Plasma testosterone-binding globulin-binding capacity in the male lesser mouse lemur (Microcebus murinus): relationship to seasonal and social factors.

Plasma testosterone-binding globulin (TeBG)-binding capacity was determined by equilibrium dialysis in the lesser mouse lemur (Microcebus murinus), a prosimian which exhibits a clear photoperiod-dependent sexual cycle. Plasma TeBG-binding capacity showed significant seasonal changes which were inversely correlated to those of plasma testosterone concentrations. The capacity of TeBG was at its maximum (1.72 +/- 0.4 mumol/l) during the sexual rest period and decreased to 0.98 +/- 0.24 mumol/l during the breeding season when testosterone concentrations were about 200-280 nmol/l. In addition, when males developed a social hierarchy in groups, the decreased sexual function in non-dominant animals was associated with a higher TeBG-binding capacity. However, although TeBG-binding capacity increased when testosterone concentrations decreased either during sexual rest or under social dominance, a positive correlation was found between TeBG-binding capacity and testosterone levels during the breeding season in males with a normal reproductive cycle, i.e. in both isolated and dominant males. This suggests that factors other than sex steroids are involved in the changes in TeBG-binding capacity occurring under seasonal and social influences.