World Animal Day | The first "person" who "fishes" in animals, seemingly standing, but actually sleeping

Every year on October 4th, World Animal Day is a special day to raise our awareness and respect for the rights of all animals. During this festival, we turn our attention to a special animal - the horse. Horses are friends and partners of humans, and they occupy an important position in our history and culture.

Millions of years ago on the North American continent, a group of creatures called "Dinohippus" were galloping wildly. When they were tired, they stopped to rest. Although they still stood, these "Dinohippus" had entered a "doze" and restored their energy briefly in this way.

From then on, the gears of fate began to turn, and until now, members of the horse family can basically fall asleep while standing.

Image credit: François Marchal

It’s not hard to guess why horses sleep standing up.

As a herbivore, horses are prey to many large predators, especially when they are resting, which is the time when predators hunt. If horses keep lying down to sleep, they will need to take some time to stand up and run when escaping. Sleeping in a standing position allows them to immediately spread their hooves and gallop the moment they wake up, thus quickly escaping from predators.

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Why can horses sleep standing up?

The horse (Equus ferus caballus) belongs to the genus Equus of the family Equidae in the order Perissodactyla. Generally speaking, modern horses can be divided into two types, namely domestic horses and wild horses.

Human domestication of horses began around 6000 BC. Thanks to the horse's powerful running ability, domesticated horses became one of the most important means of transportation in human civilization. It is no exaggeration to say that at least half of the places visited by ancient people were conquered by horse hooves.

Genghis Khan statue image source Weibo

Wild horses are literally horses that live in the wild, including those that were originally domesticated horses that gradually moved away from humans due to escaping, getting lost, or being allowed to live and reproduce in the wild. Over time, the behavior patterns of these domesticated horses and their offspring will return to a state similar to that of "wild horses."

But academically, the only wild horse that exists today is the Przewalski's horse (Equus ferus przewalskii), also known as the Mongolian wild horse, which is native to the Central Asian steppes. Gene sequencing has revealed that the number of chromosomes in Przewalski's horses is different from that in domestic horses, with Przewalski's horses having 33 pairs of chromosomes and domestic horses having only 32 pairs. This difference stems from an evolutionary divergence 45,000 years ago, when the two populations of wild horses took different development paths.

Released Przewalski's horse. Photo credit: Claudia Feh

Of course, in terms of "bloodline", the two are still close relatives, and both have retained the traditional art of sleeping standing up, for which equines have also evolved special physiological structures.

In animals that stand on all fours, there are two forces acting on the limbs: gravity due to the weight of the body and the reaction of the ground on the limbs. To prevent the limbs from bending and deforming under the pressure of these forces, the extensor muscles of the animal's joints need to remain constantly elastic.

Image source: animac

For large animals like horses, the muscle strength required to maintain a standing posture is very great. To compensate for this shortcoming, compared to small animals like cats and dogs with bent limbs, horses have straighter limbs and larger muscle proportions, which can ensure that they can stand for a long time.

In addition, sleeping while standing requires overcoming the imbalance of the body, otherwise it will occur after closing your eyes for a while. Although four-legged horses already have a good sense of balance, in order to ensure that they do not fall while sleeping, they have built a support structure in their bodies, which is called the "equine Stay apparatus" .

Image of the horned horse in the Florence Museum via Wikimedia Commons

This device is a special system of muscles, tendons and ligaments located in the horse's chest and pelvis. When the horse is at rest and its muscles are relaxed, the device will take effect. The tendons and ligaments act like a tension band, "locking" and fixing the main joints of the horse's limbs, allowing them to rest while standing without falling. If this ability appears in humans, then in the future, we will be able to touch fish while standing.

This delusion aside, the horse's hind legs are indeed "slacking off". When the device is "locked", most of the horse's weight is on one hip, while the other hip does not bear much weight, and the corresponding hind leg is in a bent state, that is, resting. After a while, the knee joint of the weight-bearing leg relaxes and the other leg can be switched. This alternating resting allows the corresponding hind leg to rest, thereby reducing overall wear and tear on the joint.

A horse sleeping upright, with one hind leg bent. Image source: insider Horse

Horses also dream when they fall asleep, so do they sleep standing up all their lives?

In fact, this is not entirely true. Although horses can sleep while standing, they also need to lie down to sleep in order to better restore their energy. An adult horse needs to spend at least twenty minutes to two hours a day lying down to sleep. A horse that falls asleep in this state can enter the rapid eye movement sleep period and even have a beautiful dream.

Image source: animac

This is necessary for horses because lack of REM sleep may cause them to suffer from sleep deprivation. A horse in this state is likely to fall down while riding because it is sleepy...

Therefore, horses must take time every day to lie down and sleep, but lying down to sleep in the wild is extremely risky. Fortunately, horses are social animals, and when a horse or a group of horses want to lie down to sleep, at least one horse will be left behind to act as a lookout to deal with any threats that may arise at any time.

Image source: mokude

This phenomenon is also very common among domestic horses. Even if they are surrounded by fences, when many horses are sleeping, there will still be one horse standing and keeping alert.

But it should be noted that the guy standing on the lookout should not fall asleep while standing...

References:

1.The equine hind limb is actively stabilized during standing

2.https://onlinelibrary.wiley.com/doi/full/10.1046/j.1469-7580.2003.00166.x

3.Equine Stay Apparatus: Active or Passive, and Why it Matters

4. https://onlinepethealth.com/equine-stay-apparatus-active-or-passive-and-why-it-matters/

5.Horses Stand Up to Sleep but Lie Down Perchance to Dream

6.https://www.scientificamerican.com/article/horses-stand-up-to-sleep-but-lie-down-perchance-to-dream/

Stay_apparatus

7.https://en.wikipedia.org/wiki/Stay_apparatus

Author: Fish

Winner of the Silver Award for Outstanding Science Popularization Works of China Science Writers Association

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Editor: Guru