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 = 201) exhibited testosterone concentrations with an upper limit of 239.0 pg/ml, and males (n = 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.
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Vol. 63 • No. 3