Seasonality in plasma thyroxine in the desert tortoise, Gopherus agassizii

To characterize seasonal changes in thyroid function in a terrestrial repti le, thyroid hormones were measured over a period of 2 years in desert torto ises, Gopherus agassizii, maintained at the Desert Tortoise Conservation Ce nter in Las Vegas, Nevada. In all samples, triiodothyronine was nondetectab le (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 emerg ence. Females exhibited only one peak in T-4, during the early spring. In m ales, T-4 levels peaked in early spring and again in late summer. The deser t tortoise has distinct activity patterns that include increased feeding, m ating, and locomotor activity in the early spring and increased mating and combat in the late summer. In an experiment to determine whether food intak e influences T-4, food was withheld for 2 weeks. Compared to continuously f ed controls, T-4 declined significantly in unfed tortoises, but increased s ignificantly within 36 h of refeeding, indicating that thyroid activity is responsive to nutrient intake. The second seasonal peak of T-4 only in male s suggests that male reproductive activity in late summer is associated wit h thyroid activation. To evaluate this possibility, adult, subadult, and ju venile males were sampled during the months of the second seasonal peak in T-4. Although all three age groups showed similar foraging and thermoregula tory 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. (C) 2001 Academic Press.