Seasonality in plasma thyroxine in the desert tortoise, Gopherus agassizii

To characterize seasonal changes in thyroid function in a terrestrial reptile, thyroid hormones were measured over a period of 2 years in desert tortoises, Gopherus agassizii, maintained at the Desert Tortoise Conservation Center in Las Vegas, Nevada. In all samples, triiodothyronine was nondetectable (less than 0.1 ng/ml). Thyroxine (T(4)) exhibited distinct cycles in both sexes, being lowest during hibernation and rising toward the time of emergence. Females exhibited only one peak in T(4), during the early spring. In males, T(4) levels peaked in early spring and again in late summer. The desert tortoise has distinct activity patterns that include increased feeding, mating, and locomotor activity in the early spring and increased mating and combat in the late summer. In an experiment to determine whether food intake influences T(4), food was withheld for 2 weeks. Compared to continuously fed controls, T(4) declined significantly in unfed tortoises, but increased significantly within 36 h of refeeding, indicating that thyroid activity is responsive to nutrient intake. The second seasonal peak of T(4) only in males suggests that male reproductive activity in late summer is associated with thyroid activation. To evaluate this possibility, adult, subadult, and juvenile males were sampled during the months of the second seasonal peak in T(4). Although all three age groups showed similar foraging and thermoregulatory behaviors, T(4) peaked in July only in the reproductively active adults, which also exhibited significantly higher testosterone levels. Elevated T(4) in desert tortoises is thus associated with periods of increased feeding and reproductive activity, supporting a role for thyroid hormones in these energy-demanding activities.