Mice bred to run three times farther show no fertility or lifespan cost

The researchers believe the absence of energetic trade-offs in the mice may reflect the fact that the mice always had enough food to meet the demands of both activity and reproduction. Rather than forcing the animals to divert resources away from one biological function to support another, the plentiful food supply may have allowed them to eat more while maintaining both athletic performance and reproductive success.>

Apparently they didn't run tests under differing environmental conditions. We don't know if these athletic mice could suffer any disadvantage in the face of a famine or infectious disease.