The effect of gonadotrophin-releasing hormone GnRH on plasma testosterone was examined in male sugar gliders Petaurus breviceps that were judged to occupy either the highest or lowest position in stable colonies consisting of up to four males and two females. Experiments were carried out at a time when breeding occurred in two of the three colonies, as evidenced by the presence of pouch young.
GnRH was either injected i.p. or as a single pulse via the i.v. route. Blood samples were collected using infra-orbital sinus puncture when GnRH was injected i.p. but samples were collected from the jugular catheter when the GnRH was administered via the i.v. route.
Males that were judged to occupy the lowest position in each colony consistently had basal plasma testosterone concentrations which were lower than males judged to occupy the highest position. After GnRH administration, the plasma testosterone concentration rose markedly in all males, however, the highest plasma testosterone concentrations in a social group were, without exception, found to occur only in socially dominant males.
GnRH challenge of a dominant male from one colony introduced into an unfamiliar colony caused a marked decline both in its basal and stimulated plasma testosterone concentration within 24 hours of introduction. The body weight of the introduced male also decreased markedly.
Analysis of plasma testosterone concentrations in serially collected blood samples following i.v. injection of a single pulse of 10 μg GnRH showed that plasma testosterone concentration reached a peak at approximately 60 minutes after injection and had returned again to pre-treatment levels after 3h. Both the basal and GnRH-stimulated plasma testosterone concentrations were higher in the socially dominant males than they were in socially subordinate males.
GnRH injected i.p. into males captured in a wild population caused plasma testosterone concentrations to rise, however, considerable differences were found between males in both the basal and GnRH-stimulated plasma testosterone concentrations. These differences in basal and stimulated testosterone concentrations are consistent with the sampling of males representing a range of positions in the hierarchy in several different social groups.