A bedtime story - That’s more or less the story of a recent publication by a team of scientists from the Muséum national d’histoire naturelle (MNHN) and the CNRS, who explain in the columns of the Journal of the Royal Society Interface 1 how birds manage to sleep and stand upright without losing their balance. A feat that we know horses and cattle are capable of because they have four legs, but which at first sight was not so obvious in birds. So where does this stability come from? From tensegrity, say the scientists. Derived from the contraction of the words "tension" and "integrity", tensegrity is none other than the property of a structure to remain stable and balanced thanks to a subtle interplay between tension and compression of its constituent elements.
A multiple balance
The starting point for this research was to understand the mechanisms of evolution via functional morphology, which is the study of the relationships between the shape of organisms and their functioning," explains Anick Abourachid, a specialist in evolutionary biology and deputy director of the Adaptive Mechanisms and Evolution 2 unit at the Muséum national d’histoire naturelle. In particular, we took a very close look at the legs of birds. The interest in studying the mechanisms of leg evolution in birds lies in the fact that they constitute a particularly homogeneous group from a structural point of view: " They are all flying bipedal dinosaurs that have shared a structure built for aerodynamics since the origin of the group. Without exception, they have a rigid trunk,Ölanding and take-off gear and wings ", explains the researcher. This extremely well-preserved organizational plan is also effective everywhere, since birds can be found in all environments. It’s this versatility that has always interested and intrigued me," shares Anick Abourachid. And it was through discussions with my colleagues Philippe Wenger and Christine Chevallereau that I learned about tensegrity, those mechanical systems maintained solely by tension. "It’s worth remembering that the legs of birds are peculiar, although structurally similar to the legs of humans. " Birds are flexed bipeds; when they’re standing, their whole body is bent. For us, this would correspond to crouching on tiptoe ", explains the researcher. A posture that, at first glance, doesn’t seem very comfortable. However, in this position, the birds relax and can even sleep standing up. In short, they rest and consequently expend less energy. To understand how birds manage this balance, the scientists devised a digital model based on the anatomy of a small bird, the Mandarin Diamond(Taeniopygia guttata), a species belonging to the passerine family. " It was by hypersimplifying the body system, to keep only what concerned standing posture, that we found postural stability ," reveals Anick Abourachid.
In this model, the body and leg bones have been replaced by bars, and the muscles and tendons by more or less rigid cables. The joints between each leg bone have been replaced by pulleys. " We began the experiments using a single cable running from the bird’s pelvis to its feet, passing through all the joints (hip, knee, ankle) and therefore all the pulleys ", explains the biologist.
In birds, a tendon passes through a ligament loop at the back of the knee to keep it in line. A unique anatomical feature.
Result - The researchers succeeded in reproducing the birds’ generic upright posture, but they were not at all stable, even though we’re still amazed to see them land on electric wires or tree branches without ever tumbling over. " Our model bird was only balanced at a single point in space, whereas they are naturally balanced in a multitude of positions. So we improved their stability by using several cables instead of just one. In particular, one that passes behind the knee rather than in front, as was the case in the initial model ", says Anick Abourachid.
In birds, however, a tendon runs along the back of the knee through a ligament loop to keep it in line. A unique anatomical feature. So, with four cables, including one passing through the back of the knee, the model became stable, meaning that even with a small disturbance the system was able to return to equilibrium on its own, passively, like a tumbler.
Stiffness and reactivity
What’s more, the scientists realized that the quality of the cables, and therefore the tendons, in the birds’ legs played a decisive role. In fact, birds are among the only animals to have calcified, almost ossified tendons," explains Christine Chevallereau, Director of Research at the Laboratoire des sciences du numérique à Nantes 3 (LS2N). In our model, we understood that this property had to be taken into account to obtain a stable equilibrium. The bird needs these stiff tendons because, when it lands, it has to counterbalance the effect of gravity so as not to tip over to one side or the other of the branch. Although this low elasticity may not seem to be a factor in favor of stability, it is, in fact, quite the opposite, as it reduces the effect of the disturbances to which the bird is subjected. It also means that the system responds much more quickly, as it is less likely to deform before returning to its equilibrium position. Whether it’s snowing, raining or winding, thanks to this tensegrity system, birds can continue to sleep without fear of falling off their perches.
" A priori, the advantage that birds derive from tensegrity in their legs is based on the passive nature of the mechanism. So, for a bird, standing upright requires no thought or concentration. It’s all down to the elasticity of the system. It’s a form of embodied intelligence where it’s the body alone that solves the problem without the intervention of the brain, and therefore without the use of energy ," shares Christine Chevallereau.
The passive nature of the mechanism is both surprising and synonymous with high-impact application. " You have to bear in mind that these mechanisms are used a lot in civil engineering, because they make structures lighter. The idea, with this discovery, is to use these mechanisms in robotics to lighten moving masses, consume less energy, use less material to build them and reduce the danger in the event of collision with an operator ", points out Philippe Wenger, Research Director at LS2N. Engineers are already imagining bipedal robots equipped with tensegrity systems enabling them to maintain upright postures, balanced on unstable supports, for long periods, without expending the slightest amount of energy. ?
