Experimental evolution of exercise physiology in house mice selectively bred for high locomotor activity

A major goal of evolutionary physiology is to understand how evolution in response to a particular selective factor causes changes at multiple levels of biological organization. More specifically, when selection acts at the level of behavioral performance, which of the many lower-level traits that may potentially determine performance abilities actually change in concert? Do all potentially relevant subordinate traits change, or only a few ones? I addressed these questions by studying aerobic performance in replicate lines of Mus domesticus which have been selectively bred for high voluntary wheel running for over 30 generations. These mice run about 170% more on wheels as compared with the four randomly bred control lines, but it remains unclear whether aerobic capacity has evolved in these lines as a correlated response. This dissertation addresses the following questions: (1) Is maximal aerobic capacity measured under different conditions higher in the selected lines? (2) How do maximal voluntary wheel speeds compare with the maximum aerobically sustainable speeds as determined on a motorized treadmill? (3) Are selected mice more efficient (i.e., 'economic') runners than control? (4) Have key subordinate traits (aspects of morphology, physiology, and biochemistry associated with maximum aerobic capacity) evolved in selected lines? (5) How does the increase in running distances in selected lines affect daily energy expenditure?; Results from two separate experiments monitoring O2 consumption during voluntary exercise over complete daily activity cycles and during forced exercise on the treadmill suggest that selected lines have evolved higher aerobic capacity than control. Biochemical measurements support that some subordinate traits associated with aerobic performance have coevolved. Selected mice do not run on wheels at their maximum aerobic levels, suggesting that additional factors are involved in the selection limit observed by generation 16. Furthermore, locomotion costs tend to be lower in selected mice, and the almost two-fold increase in running distances due to selection seemed to have increased only modestly overall daily energy expenditure of selected mice. Males and females behave quite differently on the wheels, and males may not run faster on the wheels, which may be associated with the selection limit mentioned above.