Flamingos can stand on one leg easier than two. They use almost no muscular effort for a one-legged stance, because the ‘passive posture’ has a built-in staying mechanism that keeps the leg in place. It works so well, even dead flamingos can be stood upright- but not with two legs, because an ‘active posture disengages the mechanism. It’s unclear why they do this, but studies suggest it helps them stay warm by conserving energy.