UC Riverside

I studied correlated responses to artificial selection for high voluntary wheel-running behavior. Chapter 1 used meta-analysis to summarize comparisons of four replicate High Runner (HR) lines with four non-selected Control (C) lines. Effect sizes from 34 studies demonstrate that HR mice have evolved larger hearts and smaller bodies, and that plateaus in effect sizes for both traits coincide with selection limits for wheel running. Chapter 2 tested whether HR mice would have a higher maximal rate of oxygen consumption during forced exercise (VO2max), a higher basal metabolic rate (BMR), and increases in the size of relevant organs. Although HR mice ran ~3-fold more revolutions/day and had higher VO2max, they did not have a statistically higher BMR nor differ in relative organ masses (heart, lung, liver, kidney, spleen, brain, calf muscle) or hematocrit. Thus, a large evolutionary increase in activity level has not required proportionally large changes in underlying morphological or physiological traits. Chapter 3 examined effects of maternal exercise on maternal care and offspring body composition and activity behavior. HR females ran more than C prior to breeding and before parturition, but not after. When housed with wheels, HR females had reduced maternal care compared to C during days 1-10 after parturition. HR females with wheels had fewer litters survive to weaning than those without wheels, but the opposite occurred for C females. For both HR and C dams with wheels, surviving offspring had delayed development as indicated by eye opening and lean mass at weaning. However, maternal wheel access did not alter offspring adult body mass, activity behavior or reproductive success. Overall, the evolution of high voluntary exercise did not require large changes in underlying physiology (e.g., VO2max, BMR), but morphological traits (e.g., body mass, relative heart mass) may have imposed constraints on the evolution of activity levels in these mice. Moreover, HR mothers differ in behavior and/or resource allocation resources when given the opportunity to exercise, which results in costs to offspring survival. Further research is needed to determine the underlying mechanisms of this apparent trade-off between two important biological drives.