Reproduction can impose “costs” associated with the burden of carrying developing embryos or eggs. Numerous studies using squamate reptiles have documented a reduction in locomotor performance related to reproduction. Recently several experimental studies have attempted to determine whether the reduction in locomotor performance is physical or physiological. However, no consensus has been reached, and there is evidence that effects are species specific. In addition, no previous studies have documented whether the reduction in performance is consistent from year to year. For this study, we measured the endurance performance on a motorized treadmill of Side-Blotched Lizards (Uta stansburiana) during their natural breeding season over two years. Our goals for this study were to determine (1) to what degree reproductive state reduces endurance, (2) whether females recover endurance capacity quickly (evidence of physical burden only), and (3) whether the change in performance associated with reproduction is consistent from year to year. Results from 812 trials over two years revealed a general decrease in endurance capacity associated with reproductive state such that females with shelled eggs were only able to run 80–85% as far as nongravid females. This decrement in performance is not related to relative clutch mass, suggesting a physiological effect. However, post-reproductive females recovered to nongravid levels of endurance within ∼12 h of oviposition, suggesting the decrement in performance was physical. Results were qualitatively similar in both study years indicating that changes in locomotor performance associated with reproduction may be consistently imposed from year to year.
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