Thyroid function in the desert tortoise, Gopherus agassizii

Abstract

To characterize seasonal changes in thyroid function in a terrestrial reptile, thyroid hormones were measured over a period of two years in desert tortoises, Gopherus agassizii, maintained at the Desert Tortoise Conservation Center in Las Vegas, NV. In all samples, triiodothyronine (T3) was non detectable (less than 0.01 ng/ml). Circulating amounts of T3 may reflect a low rate of T4 to T3 conversion. Thyroxine (T4) exhibited distinct cycles in both sexes, being lowest during hibernation and rising toward the time of emergence. Females exhibited only one peak in T4, during the early spring. In males, T4 levels peaked in early spring and again in late summer. The desert tortoise has been reported to have distinct activity patterns that include increased feeding, mating, and locomotor activity in the early spring and increased mating and combat in the late summer. Thus, an increase in T4 may be a result of increased food intake or metabolic rate after emergence. To determine if food intake contributes to increased T4, tortoises were fasted for two weeks. Upon refeeding, T4 levels increased, indicating that increased feeding elevates T4, possibly to support nutrient assimilation. The second peak of T4 in males may be due to increasing testosterone not seen in the females or male aggressive behavior associated with increased testosterone. To evaluate the effects of reproductive condition on thyroid function, adult, subadult, and juvenile male tortoises were sampled during the months of the second peak in male T4. T4 peaked in July only in reproductively active adults. Additionally, testosterone increased dramatically in this group. Thus, elevated T4 in desert tortoises is associated with periods of increased feeding and reproductive activity, supporting the proposal that this hormone serves to support these energy-demanding activities.