Casual movement speed but not maximal locomotor capacity predicts mate searching success.

Maximal locomotor performance is often used as a proxy for fitness. Maximal speed may be important under high-threat conditions, such as during predator escape. However, animals do not always move at a speed that reflects their maximal physiological capacities when undisturbed. The physiological factors that determine the movement speed chosen by animals, such as minimization of energy use, may be independent from maximal performance. As a result, the casual speed at which individuals move when undisturbed in a given context may better represent an individual's motivation to move. The casual speed may therefore be a better predictor of fitness in natural contexts than maximal performance capacity. We tested the hypothesis that casual movement speed rather than maximal speed predicts fitness in the golden orb-web spider, Nephila plumipes. We measured fitness in two separate contexts, mate-searching success and the positional rank near a female. We show that casual but not maximal locomotor speed predicted both aspects of fitness. Casual speed was linearly related to maximal speed, indicating that casual speed is determined by physiological optimization. Size and metabolic scope were not related to either maximal or chosen speeds, indicating that the supply of ATP does not limit locomotor performance in this species. Overall, our results demonstrate that locomotor performance is related to fitness, but suggest that different types of performance and not necessarily maximal physiological capacities are most relevant for particular ecologically relevant tasks.