Plasma testosterone titers during the annual reproductive cycle of starlings ( Sturnus vulgaris)

Abstract

To clarify the role of testosterone in the annual reproductive cycle of seasonally breeding birds, annual variations in plasma testosterone titers, the condition of the reproductive organs and behavior were monitored in adult male starlings ( Sturnus vulgaris). Wild starlings were bled to death immediately after capture, and the testosterone titers in their plasma were determined using a competitive protein-binding assay. Testicular weights, seminiferous tubule diameters, germinal epithelium condition, interstitial Leydig cell condition, and seminal sac weights were assessed using appropriate histological techniques. Plasma testosterone titers varied from 0.5 ng/10 ml during the postbreeding period to 4.2 ng/10 ml at the height of breeding. Strong correlations were found between testosterone titers and the condition of the interstitial cells of the testes and between testosterone titers and seminal sac weight. Postbreeding testicular regression and cessation of spermatogenesis corresponded temporally with ebbing testosterone titers. Expressions of masculine behavior, such as courtship and territorial aggression, corresponded with periods of elevated testosterone titers.