beefy muscles nearthe joints

Okay, David thought, maybe the Slinkies have something to do with speed. So what makes youfast? David began ticking off components. Let’s see. You need an aerodynamic body. Awesomereflexes. Power-loaded haunches. High-volume capillaries. Fast-twitch muscle fiber. Small, nimblefeet. Rubbery tendons that return elastic energy. Skinny muscles near the paws,  …Damn. It didn’t take David long to figure out he was heading toward a dead end. A lot of factorscontribute to speed, and jackrabbits share most of them with their hunters. Instead of finding outhow they were different, he was finding out how they were alike. So he tried a trick Dr. Bramblehad taught him: when you can’t answer the question, flip it over. Forget what makes something gofast— what makes it slow down? After all, it didn’t just matter how fast a rabbit could go, but howfast it could keep going until it found a hole to dive down.

Now that one was easy: other than a lasso around the leg, the quickest way to bring a fast-movingmammal to a halt is by cutting off its wind. No more air equals no more speed; try sprinting whileholding your breath sometime and see how far you get. Your muscles needs oxygen to burncalories and convert them into energy, so the better you are at exchanging gases—sucking inoxygen, blowing out carbon dioxide—the longer you can sustain your top speed. That’s why Tourde France cyclists keep getting caught with other people’s blood in their veins; those illicittransfusions pack in extra red-blood cells, which carry lots of extra oxygen to their muscles.

Wait a second … that meant that for a jackrabbit to stay one hop ahead of those snapping jaws, itwould need a little more air than the big mammal on its tail. David had a vision of a Victorianflying machine, one of those wacky but plausible contraptions rigged with pistons and steamvalves and endless mazes of wheezing levers. Levers! Those Slinkies were beginning to makesense. They had to be levers that turbocharged the rabbit’s lungs, pumping them in and out like afireplace bellows.

David ran the numbers to see if his theory held up and … bingo! There it was, as elegant andniftily balanced as an Aesop’s fable: Jackrabbits can hit forty-five miles per hour, but due to theextra energy needed to operate the levers (among other things), they can only sustain it for a halfmile. Cougars, coyotes, and foxes, on the other hand, can go a lot farther but top out at forty milesper. The Slinkies balance the game, giving the otherwise defenseless jackrabbits exactly forty-fiveseconds to either live or die. Seek shelter quickly and live long, young Thumper; or get cockyabout your speed and be dead in less than a minute.