UC Riverside

Experimental evolutionary studies are excellent approaches to study the coadaptation of the musculoskeletal with locomotor behavior in real time. My dissertation examines mice from 4 replicate High Runner (HR) lines bred for voluntary wheel running as young adults and their 4 non-selected Control (C) lines. Chapter 1 involved the analysis of skeletal data from generation 11 and found that HR mice have evolved larger hip and knee surface areas, which would lower stress (force per unit area), acting on the hindlimb during running. This chapter demonstrates that skeletal dimensions and muscle masses can evolve rapidly in response to directional selection on locomotor behavior. Chapter 2 examined the rapid and longer-term effects of selective breeding on the skeleton. HR mice reached an apparent selection limit between generations 16-28, running ~3-fold more than C mice. Analysis of bone data from generations 11, 16, 21, 37, 57, and 68 revealed unique results. I found few differences between HR and C mice for these later generations, and some of the differences in bone dimensions identified in earlier generations were no longer statistically significant. Chapter 3 highlighted aspects of phenotypic plasticity achieved through exercise. Muscle attachment site morphology reflects muscle mass and function and therefore, paleontologists have routinely used characteristics of muscle entheses to infer the past loading history of individual specimens. I used mice from generation 57 that were housed with or without wheels for throughout ontogeny to quantify the genetic differences in muscle attachment site morphology between HR and C mice, as well as plastic changes resulting from chronic exposure to exercise. My results demonstrate that muscle attachment site morphology can be (but is not always) affected by chronic exercise. Chapter 4 investigated a negative correlation between average running speed and time spent running on wheels that exists among the HR lines. I hypothesized that this trade-off may be related to evolved changes in muscle physiology and used in-situ preparations to quantify muscle contractile properties, including speed and endurance. I found that muscle-level speed and endurance do trade-off in these mice, but not in a way that maps to the observed organismal-level speed-endurance trade-off.