Image caption Comfy? Cats are evolutionarily adapted to live in trees - luxury living with humans is a recent development

A cat in the US city of Boston survived a fall from a 19-storey window and only bruised her chest. How do cats survive falls from such great heights?

The cat's owner Brittney Kirk, a nurse, left the window open a crack on Wednesday morning to give Sugar some air. Sugar got out and either fell or leapt off the ledge and hit a patch of grass and mulch.

An animal rescue service found her and traced her back to Ms Kirk through a microchip embedded in her skin.

"She's a tough little kitty," Ms Kirk told the Boston Globe newspaper.

Cats' remarkable ability to survive falls from great heights is a simple and predictable matter of physics, evolutionary biology, and physiology, veterinarians and biologists say.

"This recent story isn't much of a surprise," says Jake Socha, a biomechanist at Virginia Tech university.

"We do know that animals exhibit this behaviour, and there have been lots of records of these cats surviving."

The answer Cats have relatively large surface areas in proportion to their weight, so fall at a slower rate over a great height than larger mammals

Their bodies have evolved to allow them to survive falls from trees, their natural homes

Given the time, they twist to land on their feet

Their legs are long, muscular and extend under the body rather than straight down, allowing them to absorb the shock

But many cats who fall from heights are nevertheless severely injured and some die Lucky cat survives 19-storey fall

With scientists unwilling to toss cats off buildings for experimental observation, science has been unable systematically to study the rate at which they live after crashing to the ground.

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.

From the moment they're in the air to the instant after they hit the ground, cats' bodies are built to survive high falls, scientists say.

They have a relatively large surface area in proportion to their weight, thus reducing the force at which they hit the pavement.

Cats reach terminal velocity, the speed at which the downward tug of gravity is matched by the upward push of wind resistance, at a slow speed compared to large animals like humans and horses.

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.

This cat was lucky. But many, if not 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 Steve Dale, Cat behaviour specialist

Cats are essentially arboreal animals: when they're not living in homes or in urban alleys, they tend to live in trees.

Sooner or later, they're going to fall, biologists say. Cats, monkeys, reptiles and other creatures will jump for prey and miss, a tree limb will break, or the wind will knock them over, so evolution has rendered them supremely capable of surviving falls.

"Being able to survive falls is a critical thing for animals that live in trees, and cats are one of them," says Dr Socha. "The domestic cat still contains whatever suite of adaptations they have that have enable cats to be good up in trees."

Through natural selection, cats have developed a keen instinct for sensing which way is down, analogous to the mechanism humans use for balance, biologists say.

Then - if given enough time - they are able to twist their bodies like a gymnast, astronaut or skydiver and spin their tails in order to position their feet under their bodies and land on them.

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"Everything that lives in trees has what we call an aerial righting reflex," says Robert Dudley, a biologist at the animal flight laboratory at the University of California - Berkeley.

Cats can also spread their legs out to create a sort of parachute effect, says Andrew Biewener, a professor of organismal and evolutionary biology at Harvard University, although it is unclear how much this slows the rate of descent.

"They splay out their legs, which is going to expand their surface area of the body, and that increases the drag resistance," he says.

When they do land, cats' muscular legs - made for climbing trees - act as shock absorbers.

Image caption Cats' legs are springy and muscular, letting them absorb the shock of impact

"Cats have long, compliant legs," says Jim Usherwood of the structure and motion lab at the Royal Veterinary College. "They've got decent muscles. In that they're able to jump quite well, the same muscles divert energy into decelerating rather than breaking bones."

The springy legs increase the distance over which the force of the collision with the ground dissipates, says Dr Biewener.

"The impact forces are much higher in stiff collisions," he says. "If they can increase the collision time over a longer period, that reduces the impact force."

And a cat's legs are angled under the body rather than extended downward, like human or horse legs.

"You're not transmitting the forces really directly," says Dr Socha.

"If the cat were to land with its legs directly under him in a column and hold him stiff, those bones would all break. But they go off to the side and the joints then bend, and you're now taking that energy and putting it into the joints and you're getting less of a force at the bone itself."

However, house cats in urban or suburban areas tend to be overweight and in less than peak physical condition, warns Steve Dale, a cat behaviour consultant who is on the board of the Winn Feline Foundation, which supports cat health research.

That detracts from their ability to right themselves in midair, he says.

"This cat was lucky," he says. "But many, if not 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.

"The lessons learned: screens, please, on the windows."

Reporting byDaniel Nasaw in Washington