Plasma testosterone levels during the testicular cycle of the redwinged blackbird (Agelaius phoeniceus)

Fluctuations in plasma testosterone concentration during the testicular cycle of adult captive redwinged blackbirds were measured by radioimmunoassay. A peak in testosterone (range: 24.8-27.4 ng/10 ml) was associated with small testes that weighed less than 100 mg in the photosensitive stage. In the regressive stage, testosterone concentration was uniformly low (mean +/- SEM: 6.6 +/- 0.6 ng/10 ml) over a range of gonadal weights (range: 6.5-799.8 mg). Surprisingly, testosterone was detected in birds in the refractory stage maintained on a long photoperiod for 35 days (7.7 +/- 0.6 ng/10 ml) and in two birds kept for about 27 weeks (10.3 and 12.3 ng/10 ml). The relationship between plasma testosterone levels and the extent of testosterone synthesis by blackbird testes is discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22385/1/0000834.pd