Relation of glucocorticosteroids and testosterone to the annual cycle of free-living degus in semiarid Central Chile

We investigated seasonal patterns of plasma glucocorticosteroids (GCs) in b oth sexes and testosterone (T) in males in relation to the annual cycle in central Chile of a natural population of the degu (Octodon degus), a caviom orph rodent. We wanted to find out which GCs are present in degus, whether their seasonal variation suggests suppressive or synergistic interrelations hips with T, and whether seasonal variation in GC levels indicates a relati onship with energy mobilization and demands of reproduction. Degus mated in late autumn, and female body mass increased in pregnancy and remained high during lactation and throughout spring. Over the subsequent period of summ er drought both sexes declined to a minimal body mass before the next matin g season. Cortisol appears to be the principal GC in degus. In fact cortiso l levels were so high that the extremely low levels of corticosterone measu red were probably largely due to the cross-reactivity of our corticosterone antiserum with cortisol. Titers of cortisol in females exceeded 1000 ng/ml at lactation in the spring of 2 years; cortisol declined greatly following lactation and during the summer and reached its lowest mean level of about 500 ng/ml at mating. Males were more difficult to capture than females and thus our sampling was limited, but male cortisol levels were similar to th ose of females during the times of year when we measured them. Male T level s remained within a low range all year, but at mating, when mean T was high est (0.16 ng/ml) and when most males had detectable T, degus showed their l owest cortisol levels. The minimal cortisol level of males during mating;re presents a possible suppressive effect of T,as described in other mammals. At the time of their spring emergence, 60% of juvenile males had detectable T levels comparable to those of adults, suggesting important organizationa l effects of T at that time in their maturation. Peak cortisol titers in bo th sexes were associated with lactation in females, when energy mobilizatio n, production, and body mass were at their greatest. (C) 1999 Academic Pres s.