Why do birds sleep standing up?

Humans on one hand and birds on the other hand are the only ones who share bipedalism, but that of birds is based on a very specific mechanism, tensegrity, which could find application in robotics, according to research published on Wednesday.

The fundamental difference between humans and birds is that bipedalism in the former involves upright standing, while in the latter it visibly relies on flexion of the lower limbs.

This flexion, which humans can maintain only for a short time and at the cost of a certain effort, does not prevent ten thousand species of birds recorded in the world from sleeping upright, recalls a study published in the Interface journal of the British Royal Society.

And if we have never actually asked ourselves why, it may be because the bird is “an animal very close to us and very far away, whose flight and behavior we are particularly interested in”, according to the first author of the study book, Anick Abourachid, from the Mecadev laboratory (Adaptive Mechanisms and evolution) of the National Museum of Natural History in Paris.

In humans, balance depends on the skeleton working in compression. Forces spread vertically, by gravity, from head to toe.

The bird has a different structure, with a more horizontal body, which extends from the short bony tail, over the almost rigid spine, to the long neck and then the head.

This is the trunk as if balanced on the legs, composed of three rather long bones, which form a kind of Z before reaching the legs. A structure inherited from their dinosaur ancestors.

The Mecadev team proposes that this system be based on tensegrity. It allows the animal to remain “stable with minimal energy expenditure, i.e. with almost no muscular effort due to passive tension,” according to the study.

Tensegrity, a word derived from the English language that combines the concepts of tension and wholeness, denotes the ability of a structure to maintain its balance through the play of tension and compression.

As for a suspension bridge whose slab is held together by a balance between cables and columns, unlike a classic bridge that relies solely on the compression of the slab and its columns.

“feather layer”

In birds, “when the structure is put under tension, there is no need for energy to straighten it,” says Professor Abourachid.

Birds thus maintain their balance with minimal effort even on an electric cable or a branch shaken by the wind. A feat reserved for human “slackline” practitioners, similar to tightrope walking, but preferably without the wind.

To test their hypothesis, Mecadev researchers designed, with the help of those from the Laboratory of Digital Sciences of the University of Nantes (LS2N), a mathematical model that combines biology and robotics.

They used studies on one of the rare birds, the zebra finch, whose posture was studied with X-rays.” This is the only way to understand skeletal posture, because all “We see in a bird is a layer of feathers with a beak on one side and with one foot on the other,” says the researcher.

The model works with four cables that replace the tendons and muscles of the bird’s leg, which run from the sacrum to the leg through each joint.

The correct tension in the cables allows the modeled animal to find balance with bent legs. In reality, the bird has about forty muscles that enable it not only to stand but also, depending on the species, to run, swim, fly, grab food or defend itself.

Researchers are considering more complex models to reproduce the behavior of birds in motion. With the ambition to find application in robotics – bipedal robots are often inspired by the human model.

The bird model would allow the bipedal robot to maintain a fixed position for a long time, for example for observation, with minimal energy consumption.

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