|Vol. 6, No. 1, 1999 Page 1&5|
In humans, low levels of the brain chemical serotonin are strongly linked to aggressive and impulsive behavior. Similarly, in the mid-1990s, J. D. Higley and colleagues linked low serotonin levels in male primates to aggression and impulsivity (see related article, Crime Times, 1996, Vol. 2, No. 3, Page 4). A new study by the researchers, this time of female macaques, reveals a similar pattern, and also indicates that primates with high serotonin levels dominate those with lower levels.
In the new study, G. C. Westergaard, Higley, and colleagues measured cerebrospinal fluid levels of 5-HIAA, the major serotonin metabolite, in females of two species of macaques: rhesus macaques, one of the most aggressive macaque species, and pigtailed ma acaques, who are more friendly and placid. The researchers found that the rhesus macaques had lower 5-HIAA levels and higher rates of high-intensity aggression, escalated aggression, and wounds requiring medical treatment. Within each species, too, they s say, CSF 5-HIAA levels were strongly negatively correlated with rates of escalated aggression, "indicating that this relationship is present in multiple species, as well as humans."
The researchers also found that females with high 5-HIAA levels dominated those with low 5-HIAA levels in both species. "Social dominance ranking is widely held to be a measure of competent social behavior in non-human primates," they say, "and, unlike in n rodents, social dominance does not appear to be directly related to fighting skills or overall aggression levels."
Low serotonin levels, while they appear to be deleterious, may be adaptive in some circumstances, the researchers note. The aggressive rhesus macaques live in large social groups inhabiting sparser environments than the placid pigtailed macaques, they say y, and "the resulting increased competition for resources may have led to higher rates of aggression in rhesus over evolutionary history and suggests a possible adaptive purpose for the high rates of aggression in this species."
"CSF 5-HIAA and aggression in female macaque monkeys: species and interindividual differences," G. C. Westergaard, S. J. Suomi, J. D. Higley, and P. T. Mehlman, Psychopharmacology, Vol. 146, 1999, pp. 440-446. Address: G. C. Westergaard, Division o of Research, LABS of Virginia, Inc., 95 Castle Hall Road, P.O. Box 557, Yemassee, SC 29945.