Asked by: Anonymous
To pull off this trick, you need legs that can be aligned vertically, so you don’t need to use muscular effort to keep them in place. You also need knees that ‘lock’ in place. Sleeping upright is advantageous for large animals because they would be slow to lumber to their feet if attacked. For smaller animals the reduction in leg springiness outweighs this benefit.
Horses, zebras and elephants sleep standing up. Cows can too, but mostly choose to lie down. Some birds also sleep standing up. Flamingos live on caustic salt flats, where there’s nowhere they can sit down.
Many birds roost in trees at night using an arrangement of their leg tendons that causes their body weight to pull the claw shut around the branch. Whether this counts as standing up is a matter of semantics.