Why do horses doze off while on their feet?

Anybody who knows anything about horses is aware of the fact that they can fall asleep in standing posture. However, the assumption that they sleep only when standing, is false. When horses need to catch up on REM sleep, they lie down in a series of short intervals that total up to two or three hours a day. When horses need short naps, they doze off while on their feet. 

Why do horses do this? 
Once they have laid down for rest, horses take quite some time to get up. This gives ample opportunity for predators to attack them. So, horses evolved to be able to doze off while standing up so that they can make a quick escape if they sense a nearby threat. Even when they lie down for deeper sleep, there is always at least one other horse standing by on the lookout for danger. Horses developed this ability for natural survival in the wild as they first evolved in open plains. 

How do they do this? 
Horses have a feature called the stay apparatus. It is a special arrangement of muscles, tendons and ligaments (tissues that connect the muscles and bones) that allows them to lock their legs in place and doze off without the risk of falling. It helps them balance their weight on three legs while the remaining one can rest. Horses can alternatively switch legs while resting so that all of them can regain energy. 

Other interesting animal facts! 

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• A rhinoceros horn is made of compacted hair. 
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