Recently, a small cat living in Boston survived a 19-storey fall from an apartment building. It was surely a bit of luck, but cats are evolutionarily adapted to survive such falls. Cats’ remarkable ability to survive falls from great heights is a simple and predictable matter of physics, evolutionary biology, and physiology.

Adorable, and perfectly adapted to survive a fall

From a great height

There are numerous accounts of cats surviving falls from seemingly deadly heights. However, as it is fairly unethical to test this ability by experimentally throwing cats out of high windows and seeing what happens, it has been hard to study this phenomena. Even so, by looking at the numbers of vet visits for such events, we get an idea of how successful a cat’s biology is at bracing a serious fall. Via the BBC:

In a 1987 study of 132 cats brought to a New York City emergency veterinary clinic after falls from high-rise buildings, 90% of treated cats survived and only 37% needed emergency treatment to keep them alive. One that fell 32 stories onto concrete suffered only a chipped tooth and a collapsed lung and was released after 48 hours.

It is pretty apparent from stories and reports like these that cats know how to fall effectively, but how do they do it?

The science of falling cats

First, cats have a high body surface area in proportion to their weight, reducing the pressure to their bodies when they hit the ground (because pressure equals force divided by area).

Second, because of their surface area and their low weight they reach terminal velocity, or the speed at which the pull of gravity is matched by wind resistance, much quicker than larger animals like humans. This means that they fall at a slower speed and subsequently hit the ground with less force. Again via the BBC:

For instance, an average-sized cat with its limbs extended achieves a terminal velocity of about 60mph (97km/h), while an average-sized man reaches a terminal velocity of about 120mph (193km/h), according to the 1987 study by veterinarians Wayne Whitney and Cheryl Mehlhaff.

Though we see cats usually living in someone’s house, cats are traditionally arboreal animals (they live in trees). Because sooner of later they are bound to fall, and any tree-dwelling animal will eventually pounce for prey and miss, cats are evolutionarily adapted to survive falls. That is to say, it is evolutionarily beneficial to be able to survive a fall, and as such modern house cats retain this ancestral adaptation.

Third, again through natural selection, cats have what is called aerial righting reflex which allows them to, given enough time, sense that they are falling incorrectly and spin themselves around like a sky diver or astronaut so that their feet are facing the ground. Having your main shock absorbers hit the ground instead of your back is crucial for fall survival. Interestingly, every animal that lives in the trees has a similar reflex, suggesting the same evolutionary process. You can see a shot by shot view of the aerial righting reflex below.

The shot by shot view of the aerial righting reflex that allows cats to pretty much always land on their feet. Also notice how in the last section of the fall pictured about that the cat is extending its legs and arching the back. This allows the cat to absorb more force once it hits the ground. Both a straight back and unprepared legs will most likely increase the risk of injury for the cat.

Fourth, cats also spread out their legs when they fall (as seen in the first picture above), increasing their surface area and slowing their descent by increasing the air’s drag on their bodies. Just how much this action slows their descent is unclear.

Fifth, cats have long muscular legs, adapted for climbing trees, that act as great shock absorbers. The same large muscles that allow them to jump many times their own height allows them to divert energy into decelerating them once they hit the ground instead of breaking bones. Because much of what is destructive about a collision is how rapidly a body decelerates, having long, springy legs allows cats to decelerate more slowly, thus reducing the seriousness of the collision.

WARNING PHYSICS CONTENT: For an example, think of an egg falling either onto a sheet held above the ground or onto pavement. In both cases the egg will come to a stop, meaning that the change in momentum (from some quantity during the fall to zero) is the same for both cases. However, because this change in momentum is dependent on the force applied to the egg and how long the force is applied to the egg, extending the amount of time the force is applied thus reduces the force applied.

So, why does the egg break on the pavement and not on the sheet even though the change in momentum is the same? The sheet is flexible and allows the time of collision to be extended, therefore reducing the force imparted to the egg. The pavement has an almost instant time of collision, therefore increasing the force and breaking the egg. You can also see this happening with your own body. If you jump to the ground from any height, you bend your knees upon impact, extending the time of collision and lessening the impact force. Learn more here.

Cats reduce the force imparted to their legs by having strong and springy legs in the first place. If cats landed from with their legs completely straight and rigid however, all of their bones would most likely break. Cat’s legs also have joints that can bend off to the sides of their bodies when they fall, unlike humans, which further reduces the force to their bodies.

Even given all of this adaptation, many house cats are estimated to be overweight and under-exercised and this reduces their ability to right themselves in mid-air and increases the speed at which they hit the ground. In all of their natural glory however, cats are purrfectly adapted to survive high falls.

The lesson from all of this? Cats that survive these incredibly high falls are lucky. Though many cats would survive, most would have severe lung damage, would have a broken leg or two or three or four, maybe have damage to the tail, and maybe more likely than any of that a broken jaw or dental damage. So put screens on your windows if you have a cat. They could probably handle a tumble from a tree, but an apartment building was never selected for.

[Via the BBC, HyperPhysics]

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