Annual and diurnal cycles in plasma testosterone and thyroxine in the male green sea turtle Chelonia mydas.

Male plasma testosterone (T) and thyroxine (T4) were monitored over several annual cycles in a captive breeding colony of green sea turtles, Chelonia mydas. Daily and annual water temperatures varied by only approximately 1 and 3 degrees, respectively. A pronounced season cycle in plasma T was evident in the population as a whole and in individual animals: plasma T was at a nadir (approximately 3 ng/ml) in September-November and then increased progressively to a peak (27-39 ng/ml) in April; levels began declining immediately thereafter, coincident with the onset of copulatory behavior. By contrast, plasma T4 remained uniform (approximately 9 ng/ml) throughout the year and, thus, could not readily account for the decline in androgen levels. Plasma hormones were relatively stable over a 24-hr period at three times a year, and there was a correlation for individual plasma T levels sampled in April and May. Thus, limited sampling should allow identification of seasonal rhythms and individual variability in plasma T levels. Testis mass and spermatogenic activity were significantly greater in January than in September; i.e., spermatogenesis and androgen secretion were not "uncoupled." Copulatory activity began in April but did not peak until May-June, after plasma T had significantly declined. However, there was a significant (but weak) correlation between individual peak levels of plasma T (i.e., in April) and the quantitative level of mating activity (time spent mounting and number of mates) measured for the entire subsequent season. Thus, green turtles do not exhibit the "postnuptial" type of testis cycle typical of many temperate-zone turtles, and the levels of plasma androgen may be important for initiating and maintaining sex behavior, although they are not tightly linked during the mating season.