Steroid Hormone Profiles and Demographic Data for Green Turtles (Chelonia mydas) Inhabiting the Mexican Caribbean Coast

Abstract

Abstract. – This study provides, for the first time, reference intervals of sex steroids and thyroxine concentrations from free-living green sea turtles (Chelonia mydas) inhabiting the coast of Quintana Roo, Mexico, determined in serum samples (n = 150) collected during 2013–2019 and characterized by endogenous (sex, size, and reproductive stages) and exogenous (foraging site) factors. A new demographic classification is proposed based on sea turtle size and habit preferences. Enzyme-linked immunosorbent assays were used to quantify serum estradiol, testosterone, and free thyroxine concentrations of green sea turtles captured in 7 foraging localities along the coast. The mean curved carapace length (CCL) of individuals (n = 150) was 66.60 ± 21.26 cm (range: 26.2–116.4 cm); immature turtles of undetermined sex were the most frequent age class (79.3%). The mean steroid values were 0.33 ± 0.416 ng ml–1 (0.001–2.85 ng ml–1) for estradiol, 1.41 ± 1.83 ng ml–1 (0.015–9.21 ng ml–1) for testosterone, and 2.62 ± 1.89 µg 100 ml–1 (0.001–9.22 µg 100 ml–1) for thyroxine. The sex ratio estimated for adults (n = 31) was 1:3 (M:F) in deep near-shore habitats in the northern region. A bimodal distribution in the frequency of sea turtles grouped by size classes (CCL) was found, based on which a new theoretical size class classification is proposed for the Mexican Caribbean population. The 2 size classes were resident green turtles (26.2–80 cm CCL) and transient individuals (66.5–116.4 cm CCL), which also differed significantly in testosterone concentration, residents 1.6 ± 1.9 ng/ml (0.01–9.2 ng/ml) and transients 1.0 ± 1.3 ng/ml (0.08–5.1 ng/ml) (Mann-Whitney U-test = 1970, p = 0.04). Resident green turtles found along the central region of the Quintana Roo coast presented higher concentrations of testosterone than transients from the northern region (Kruskal–Wallis H-test = 6.8, p = 0.03). With the exception of testosterone, differences in steroid hormone levels between life-history stages, reproductive state, or sex were not found. Sex steroid concentrations did not differ significantly from reports worldwide, and thyroxine levels suggest an optimal nutritional condition of the individuals. Data presented in this work could be used as the basis for future health monitoring programs and demographic studies required to conserve this sea turtle stock.