SEASONAL RELIABILITY OF TESTOSTERONE RADIOIMMUNOASSAY (RIA) FOR PREDICTING SEX RATIOS OF JUVENILE LOGGERHEAD (CARETTA CARETTA) TURTLES JOANNE BRAUN-MCNEILL 1,5 ,SHERYAN P. EPPERLY 2 ,DAVID W. OWENS 3 ,LARISA AVENS 1 , ERIK WILLIAMS 1 , AND CRAIG A. HARMS 4 1 NOAA, National Marine Fisheries Service, 101 Pivers Island Road, Beaufort, North Carolina 28516, USA 2 NOAA, National Marine Fisheries Service, 75 Virginia Beach Drive, Miami, Florida 33149, USA 3 College of Charleston, 205 Fort Johnson, Charleston, South Carolina 29412, USA 4 North Carolina State University, College of Veterinary Medicine, Center for Marine Sciences and Technology, 303 College Circle, Morehead City, NC 28557, USA ABSTRACT: Because sex is determined by incubation temperatures in sea turtles and immature animals are not sexually dimorphic externally, circulating levels of testosterone measured with radioimmunoassay (RIA), in conjunction with laparoscopies, have been used to estimate sex ratios. From September to December 1995 to 1997, and from June to December 1998 to 2002, we sampled blood from 1106 juvenile loggerhead sea turtles (Caretta caretta) incidentally captured in pound nets set in Core and Pamlico Sounds, North Carolina to measure testosterone levels. Laparoscopies of 89 of these turtles revealed a sex ratio of 2.1F:1M, similar to other juvenile loggerhead populations along the southeastern coast of the USA. Laparoscopies demonstrated that testosterone levels correctly identified males during summer months (water temperatures .23 C), but were unreliable during late autumn/winter months (water temperatures #16 C). During the summer months, females (n 5 201) exhibited testosterone concentrations with an upper limit of 239.0 pg/ml, and males (n 5 69) exhibited a lower limit of 372.0 pg/ml, for a sex ratio of 2.9F:1.0M. We recommend that verification of the RIA should be conducted by laparoscoping a subset of turtles sampled in all sex ratio studies. In addition, this verification should be conducted at several different times throughout the year to evaluate any possible seasonal effects on testosterone concentrations. Key words: Caretta caretta; Laparoscopy; Loggerhead sea turtle; Radioimmunoassay; Sex ratio; Testosterone LIKE many reptile species, the temperature at which sea turtle eggs are incubated determines the sex of the hatchlings, a process known as temperature-dependent sex deter- mination (TSD) (reviewed by Janzen and Paukstis, 1991). This lack of heteromorphic sex chromosomes combined with the absence of dimorphic secondary traits in juveniles can make it challenging to determine the sex ratio within a population. To add to the difficulty, apparent sex ratios may vary depending on the life stage being studied (Wibbels et al., 1987b; Wibbels et al., 1991). For example, the sex ratios of hatchling populations can vary from year to year depending upon the geographic location of nests, time of year the eggs are laid, and annual weather patterns (Mrosovsky, 1994; Mrosovsky et al., 1984). Ascertaining adult sex ratios can be problematic because various sex-specific behaviors (i.e., mating, remigration interval) make it difficult to randomly sample the adult population (Wib- bels et al., 1987b; Wibbels et al., 1991). With all of the difficulties inherent in studying hatchlings and adults, it is possible that population sex ratios might be assessed more accurately by evaluating neritic juveniles (Shoop et al., 1998; Wibbels et al., 1987b). Because of the long duration of the neritic life stage, it represents many cohorts and is therefore a condensation of many years of hatchling production, integrating sex ratio variability over time (Wibbels et al., 1987b; Wibbels et al., 1991). In addition, neritic juveniles are less likely to have developed the sex-specific behavioral biases exhibited by adults. As a result, studies of this stage are more likely to reveal the actual secondary sex ratio for a given population, (Shoop et al., 1998; Wibbels et al., 1987b), which, in turn, is a better indicator of the population’s future reproductive potential (Wibbels et al., 2000). Several sex determination techniques for sea turtles are available to researchers (Wib- bels, 1999). Although laparoscopy is an 5 CORRESPONDENCE: e-mail, Joanne.B.McNeill@noaa. gov Herpetologica, 63(3), 2007, 275–284 E 2007 by The Herpetologists’ League, Inc. 275