similar to our cousins. In fact, genetically we are about 98% the same as our primate cousins. Although research that involves monkeys demonstrates the same dose-dependent effects of amphetamine as shown with rodent subjects, the resultant effects on aggressive behavior favor a positive rather than negative relationship (). Primarily, the effects that amphetamine has on primates’ dominance rank have been examined. Analysis has suggested that these effects are a function of social status and group dynamics ().

Differences of Effects between Ranks

The behavior of dominant animals differs drastically from that of subordinate animals (). We tend to categorize dominant styles of behavior as aggressive and subordinate styles of behavior as defensive. Dominant and subordinate animals also differ from each other neurochemically and hormonally. We can identify the rank of a primate within its hierarchy by observing behavior. When amphetamine is administered to monkeys of different social status within an established colony, the subjects express behavior dependent on their position in the hierarchy. For example, treatment of d-amphetamine causes an increase in aggressive behavior — open mouth threats, biting, chasing — in low- and high-ranking monkeys, with little or no effect on those in the mid-ranks (). Similar effects were observed when measuring rates of affiliative behaviors — grooming, holding, huddling — between the subjects: high-ranking monkeys showed decreases in affiliation with little variance, low-ranking monkeys also displayed reductions but with a larger range of variance, and mid-ranking monkeys conveyed no significant decline in affiliative behaviors (). These findings are extremely important. They suggest that the effect of amphetamine on aggressive behavior is linked to the initial level of aggressiveness of the individual.

Additional discoveries have been made that further illustrate the diverse effects amphetamine has on the dynamics and hierarchy of primate interaction. Along with the fact that low- and high-ranking monkeys are principally affected comes a certain directionality of their aggressive displays. High-ranking subjects treated with d-amphetamine were more aggressive to adults and other superior members of the group, whereas those in the lower ranks displayed greater aggression to juveniles and those with inferior positions in the dominance hierarchy ().

This type of effect, a drug causing individuals to act differently depending on preexisting personality traits, is not unique to amphetamine. A comparison of the animal and human literature on the effects of alcohol on aggressive behavior yields a similar result (). In fact, the effect of alcohol on aggressive behavior may be bidirectional. In other words, alcohol can cause aggressive behavior in a subclass of the population (i.e., in highly aggressive individuals but not in low to moderately aggressive individuals) and alcohol use can be caused by the need of the highly aggressive individual to self-medicate (this theory assumes that being very aggressive causes increased levels of stress and that alcohol is used by these individuals to alleviate that stress). This type of behavior can lead to a cyclic pattern in which the highly aggressive individual becomes involved in situations that cause stress. This person then drinks alcohol to alleviate that stress. The alcohol causes that person to become more aggressive, which causes more problems in the individual’s life (brought about by aggressive behavior leading to negative outcomes), which causes more stress, which leads to more drinking. These results taken together with the above analysis of the effect of amphetamine on primate behavior suggest that the reports of human aggressive behavior being increased by amphetamine use are simply an artifact of highly aggressive individuals tending to take and be made more noticeably aggressive by amphetamine.

Dose-Dependent Effects

In contrast to the results reported from rodent research, greater rates of aggressive behavior were observed in correspondence with increases in dosage. Low doses (0.01 mg/kg) produced very little change in aggression whereas a rapid escalation was observed with subsequent increases (up to 1 mg/kg). In Smith and Byrd’s (1984) study, the highest-ranking monkey displayed the largest increase in aggressive behavior in direct relation to the increase in dose, with rates of more than 30 times that of the control group at the highest dose (0.56 mg/kg). According to previous literature and current speculation, an adequately broad range of doses will yield an inverted U-shaped dose-effect curve that is typical of the behavioral effects of psycho motor stimulants (). This finding indicates that increasingly larger doses of amphetamine would eventually lead to the reduction of aggressive behavior.

In regard to affiliative behaviors, one notices a dose-dependent effect on the rate of occurrence that almost parallels that of aggressive behavior. Over a range of doses (0.003 to 0.56 mg/kg) a considerable majority of subjects demonstrate a dose-related pattern of affiliative behavior with little or no effect at lower doses and large decreases at higher levels (0.3 to 0.56 mg/kg). These results are also contrary to those found in rodent experimentation, where clear increases of social exploration were observed in male mice following acute doses of amphetamine.