Plasma steroid concentrations in relation to size and age in juvenile alligators from two Florida lakes

Previous studies have reported a number of physiological differences among juvenile alligators from two well-studied populations (Lake Apopka and Lake Woodruff) in north central Florida. These studies obtained alligators of similar size from each lake under the assumption that the animals were of similar age. Lake Apopka is a hypertrophic lake with a 50-year history of contamination from agricultural and municipal operations, whereas Lake Woodruff is a eutrophic lake and part of a National Wildlife Refuge that receives little point source pollution. If growth rates differ among these areas, it could be argued that differences in endocrine parameters reported previously (e.g. steroid or thyroid hormone concentrations) could be the result of differences in the animals' ages. Using growth annuli in cross-sections of femurs, we estimated the ages of juvenile alligators and compared the relationship of estradiol-17beta (E(2)) and testosterone (T) to size and age within each lake and sex. No differences were detected in the relationship between size and age between the two areas indicating similar growth rates between lakes. Plasma E(2) was positively related to size in females from Lake Apopka, and age in Woodruff females. Males from Lake Apopka had elevated plasma E(2) compared with Lake Woodruff males and did not differ from Woodruff females. No significant relationships were detected for T from either lake, and no differences in plasma T were detected among lakes or sexes. Our data indicate that both size and age can have a significant relationship with steroid concentrations. However, the relationship between steroid concentrations and size or age differed between lakes. We suggest both factors should be considered when conducting physiological studies where there is evidence to suggest growth rates may differ among populations.