Which animal can sleep standing up by locking its leg joints?

A horse can doze standing because of an arrangement of tendons and ligaments called the stay apparatus. By shifting the kneecap over a ridge on the thigh bone, the animal locks the joint in place, and a system of tendons down the leg then holds the whole limb rigid with almost no muscular effort. Horses commonly rest one hind leg while standing on the other three and swap over, which is why a resting horse often looks lopsided. The reason is that a horse is a prey animal of open grassland with no burrow or den to hide in, and getting a heavy body up from lying down takes precious seconds. Standing sleep, however, only allows light dozing. Deep REM sleep requires full muscle relaxation, so horses must lie flat for short periods to get it, and they usually do this in short spells while others in the group stay alert.

Cattle and some birds have their own versions of standing or perching locks, though the anatomy differs.