Delayed dispersal and elevated monoaminergic activity in free‐ranging rhesus monkeys

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Abstract
Male rhesus monkeys typically disperse from their groups of birth when they are between 3 and 5 years of age. Some males, however, delay dispersal from their natal groups until after they are 5 years old. The current study evaluated central monoaminergic neurotransmitter activity as a potential correlate of such “delayed” dispersal among 54 randomly selected adolescent and adult male rhesus monkeys (Macaca mulatta) captured on Cayo Santiago during an annual trapping season. Specifically, cerebrospinal fluid (CSF) concentrations of 5‐hydroxyindoleacetic acid (5‐HIAA, a serotonin metabolite), 3‐methoxy‐4‐hydroxyphenylglycol (MHPG, a norepinephrine metabolite), and homovanillic acid (HVA, a dopamine metabolite) were compared in monkeys 60 months of age or more that had either dispersed (n = 33) or were still in their natal groups (n = 5). The monkeys still in their natal groups had higher CSF concentrations of both 5‐HIAA and HVA (but not MHPG) than did the animals that had emigrated (Ps < 0.05). Subsequent analysis indicated that only 5‐HIAA independently differentiated dispersing monkeys from delayed dispensers. Of monkeys less than 60 months of age (n = 16), only two had dispersed from their natal groups; in this age class, there were no significant differences between dispersing and natal individuals in any CSF monoaminergic metabolite (all Ps = NS). Finally, there was no difference in the CSF 5‐HIAA concentrations of the five delayed dispersers and those of younger animals (P = NS), suggesting a failure to experience the frequently reported adolescent decline in serotonergic activity. In contrast, the CSF 5‐HIAA concentrations of the dispersing animals were lower than those of the younger animals (P < 0.05), consistent with either an agerelated decline or an effect of dispersal per se.