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Leptin is a hormone that regulates energy expenditure and body mass in mammals, and it has attracted considerable attention because of its potential in treating human obesity. Comprehensive data from both pathological and non-pathological systems strongly support a role for leptin in regulating energy metabolism, in thermoregulation and in regulating the onset of puberty. We report here that daily injections of recombinant murine leptin in fence lizards (Sceloporus undulatus) produce phenotypic effects similar to those observed when leptin injections are given to mice. Lizards injected with leptin had body temperatures 0.6 degrees C higher, ate 30 % less food and showed a 14 % reduction in activity rates, and females showed a 2. 5-fold increase in resting metabolic rates, compared with lizards injected with vehicle only (phosphate-buffered saline). We also detected native lizard leptin using an immunoassay. Our results indicate that leptin is expressed in ectotherms and may be conserved both functionally and structurally. In the wake of unprecedented research activity on the role of leptin as a cause of, and potential treatment for, human obesity, we believe that other applications of leptin research have been ignored. For example, the response of lizards to leptin injection in our study has important implications for two broad areas of research in evolutionary biology: the evolution of age at first reproduction and of endothermy. We argue that research in these areas, previously limited to comparative approaches, may now benefit from experimental manipulations using leptin.

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