Herbivorous turtle, Chelonia mydas, inhabiting the south China Sea and breeding in Peninsular Malaysia, and Natator depressus, a carnivorous turtle inhabiting the Great Barrier Reef and breeding at Curtis Island in Queensland, Australia, differ both in diet and life history. Analysis of plasma metabolites levels and six sex steroid hormones during the peak of their nesting season in both species showed hormonal and metabolite variations. When compared with results from other studies progesterone levels were the highest whereas dihydrotestosterone was the plasma steroid hormone present at the lowest concentration in both C. mydas and N. depressus plasma. Interestingly, oestrone was observed at relatively high concentrations in comparison to oestradiol levels recorded in previous studies suggesting that it plays a significant role in nesting turtles. Also, hormonal correlations between the studied species indicate unique physiological interactions during nesting. Pearson correlation analysis showed that in N. depressus the time of oviposition was associated with elevations in both plasma corticosterone and oestrone levels. Therefore, we conclude that corticosterone and oestrone may influence nesting behaviour and physiology in N. depressus. To summarise, these two nesting turtle species can be distinguished based on the hormonal profile of oestrone, progesterone, and testosterone using discriminant analysis. 1. Introduction Hormones have various functions across animal taxa. In birds, for example, testosterone is linked to mating success, while corticosterone has been shown to support energetically demanding processes . In vertebrates glucocorticoid hormones can assist in the modulation of life-history events, survival probabilities, and fecundity . Similarly steroid hormones play an essential role in the reproductive behaviour and sexual development in marine turtles . Sex determination in sea turtles is regulated by incubation temperatures and by exogenous steroid hormones [4, 5]. In addition to determining sex, steroids have also been causally related to various physiological and behavioural actions in sea turtles. For example, corticosterone and testosterone have been associated with hepatic regulation and ovarian function during nesting activities in Caretta caretta [6–8]. Corticosterone concentration decreases when a turtle is subjected to stress or drops to low levels when a female turtle is breeding [9–11]. Nesting Lepidochelys olivacea during arribadas (mass nesting behaviour) maintain low corticosterone level as a mechanism of
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