Effects of long-term grouping on serum cortisol levels in Microcebus murinus (Prosimii).

The influence of social interactions on plasma cortisol was studied in captive Microcebus murinus, a prosimian primate. Regardless the group composition, there is an annual variation in cortisol levels characterized by an autumn peak that coincides with the fattening and sexual resting period. The beginning of activity, early in January, is accompanied by a fall in cortisol levels. Cortisol levels are higher in females than in males both in pairs and in heterosexual groups. In paired animals, mean cortisol levels vary around 140 ng/ml in males and 230 ng/ml in females. In all grouped animals, a persistent adrenocortical activation was found and represents a 75 to 120% increase compared to paired animals. Due to the fact that M. murinus, the mouse lemur, is a solitary species with a primitive social structure, the high concentrations of cortisol found in grouped mouse lemurs could be a reaction to persistent social stress. Such high concentrations of cortisol may be related to poor reproductive success of grouped females and could play a major part in the pathological lesions found in males that die while maintained in social groups.