Group formation of female pigtail macaques (Macaca nemestrina)

Human epidemiological studies have suggested that social variables can modulate the effects of stress on the immune system, and this concept has been gaining increasing attention with positive results emerging from empirical studies using nonhuman primates over the last two decades. Results from a previous study in rhesus monkeys suggested that receiving grooming positively affected recovery of T-helper and T-suppressor cells following the initial stress associated with group formation, and this co-varied with high dominance rank. Thus, the present study was undertaken in order to determine: (1) if the stress effect of formation could be replicated in another species and (2) if social behaviors or dominance rank, given that formation is a stressor, might independently correlate with physiological recovery from the stressor. Eight adult female pigtail macaques were moved from individual cages and simultaneously introduced into an outdoor enclosure along with an adult male, while eight weight-matched controls remained in individual caging. Behavioral data were collected during the introduction and over 4 weeks thereafter. Blood samples were collected prior to and at intervals for 4 weeks following formation. Compared to control subjects, the test subjects showed an increase in basal cortisol secretion (+28.9%) and a significant decrease in T-helper cells (-33.6%), T-suppressor cells (-30.8%), and B cells (-22.5%), while there was a significant increase in white blood cells (+29.5%) 24 hr following formation. When dominance rank and seven behavioral categories were analyzed, only the frequency of receiving grooming significantly predicted change, with animals who received a greater frequency of grooms showing a lesser negative percent change from baseline in the absolute number of T-helper cells 1 week following formation. The establishment of a dominance hierarchy, apparent within 1 week, was accomplished with no serious fighting and a complete absence of wounding or trauma, suggesting that psychosocial stress was responsible for the physiological changes observed. © 1996 Wiley-Liss, Inc.