SPANOVICH, S; NIEWIAROWSKI, PH*; LONDRAVILLE, RL; University of Akron: Seasonal Variation in Circulating Leptin in Fence Lizards, Sceloporus undulatus, From Two Populations

We showed in a previous study that the hormone leptin, when injected into fence lizards, evoked the same short-term phenotypic responses as observed in laboratory mammals (changes in energy metabolism, thermoregulatory behavior, and activity). We have also demonstrated that lizards have a native �leptin like� protein. As part of our program to develop a method of experimentally manipulating age at maturity, we report here on seasonal variation in circulating levels of native leptin in lizards from two populations, one where individuals mature early (<1year) and one where individuals mature late (2 years). Consistent with our understanding of leptin function in the reproductive access of mammals, we found that lizards from both populations had the highest concentrations of circulating leptin during the reproductive season (spring: 2.6232 +/- 0.3051; summer: 2.6223 +/- 0.3063; fall: 1.8018 +/- 0.2537; all micrograms/ml blood; mean +/- 2SE). In contrast to our expectations however, lizards from both populations had the lowest circulating levels of leptin just prior to the onset of winter hibernation, when fat stores are at their highest levels. Although unexpected, such a result is consistent with several studies showing seasonal mammals can exhibit leptin dissociation during periods of fattening prior to hibernation. Finally, lizards from the late maturing population had lower overall circulating leptin concentrations throughout the year (early maturity, spring: 2.8267 +/- 0.308; early maturity, summer: 2.8906 +/- 0.3723; early maturity, fall: 1.874 +/- 0.4357; late maturity, spring: 2.4198 + 0.5223; late maturity, summer: 2.354 +/- 0.4645; late maturity, fall: 1.7295 +/- 0.4583, suggesting that administration of exogenous leptin might reduce age at maturity in fence lizards. If so, fitness consequences of variation in age at maturity could be directly tested, addressing an important theoretical and empirical problem in the evolution of life history traits.