Study: Sleep position determined by body size

Cows always lie in sternal position so as not to interrupt their digestive processes. Rodents also rest while sitting, and giant kangaroos sometimes rest on their backs. A team from UZH has studied the resting positions of mammals. The larger an animal, the less often it lies down – and when it does, it tends to lie on its side. But there are exceptions.

Why do you never see cows lying on their side in a pasture? In ruminants such as cows, sheep, antelopes, deer, or giraffes, the portions of food that need to be chewed again are sorted out in the stomach by the force of gravity. For this to work smoothly at all times, the stomach must maintain the same position relative to gravity whether the animal is standing or lying down. This is why cows always rest in sternal position and almost never lie on their side. It is therefore assumed that animals which digest differently are more able to lie on their side. To examine the relationship between the digestive system and resting position more closely, researchers at the University of Zurich observed 250 mammals in zoos across more than 30,000 resting phases.

Large animals rest standing or on their side

They found that their assumption was not entirely correct: among other factors, body size influences an animal’s resting position more than the type of digestive system. Small animals with short legs spend a lot of time in sternal position – their body shape is ideal for it. One example is the rock hyrax from Africa, which resembles a guinea pig. “The shorter the distance between the animal’s centre of mass and the ground, the more likely it is to lie down», says Prof. Marcus Clauss of the Clinic for Zoo Animals, Exotic Pets and Wildlife at UZH. The larger the animals, the more frequently they lie fully on their side, which is more comfortable for the legs of bulkier animals.

But there are exceptions: large animals also rest while standing. Horses do this far more frequently than their closest relatives, the tapirs or rhinoceroses. To do so, they “lock” one of their hind legs in place using their kneecap, without needing to engage any muscles. Camelids such as llamas or dromedaries regurgitate part of their stomach contents like ruminants; unlike cows, however, they can sometimes lie on their side and briefly interrupt their digestive process.

Elephants lie on their side

Of all herbivores, elephants lie on their side most frequently. However, as they grow older and can no longer get up as easily, they tend to avoid lying down. «That is why it is important to provide elephants in zoos with mounds of sand. When they lie at a slight angle, even older animals can get back up much more easily», explains Christian Schiffmann, the elephant specialist in the research team. If the animals do not have this option, they increasingly lean against walls, posts, or tree trunks.

Hippos, on the other hand, appear to remain mobile well into old age and do lie on their side. Rodents also enjoy taking a rest in a seated position. As the only species studied, red kangaroos sometimes rest on their backs as well.