Seasonal Changes in Serum Testosterone in Adult Rams

Monthly blood samples collected from seven rams were assayed for serum testosterone concentrations. Testosterone concentrations were lowest in December (.76 ng testosterone/ml serum), increased gradually through April (3.88 ng/ml), and reached 7.42 ng/ml in May. After a transient decrease in June (to 4.15 ng/ml), testosterone concentrations reached a peak of 8.31 ng/ml in July; concentrations then decreased in August and September. It is suggested that light is the major stimulus to testicular steroidogenesis where summer temperatures are moderate; in hotter climates or during acute temperature increases, thermal factors may override photoperiodic effects.