Environmental and endocrine control of reproduction in the song sparrow, Melospiza melodia. II. Agonistic interactions as environmental information stimulating secretion of testosterone.

A group of eight free-living adult male song sparrows, selected at random, were given subcutaneous implants of testosterone (T) in Silastic tubing. In a separate area, 1 km distant, a second group of eight free-living males were given empty implants as controls. All implants were administered in late March and early April and in the experimental group maintained plasma levels of T at the springtime peak. To test whether prolonged high levels of T maintained aggressive territorial behavior at the springtime maximum, each bird was "challenged" by stimulating a territorial intrusion achieved by playing tape-recorded conspecific songs on their territories. Sparrows with T implants were more aggressive than controls in accordance with other studies showing that T increases frequency and intensity of aggression. Blood samples were collected from all implanted birds, and their untreated neighbors, at intervals throughout the breeding season, and plasma levels of luteinizing hormone (LH) and T were measured. Circulating T levels in males with territories next to control males show declines in T levels in April, increase again in May, and are low during the parental phase in June and July. However, the decline of T in males with territories next to males implanted with T did not occur, and levels remained significantly higher than those of males in the control area. Plasma T levels did decrease in May, however, but rose again in June, and in July were significantly higher than controls. Curiously, plasma levels of luteinizing hormone did not differ between controls and experimentals.