Insulin-like growth factor I is suppressed in socially subordinate male baboons

Insulin-like growth factor (IGF) I is a potent growth-promoting and anabolic hormone with major roles in cellular growth and differentiation, protein metabolism and muscle physiology, wound healing, erythropoiesis, and immune stimulation. Few, if any, studies have examined social or psychological factors that could give rise to individual differences in IGF-I levels. As part of a long-term psychoendocrine study of a population of male baboons living freely in a national reserve in East Africa, we examined the relationship between social rank and IGF-I concentrations. We observed that social subordinance was associated with a relative suppression of IGF-I concentrations; no rank-related differences in concentrations of IGF-II or IGF-binding protein were observed. Extensive psychoendocrine literature suggests that the individual differences in IGF-I profiles were a consequence, rather than a cause, of the rank difference. We were able to rule out a number of possible proximal explanations for the rank-IGF-I correlation: 1) the correlation was not a function of age (which involves both an adolescent spurt in IGF-I concentrations as well as a decline in concentrations in aged individuals); 2) the IGF-I suppression in subordinate individuals could not be explained by the basal hypercortisolism typical of such subordinate animals; and 3) neither differences in the quality or quantity of food consumed, in basal testosterone concentrations, nor in genetics could explain the rank difference. Although the mediating mechanisms responsible for this rank difference were not discernible in this study, the magnitude of difference in IGF-I levels among baboons of differing ranks might be of physiological significance.