Steroid hormones in the blood plasma of Townsend's vole (Microtus townsendii)

Measurements of the concentrations of total corticosterone, corticosteroid-binding globulin (CGB), and total androgen in the plasma of male and female Microtus townsendii were made on two occasions separated by 13 days during a spring decline in numbers. The females had a much higher (7- to 12-fold) total and free corticosterone concentrations in their plasma than the males. Also, with the single exception of a female with no evidence of preceding pregnancy, which had a very low plasma androgen concentration, the concentration of steroids, resembling testosterone and dihydrotestosterone, in the plasma of the females was about double that in males with capture weights less than 80 g. Males, both wild and captive, with body weights in excess of 80 g at some time had plasma androgen concentrations up to fourfold those of the smaller males. Castration of males had no effect on total corticosterone or CBG concentration of the plasma. It is suggested that, at the time of sampling, the females were more readily stressed than the males and, hence, were at risk from the deleterious effects of excessive adrenocortical activity. The males taken from the wild population probably represented displaced animals, rather than more reproductively successful resident males. The source of the high total androgen concentration in the plasma of the females could be a special zone in the adrenal cortex which is known to appear after the first pregnancy in female Microtus agrestis.