Ever wondered why horses sleep standing up?

Baby Przewalski's horses try out their shaky legs at the National Zoo's Conservation and Research Center in Virginia.
Baby Przewalski's horses try out their shaky legs at the National Zoo's Conservation and Research Center in Virginia. (By Mehgan Murphy -- National Zoo)
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Monday, July 14, 2008

· It wouldn't be terribly comfortable for you to snooze standing up, but the opposite is true for horses. That's because of the way their bodies are built.

Horses have bones and ligaments (the elastic bands that connect bones at the joints) in their legs that can lock together in a special way. That allows the animals to be completely relaxed while standing.

Because they don't have to exert any energy, sleeping while upright poses no problem for a horse. In fact, it is preferable to lying down. Horses are heavy animals with big muscles, but their bones are surprisingly delicate. Lying in one position for too long could injure a horse.

Horses do, however, sometimes take short naps while lying on the ground in an area where they feel safe.

How did this happen? Scientists say it evolved over time. Horses probably developed their habit of sleeping upright as a defense against predators.

Their speed is their main defense in the wild, and it takes horses longer than most animals to pull themselves upright. A standing position keeps a horse constantly ready to race away if danger approaches.

Horses' ability to stand in that special way is called the "stay apparatus." They use it even while they are awake to avoid getting tired. Horses can stand for a very long time -- a few years -- without getting tired.

So how can you tell whether a horse is awake or asleep? It's probably napping when its head and bottom lip are drooping.

-- Valerie Strauss

SOURCE: "The Handy Answer Book for Kids (and Parents)," by Judy Galens and Nancy Pear.


© 2008 The Washington Post Company

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