Performance trade-offs in wild mice.

Various aspects of performance (e.g., speed, strength, endurance) are thought to be important determinants of the success of animals in natural activities such as foraging, mating, and escaping from predators. However, it is generally known that morphological properties enhancing one type of performance (e.g., strength) can lead to a reduction in another (e.g., speed). Such performance trade-offs have been quantified at the inter-specific level, but evidence at the individual level remains equivocal. To test for the presence of a performance trade-off, we repeatedly captured a total of 189 wild white-footed mice (Peromyscus leucopus) and measured their grip strength and sprint speed. Using the maximum performance score obtained for each individual across all their repeated tests, we obtained a counter-intuitive (and biased) positive and highly significant phenotypic correlation. Using a bivariate mixed model, we detected a significant negative among-individual correlation between grip strength and sprint speed. By contrast, the within-individual correlation was positive but non-significant, thus illustrating the importance of properly partitioning the correlations at the among- and within-individual levels when testing for the presence of a performance trade-off. This study is one of the first to detect a performance trade-off at the among-individual level in a wild animal population. Such a trade-off may be caused by individual differences in muscle physiology and scapular morphology resulting from genetic differences and/or plastic responses to differential use of the arboreal vs. terrestrial parts of the environment.