Summary
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.
Methodology
The study site was located on Cow Island at the Queen’s University Biological Station (44°34′08″N; 76°19′08″W), a 7 ha island mostly covered with deciduous forest dominated by sugar maple (Acer saccharum) and oaks (Quercus rubra and Q. alba), which has been used before as a study site for this species (Bendell 1959). We installed 214 Longworth traps and 200 nest boxes, overlaid onto the island in a 15 × 15 m sampling grid alternating between 2 Longworth traps or 2 nest boxes per sampling location. Trapping occurred daily from 23 Apr 2016 to 18 Aug 2016. Traps were set at sunset, insulated with polyester batting and baited with sunflower seeds, oats, and a piece of apple. Traps were checked at sunrise the following day. Newly captured individuals were assigned a unique identification code and permanently marked with two uniquely numbered ear tags. For each capture, sex, age, and reproductive status were recorded. Age was categorized as either adult or juvenile based on pelage. Adult mice had complete brown pelage above the middorsal molt line, while juvenile mice had brown and grey pelage above the molt line (Collins 1923). Reproductive status was recorded as a binary variable of reproductively active or not based on external sexual characteristics.
After standard manipulations were completed at the capture site, up to 16 individuals per day were placed back into their trap and transported to a laboratory located 600 m away from the study site, where performance was measured (see below). Additional mice were released immediately, as well as mice that had been to the laboratory less than 7 days prior. Body mass was measured in the laboratory using a precision scale (Mettler Toledo, Model ML1602T/00). All mice were released at the end of the day directly at their capture location. A total of 189 individuals (93 males, 93 females, and 3 individuals of unknown sex) were captured and brought into the laboratory at least once for performance measurements. On average, individuals experienced two tests in the laboratory, ranging from one to eight tests (Fig. 1). Due to natural mortality and migration, some individuals were only tested within a restricted time window, while some individuals were repeatedly tested throughout the season (Fig. 1).