Behavioral and physiological characteristics of Indian and Chinese-Indian hybrid rhesus macaque infants

Strain differences in temperament and physiology have been reported for several animal species, but nonhuman primates have not been well studied in this regard. We assessed behavior and physiology in Chinese–Indian hybrid (n = 13) and Indian-origin (n = 29) nursery-reared rhesus monkey infants. Previous data indicate that Chinese-origin and Chinese–Indian hybrid rhesus exhibit more aggression directed toward humans and conspecifics and are more irritable in response to neonatal assessment procedures than are Indian-derived rhesus. In addition, in rhesus adults, low levels of cerebrospinal fluid 5-HIAA have been correlated with impulsivity, aggressive behavior, and diminished social competence. We therefore hypothesized that hybrid infants would exhibit more behavioral and adrenocortical reactivity in the home cage and during social separations, would be less sociable in their peer groups, and would exhibit lower CSF 5-HIAA levels than Indian-derived monkeys. Blood and cerebrospinal fluid samples were obtained on Days 14, 30, 60, 90, 120, and 150 of life, and prior to and during social separations at 6 months of age. Behavioral observations were conducted in the home cage and during the separation condition. No differences in behavior were observed between hybrid and Indian-derived animals in the home cage. Indian-derived and hybrid infants exhibited diverging patterns of behavioral reactivity across the 4 weeks of the repetitive social-separation procedure, and during reunion periods. Although plasma cortisol levels were sensitive to the testing conditions, no group differences were observed. CSF 5-HIAA declined over time for all monkeys, and hybrid animals exhibited significantly lower 5-HIAA levels than Indian monkeys beginning at 6 months of age. These findings are consistent with the known behavioral and physiological characteristics of Chinese-origin adult rhesus. ©1997 John Wiley & Sons, Inc. Dev Psychobiol 31: 49–63, 1997