Thesis

Evolution of temperature-dependent sprint performance in nocturnal and secondarily diurnal geckos

Temperature has a profound influence on most aspects of an ectotherm's life. Most of the world's 1500+ gecko species are nocturnal and therefore presumably active at body temperatures (Tbs) that are suboptimal for performance. However, several clades and numerous gecko species have independently reverted to diurnality and are therefore active at warmer Tbs allowing for comparisons of temperature-dependent sprint performance between nocturnal and secondarily diurnal species. Following the hotter-is-better hypothesis, which states that warm-adapted species will have better performance than cold-adapted species, we hypothesized that diurnal geckos would have faster maximum speeds than nocturnal species at their thermal optimum. Likewise, the optimality hypothesis predicts co-adaptation between environmental temperatures and optimal Tbs for performance. Thus, we hypothesized that diurnal geckos would sprint faster at warmer Tbs and slower at cooler Tbs relative to nocturnal geckos. We tested these hypotheses by comparing the temperature-dependent sprint performance of 27 gecko species over a range of ecologically relevant Tbs to determine how activity time (nocturnal/diurnal) affected sprint performance. Conventional statistics and those that accounted for the genetic distances among the species were used to compare the sprint performance of nocturnal and diurnal geckos. Additionally, two clades with closely related nocturnal and secondarily diurnal species were compared using conventional statistics. Nocturnal geckos had slower sprint speeds than diurnal geckos overall, which is consistent with the hotter-is-better hypothesis. In accord with the optimality hypothesis, diurnal species sprinted relatively faster at higher Tbs and slower at cooler Tbs relative to nocturnal species suggesting that there has been an evolutionary shift in the temperature-dependent sprint performance of diurnal species with the higher activity temperatures associated with diurnality. Phylogenetic history did not influence temperature-dependent sprint performance because diurnality has evolved independently several times among the species included in this study and clades with diurnal species are spread throughout the gekkotan tree. Our study provides insight into the ways in which an evolutionary change in activity time can affect the evolution of temperature-dependent sprint performance in geckos and illustrates the likely constraints on performance for other cold-adapted nocturnal species.

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