Seasonal variation in plasma thyroxine concentrations in juvenile alligators (Alligators mississippiensis) from three Florida Lakes

Circulating concentrations of thyroxine (T4) vary seasonally in many vertebrates. This study examined the seasonal variation in plasma concentrations of T4 in juvenile American alligators (Alligator mississippiensis) from three populations in central Florida, USA. One site, Lake Woodruff National Wildlife Refuge, is considered a reference site whereas the other two lakes, Lake Apopka and Orange Lake, are significantly impacted by human activity. Juvenile American alligators ranging from 75-150 cm in total length were hand-captured at night from November 2000-April 2002. Plasma thyroxine concentrations were analyzed using a radioimmunoassay (RIA) previously validated for alligator plasma. Juvenile American alligators display seasonal variation in circulating T4 concentrations. Plasma T4 concentrations decrease from August/September to November and then begin a slow rise until April, at which point they plateau. Sex of juveniles influenced plasma concentrations of T4 in some months but did not appear to alter the pattern in seasonal variation. The pattern we observed in plasma T4 concentrations is not directly related to an environmental factor such as ambient temperature but is similar to that seen in plasma sex steroid concentrations during the reproductive cycle of adult alligators. Although the pattern and plasma concentration of T4 exhibits significant variation among the three lakes studied, the pattern in seasonal variation appears similar. Comparing the seasonal pattern in plasma T4 with plasma concentrations of sex steroids (testosterone and estradiol-17β) or corticosterone could provide important information on the peripubescent life stage of the American alligator.