Many male animals are territorial in the breeding season, when plasma testosterone (T) levels are high, and nonterritorial in the nonbreeding season, when plasma T levels are basal. In contrast to this common pattern, male song sparrows (Melospiza melodia morphna) are territorial year-round, except briefly during molt. Song sparrows are highly aggressive in the nonbreeding season (autumn and winter), even though plasma T, 5 alpha-dihydrotestosterone, androstenedione (AE), and 17beta-estradiol levels are undetectable (<or=0.1 ng/ml). Castration has no effect on nonbreeding territoriality. However, aromatase inhibitors decrease aggression in the nonbreeding season, indicating a role for estrogens in winter. The androgenic substrate for brain aromatase in winter is unclear, because plasma T and AE levels are basal. Here, we measured plasma levels of dehydroepiandrosterone (DHEA). DHEA is a precursor to AE and T, and the avian brain can convert DHEA into sex steroids. In nonbreeding male song sparrows, plasma levels of DHEA were detectable and several times higher than plasma AE and T levels. Plasma DHEA levels were similar in the breeding and nonbreeding seasons, but significantly lower during molt, which parallels seasonal changes in male aggression. Adrenal glands and testes from nonbreeding males had high concentrations of DHEA, suggesting that both tissues may secrete DHEA. However, stress and gonadotropin-releasing hormone (GnRH) did not increase plasma DHEA in nonbreeding birds. We hypothesize that in the nonbreeding season, circulating DHEA, possibly of adrenal origin, is converted into active sex steroids by steroidogenic enzymes in the brain. This mechanism would create high local levels of sex steroids in the brain to support winter aggression.
Copyright 2001 Academic Press.