Seasonal variation in thyroxine in the American alligator (Alligator mississippiensis)

Abstract

Male and female alligators ranging from 58 cm to 361 cm in length (N=1054) were captured and their blood was sampled between 9 A.M. and midnight from June 2000 - June 2002 at the Rockefeller Wildlife Refuge in Grand Chenier, LA. Animals were captured in a variety of habitats including marsh, open water, canals, and ponds. Total thyroid hormones were measured by radioimmunoassay. Four criteria were used to evaluate changes in hormone levels: sex, total length, season, and time of day. Thyroxine (T4) varied over a wide range from 0.5 to 57 ng/mL. Triiodothyronine (T3) levels were below the sensitivity of the assay (<0.6ng/mL). A distinct seasonal peak in T4 was observed between December and April, peaking in March with highest mean T4 levels observed in the spring. Mean T4 declined significantly from the spring to an intermediate summer mean and declined significantly again in the winter. No T4 differences between the sexes were observed. Mean T4 was highest in juveniles, significantly lower in subadults, and significantly lower still in adults. Highest levels were found between 5 P. M. and midnight. With values up to 57 ng/ml in the spring, alligators exhibit T4 levels among the highest recorded for reptiles.