The South American tegu lizard, Salvator merianae, has annual cycles of high activity during the spring and summer and hibernation during winter, a seasonal rhythm that is independent of ambient temperature (Ta). Furthermore, during the breeding season in spring, animals are able to maintain core body temperature (Tb) above Ta using increased endogenous heat production, making S. merianae the only known facultative endothermic lizard. S. merianae is, therefore, a unique platform to investigate mechanisms of seasonal variation in behavior and physiology and to generate insights for the evolution of endothermy. In this context, the mechanisms underlying endogenous seasonal changes are not yet known. The first candidates we propose to investigate are the hormones known to be involved in energy metabolism (thyroid hormones) and reproduction (gonadal steroids). Thus, the aim of this project is to evaluate the interrelationships of seasonal changes in T3, T4, testosterone, estradiol, progesterone and corticosterone with seasonal changes in activity levels and body temperature of S. merianae in the same individual over a year. We hypothesize that endocrine changes will precede the observed behavioral and metabolic changes and predict that reduced plasma thyroid hormone concentrations will be associated with decreased motor activity during hibernation, independent of Tb, and increased concentrations of thyroid and sex hormones will precede the post-hibernation increases in motor activity, thermogenesis and reproduction. To test this hypothesis, each adult animal (males and females) will be marked with a subcutaneous microchip, implanted with a temperature logger into the coelomic cavity for semi-continuous measurements of core Tb (each 10 mins) and will be affixed with an accelerometer for three-dimensional measurements of movement (sampled at 1 Hz). Devices will be deployed for an entire year; blood samples will be collected monthly and shed skin opportunistically collected from individuals when available for later analysis of hormone concentrations. The animals will be kept in a semi-natural habitat under the influence of natural changes in temperature and photoperiod. The collection of data from the animals will be performed at UNESP, Jaboticabal, Brazil, while the hormone analysis and accelerometer data analysis and interpretation will be performed at NAU, Flagstaff, USA (under C. L. Buck supervision). After the training and analysis of data at NAU, the candidate returns to Brazil to finish all the analysis and write the manuscripts for submission for publication.
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