When cats fall, they flip themselves rightside-up with seemingly little effort — which has perplexed scientists for decades. Our feline friends appear to defy the laws of physics by reconfiguring themselves mid-air without intervention from another force. So how do they do it?
Several factors affect how a cat is able to land on all fours, but simply put, there are two main forces at play: physics and neurology.
"One of the reasons that physicists were surprised that cats could rotate to always land on their feet is the conservation of angular momentum," Greg Gbur, a physicist at the University of North Carolina at Charlotte, told Live Science.
Essentially, this means that if something twists clockwise, something else has to twist counterclockwise. Imagine a cat falling from a stationary upside-down position. By bending at the waist, the cat can twist the front half of its body in one direction and the back half in the opposite direction. By the time it unfolds at the waist, the cat is rightside-up. Gbur dubbed this the "bend and twist" model in his book, "Falling Felines and Fundamental Physics" (Yale University Press, 2019).
But there are other techniques that can help cats right themselves mid-air, and it's likely they employ more than one. In the "tuck and turn" method, a cat extends its front legs and tucks in its back legs, giving the backside a lower moment of inertia, meaning a lower amount of resistance to changes in rotational motion. Then, it does the opposite, tucking in its front legs and extending the hind legs. It has an effect similar to a spinning figure skater: Extending the arms out wide increases the moment of inertia, while drawing the arms close to the body decreases it. This inversely correlates with speed. As inertia goes up, speed goes down, and vice-versa. Cats can also use what Gbur calls "the propeller tail," which works similarly. As the tail spins in one direction, the body can rotate in the other.
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Such contortions are possible thanks to cats' flexible lumbar region — the space between their pelvis and ribcage — John Hutchinson, a professor of evolutionary biomechanics at the Royal Veterinary College at the University of London, told Live Science. Cats have skinny vertebrae, which makes them more flexible than other four-legged vertebrates.
This ability to land on all fours also has a neuroanatomical explanation: the righting reflex or response. Unlike a simple reflex, like a knee jerk, righting in animals is a complex reflex, meaning it's tied to the conscious brain, Gbur said.
Righting is a behavioral response to gravity that relies on the vestibular system, which controls balance. Semicircular canals and sensors called otoliths in a cat's inner ear detect changes in its acceleration and position relative to the ground, prompting its muscles to move in a way that helps it land on its paws. Interestingly, experiments in the 1950s showed that this righting response isn't simply ingrained in cats from birth. When adult cats and newborn kittens were flown on jets in zero-gravity conditions, the kittens weren't able to right themselves but the adult cats did. Because otoliths are made of dense bone, it's possible that they need to develop fully before an animal can properly right itself, Hutchinson noted, but scientists aren't completely sure.
However, the height of the fall also matters. Studies have shown that cats land with less impact when they fall from higher than the seventh floor, for example, than lower heights. This is mostly because of air resistance, which slows the cat's body enough to allow it to turn. Further, cats are unlikely to turn properly from less than 1 or 2 feet (0.3 to 0.6 meters), according to research published in Annals of Improbable Research. (It is not safe for cat owners to purposely drop their cats to test their righting response; they can get hurt, especially if they have a vestibular disease.)
Righting responses aren't unique to domestic cats. Many wild cats exhibit the same behavior, as do rats and rabbits. For cats, the most plausible evolutionary explanation is their tendency to climb trees and other spots high off the ground. For rabbits, predation is a likely evolutionary force. As a hawk swoops toward a rabbit horizontally, for example, the rabbit will jump up vertically, causing the hawk to fumble for the rabbit, knocking it off its linear path. Thus, the rabbit evolved a way to land upright and unharmed.
For a behavior scientists once thought of only as an instinct for many years, Gbur said that multiple techniques can be the answer.
"We have it sort of built into our DNA to look for the simplest solution to a problem, but nature just cares about the most effective solution," he said. "Anything that gets it on its feet faster is better."
