Vol. 2, No. 3 , 1996, Page 4


An intriguing new primate study supports the argument that low levels of the neurotransmitter serotonin are a risk factor for dangerous behaviors. (See related articles in Crime Times, Vol. 2, No. 1, Page 4, and Crime Times, Vol. 1, No. 1/2, Page 7.)

Dee Higley and colleagues studied 49 male rhesus monkeys for four years. During this period, the young monkeys were undergoing a very dangerous period of life, during which they migrated from their own groups to new social groups. Between 30% and 50% of monkeys die during this period, often from violent encounters with other monkeys. (No other species prey on the island monkey colony.)

At the beginning of the study, the researchers measured levels of 5-HIAA (the principal metabolite of serotonin) in the monkeys' cerebrospinal fluid, dividing the monkeys into four groups: low, mid- low, mid-high, and high 5-HIAA. Twenty-seven of the monkeys were observed in the wild by researchers who recorded the monkeys' aggressive acts. The researchers also recorded the aggressive acts of all 49 monkeys while the monkeys were in captivity, and tallied the monkeys' fight-related scars and wounds.

After four years, the researchers say, 11 members of the monkey group were either known or presumed dead. "CSF 5-HIAA concentrations," they say, "were predictive of these early deaths."

"The 11 subjects that died or [were presumed dead] had significantly lower CSF 5-HIAA levels during their first capture than the subjects that remained alive," Higley et al. say. "Direct observations of aggressive behavior showed that subjects that had died engaged in high rates of escalated aggression; they also exhibited a trend to engage in more overall aggression." Of the six dead monkeys whose bodies were recovered, all four who died violently had low 5-HIAA levels, while the two monkeys who died of illnesses had 5-IAA levels similar to those of the surviving monkeys.

"Our findings," the researchers say, ".suggest that the rate of death is not randomly distributed across the overall male population. Instead, it appears that subjects with low CSF 5-HIAA concentrations are much more likely to die during this period of life than the rest of the male population."

While these monkeys were more violent than the high 5-HIAA monkeys, the researchers say, they also had other dangerous personality traits. They migrated at earlier ages, when they were less prepared to defend themselves, and they were more likely to take life-threatening risks, "such as spontaneous jumping at dangerous heights when moving from tree to tree." In addition, the low 5- HIAA monkeys were the most likely to be caught repeatedly in traps-more evidence of high-risk behavior.

As an example of the low 5-HIAA monkeys' aggressive, risky behavior, the researchers cite the male with the lowest 5-HIAA level, who was also the first monkey to be killed. "He was three years old and weighed less than half of a mature adult male," they say. "The night before his death, two of us observed this young male repeatedly attack a pair of fully mature males."

Similar pattern seen in humans

The increased death rate seen among monkeys with low 5-HIAA levels, Higley et al. say, is consistent with results of a 1993 study of a mixed-diagnosis group of 73 male psychiatric patients examined between 1976 and 1990. Seven of these patients later died before the age of 40. All seven had markedly lower CSF 5-HIAA levels than the surviving patients, and six of the seven died either in homicides or suicides, or in "suspicious" accidents. "These findings," the researchers say, "suggest that low CSF 5-HIAA concentrations may be a marker for early death among humans as well."


"Excessive mortality in young free-ranging male nonhuman primates with low cerebrospinal fluid 5-hydroxyindoleacetic acid concentrations," J. Dee Higley, Patrick T. Mehlman, Sue B. Higley, Beth Fernald, James Vickers, Stephen G. Lindell, David M. Taub, Stephen J. Suomi, and Markku Linnoila, Archives of General Psychiatry, Vol. 53, June 1996. Address: J. Dee Higley, NIH Animal Center, Building 112, P.O. Box 529, Poolesville, MD 20837.

Related Articles: [1997, Vol. 4] [2000, Vol. 6]

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