Does speed help you survive? A test with Collared Lizards of different ages

1. While it is generally accepted that the evolution of morphological traits is mediated by selection on whole-animal performance, few studies have examined how variation in performance explains variation in fitness. Most studies to date have studied how performance predicts survival, and these typically examine only a single age class. 2. Maximal sprint speed was measured for hatchling and adult Collared Lizards (Crotaphytus collaris), in addition to morphological measurements, in order to determine how well annual survival could be predicted by performance and whether the relationship between performance and survival differed between age classes. 3. Logistic regression revealed that no morphological trait or sprint speed performance predicted survival in adults. However, sprint speed predicted survival in hatchlings. Further, hindlimb length differed between survivors and non-survivors and correlated with sprint speed, suggesting that the evolution of hindlimb length in hatchlings may be driven by selection on sprint speed performance. 4. Selection on sprint speed may be mediated directly by predators where hatchlings cannot attain speeds sufficient to escape predators or indirectly where slower individuals use a greater proportion of their maximal capacity than faster individuals while escaping predators, perhaps causing elevated stress levels or a higher expenditure of energy stores.