Why do cats' claws retract but dogs' claws don't?

Imagine the sound of a dog walking across a tile floor, the "click, click, click" of its claws tapping against the ground. Now, imagine a cat padding across the same floor — the difference is the cat moves in total silence. Cats can fully retract their claws, giving them superior sneaking ability.

But why do cats have retractable claws, whereas dogs do not?

It all comes down to how cats and dogs use their claws. Cats use their claws primarily to attack their prey, said Anthony Russell, an evolutionary biologist at the University of Calgary in Alberta, Canada. They use their front claws to grab and hold on to their quarry and their back claws to rake open the prey's belly.

Cats need to keep their claws sharp to secure that next meal. If their claws were extended all the time, they would wear down as they scraped the ground, much like how a nail file wears down sharp points on a human fingernail.

"So the reason for this type of retraction is to keep them really sharp," Russell said. "Keeping these very sharp tips means that [cats] can use them when they're needed, then store them away for the next time."

Related: Why do cats wiggle their butts before they pounce?

To keep those tiny toe weapons in prime slicing condition, cats' claws are retracted by default, Russell told Live Science. "It would be like having the end of the finger on a rubber band," he said. "It costs energy to actually extend those claws, and once you relax those muscles, the claws flip back on their own."