Comparative Biochemistry and Physiology Part A 131 (2002) 923–930 1095-6433/02/$ - see front matter  2002 Elsevier Science Inc. All rights reserved. PII:S1095-6433 Ž 02 . 00025-9 Plasma steroid concentrations in relation to size and age in juvenile alligators from two Florida lakes Matthew R. Milnes *, Allan R. Woodward , Andrew A. Rooney , Louis J. Guillette a, b c,d a Department of Zoology, 223 Bartram Hall, University of Florida, Gainesville, FL 32611-8525, USA a Florida Fish and Wildlife Conservation Commission, 4005 South Main Street, Gainesville, FL 32601, USA b US Environmental Protection Agency, NHEERL, Research Triangle Park, NC 27711, USA c Department of Anatomy, North Carolina State University CVM, Physiological Sciences and Radiology, Raleigh, NC 27711, USA d Received 27 September 2001; received in revised form 27 December 2001; accepted 3 January 2002 Abstract 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-17b (E ) and testosterone (T) to 2 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 was positively related to size in females from 2 Lake Apopka, and age in Woodruff females. Males from Lake Apopka had elevated plasma E compared with Lake 2 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.  2002 Elsevier Science Inc. All rights reserved. Keywords: Alligators; Reptiles; Aging; Eutrophication; Growth; Steroids; Contaminants; Hormones 1. Introduction Nutrient availability is one of numerous factors that affect growth rates of crocodilians. Other important factors include age, size, sex, habitat, and geographic area. Among populations that have been studied, differences in crocodilian growth *Corresponding author. Tel.: q1-352-392-1098; fax: q 10352-392-3704. E-mail address: mrmilnes@zoo.ufl.edu (M.R. Milnes). rates have been attributed largely to climatic dif- ferences, diet, and prey availability. For instance in South Carolina where non-growing periods are more extended, age and size at sexual maturity is estimated to be greater than alligators (Alligator mississippiensis) found in Florida and Louisiana (Wilkinson and Rhodes, 1997). Populations of alligators in Florida vary in growth rates, with animals from the Everglades showing slower rates due to apparent increases in metabolic costs asso-