Running around on fairly stiff, upright legs like those of a horse is economical: little energy is wasted slowing down and speeding up within each step. So why are small animals crouched? Why do mice scurry around on very bent legs? This may be down to simple mechanics. Small animals have short legs, so do not have long to push against the ground before they take off. This is why a leaping grasshopper, even with its disproportionately long legs, cannot jump as high as a human can. So, the mouse’s crouched posture is a simple trick to give it longer to push against the ground each step. The down-side? Running with crouched legs requires more effort – try for yourself!