Summary: movement rate increases.

Observation: "You'd think this would be scarier on the small critters – and don't get me wrong, a Lerk with celerity is like trying to whack a hummingbird with a baseball bat. But it's a little scarier when one of those three-ton freaks is pounding along behind you, and it's comin' on like a freight train. I've never been so happy to see a ventilation shaft in my whole life."

Thesis: the tensile strength of all muscles and tendons in the "running" group is increased, cores of flexible and strong material form in the bones. This is probably a relatively simple change, but the results are dramatic. At their peak, our simulations cannot even speculate how they can maintain such speeds – the stress and impact alone should shatter their bones and snap their muscles, no matter how strong.