Scientists reveal how cats' spines help them land on their feet

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The mystery of how cats are able to right themselves in the air after a fall and land on all fours has been debated for over a century. Researchers finally found the reason: the upper part of a cat's spine is highly flexible and twists into place, with the stiffer lower spine following. Feline body-mass distribution also helps, with the much lighter upper body allowing for easier twisting than the heavier lower body.

As any ailurophile-the unnecessarily fancy way to reference a cat lover-knows, felines have the almost supernatural ability to pull off a hushed landing on all four paws even if they fall suddenly or awkwardly. They're able to turn to right themselves in midair, but until now, no one seemed to knew exactly how they managed to do this before touching terra firma.

Feline agility is far superior to that of humans. Cats can be dropped on their backs and still reorient their bodies to land on their feet and absorb most of the impact, which explains why they often emerge from falls unscathed. During the feline aerial twisting maneuver, one part of the spine is more flexible than the other. While this was first investigated by French physiologist Étienne-Jules Marey in 1894, a recent physics-based investigation has revealed far more than those early anatomical efforts did. As researchers from Yamaguchi University in Japan found out, the thoracic spine of a cat (neko in Japanese), or upper half, has a much wider range of motion than the lumbar spine, or lower half.

"In axial torsion, the thoracic spine had a larger [range of motion], larger neutral zone, and lower stiffness than the lumbar spine, suggesting greater flexibility," the researchers said in a study recently published in The Anatomical Record. "Furthermore, maximum torque was lower in the thoracic spine than in the lumbar spine, suggesting that the thoracic spine is more fragile."

Felis silvestris catus twisting as it falls is an example of axial torsion. This merges torque, the twisting force the makes rotation possible, with axial forces, which act parallel to the central axis of the animal. Because the research team was not about to traumatize any living cats, they dissected spines from cat cadavers that had been donated to them post-necropsy and tested the limitations of the thoracic and lumbar spine. Each region of the spine was held in place by a torsion testing device that allowed them to manually twist that region as much as they could before it showed any signs of dislocation.

Unsurprisingly, the thoracic spine was much more flexible than the stiffer lumbar spine. The neutral zone was the segment that twisted with minimal torque-and the thoracic spine was found to have such a neutral zone, while none was seen in the lumbar spine.

Of course, the researchers also needed to see the twist in action with live cats. Their two furry test subjects were dropped from a height of 3.3 feet (1 meter) onto a soft cushion. Even when dropped with their backs facing downward, they turned and landed flawlessly, at least if the gray feline subject's derpy way of sticking out its tongue upon landing doesn't count as a flaw. Video files of the falling cats were then analyzed frame by frame. Zero marked the start-time of the free fall. The time of landing was defined as the moment any limb made contact with the cushion, and the amount of time needed complete the twist before landing was also determined. As hypothesized, the thoracic spine rotated before the lumbar spine for a successful landing.

It is thought that the rotation of the thoracic spine, followed by the lumbar spine, may have something to do with the landing being guided by both vision and distribution of body mass. The anterior part of a cat, including its head, neck, and forelimbs, carries 26.4% of its body mass, while 49.3% is in the hind limbs, rear trunk and tail. Because the thoracic spine and the rest of the anterior are lighter and more flexible, they rotate more easily than the stiffer and heavier posterior. This comes as a correction to earlier interpretations of how cats fall that were based on assumptions of symmetrical mass distribution.

While some animals, such as squirrels, have tails substantial enough to help them right themselves during a fall, a cat's tail is too light to have much of an effect. Whether those with extra floof have an advantage is unknown.

"Future research is necessary to investigate interspecific differences in the mechanical properties of the spine and obtain a more detailed understanding of its behavior in living animals," the researchers said.