Seasonal cycles in testicular activity, gonadotropin, and thyroxine in the painted turtle, Chrysemys picta, under natural conditions.

Plasma levels of testosterone (T), thyroxine (T4) and follicle-stimulating hormone (FSH) were measured in a field population of the painted turtle, Chrysemys picta, in Michigan. All three hormones showed pronounced seasonality; plasma luteinizing hormone (LH) remained nondetectable. Plasma T and FSH concentrations were highly correlated and exhibited biphasic cycles with peaks in spring and fall, whereas T4 showed a single peak in summer, coincident with the nadir in T. Hormone levels were minimal on the day that animals emerged from hibernation at the end of March; T and FSH then increased rapidly (in ca. 1 week after emergence) to a transient peak that persisted for only ca. 2 weeks. Plasma T and FSH rose again in September before the onset of hibernation and shortly after the late summer peak in spermatogenetic recrudescence. Plasma T4 increased more gradually after emergence in spring and did not peak for 1-2 months, coincident with the nadir in T. The profile in plasma FSH is consistent with the view that this gonadotropin may regulate androgen secretion but it did not show the expected relationship to the spermatogenetic cycle. Separate studies indicated that the turtles are sensitive to capture and handling; hormone levels (especially androgen) fall precipitously within a day of capture. These "stress" effects may account for discrepancies with previous studies of plasma T in this species.