Locomotion is an integrative trait, critical for aspects of Darwinian fitness in many organisms. Selection resulting from locomotor demands can greatly influence morphology, physiology, and behavior. Locomotion can be energetically costly, and locomotor performance involves the function of many underlying physiological systems. When organisms are faced with multiple energetic demands, it is reasonable to expect trade-offs related to energy allocation.
My dissertation research investigated the effects of environmental stressors on exercise. Using four unique High Runner (HR) lines of mice (Mus domesticus) bred for voluntary wheel running, my first chapter investigated a hindlimb `mini-muscle' phenotype that increased in frequency in response to selection in two of four selected lines. As compared with normal HR mice, mini-muscle individuals ran as much on wheels, but had reduced sprint speed, suggesting a trade-off with stamina, and, unexpectedly, a higher cost of transport.
My second chapter investigated the response to a mammalian endoparasite (Trichinella spiralis) in HR and non-selected control mice, particularly possible trade-offs between wheel running and immune function. Contrary to our expectations, HR mice apparently did not have an important reduction in immune function. Moreover, our results suggest a similar response to infection in both HR and control mice.
My third dissertation chapter examined the physiological, morphological and behavioral correlates of circulating corticosterone (a steroid "stress" hormone) levels in captive-bred California mice (Peromyscus californicus), a species that has unusually high corticosterone levels. Contrary to expectations based on the role of corticosteroids in energy metabolism, we found surprisingly few statistically significant relationships at the level of individual variation, suggesting that corticosterone is not always a critical contributor to voluntary exercise behavior or exercise performance. However, we did find differences in correlations between the sexes.
My final chapter analyzed the upper limit to aerobic metabolism (V. O2max) during maximal locomotor exercise for 77 species of mammals. V. O2max is a physiologically and ecologically relevant trait for many animals, and it is generally considered to be the single best indicator of cardiopulmonary function and capacity for sustained intense exercise. Using conventional and phylogenetic statistics, we estimated allometric scaling exponents and demonstrated phylogenetic signal in mass-adjusted V. O2max.
Together, these results point to the importance of understanding the complex, integrative physiological mechanisms contributing to exercise physiology and the limits to performance. Moreover, they drive home the importance of both evolutionary history and adaptation in response to natural selection as contributors to behavior and whole-organism performance.