So That's Neat! What's up with retractable claws?
(Or, 'Why Don't Cats Have Hooves?')
I was sitting at the kitchen table the other day, Audrey (my cat)(the nice one) sitting happily in my lap and kneading away, when some sparrows flitted past the window. She sprang up and, in her rush to keep an eye on the situation, one of her claws predictably got stuck in my jeans. As I pulled it out and inspected the damage (minor, and they were old jeans, anyway), I wondered about other cats. Do lions have retractable claws too? Tigers? And what about other animals? Thinking back, it seemed like dogs' nails are always tip-tapping on the floor. And (then I was really wandering) what determines if an animal – a mammal in particular – evolves hooves vs. claws vs. our comparatively weak little fingernails?
To the literature!
What is a cat? What's up with retractable claws?
I knew that cats, along with mammals in general, essentially took off in the Cenozoic, once the dinosaurs were cleared out. Apparently, the origin of modern cats is still a little unclear, but we think it's sometime in the Oligocene epoch (34-23 million years ago), as global temperatures were cooling and polar ice sheets were starting to be A Thing, with some distant relatives all the way back in the Eocene.
Modern cats are in Felidae's subfamily Felinae; their extinct relatives (like sabre-tooth cats) are in the family Nimravidae as well as a different subfamily of Felidae, Machairodontinae, which from the name I'm assuming means something like "big teeth." Today, all the cats, from the heftiest tiger to the tiniest two-pound rusty-spotted cat, have retractable claws, with one notable exception: the cheetah (only semi-retractable, poor guys, they'll be laughed out of the family reunion). Sand cats' and a few other cats' claws don't retract fully, either. But overall, modern cats' claws can retract at least partially.
A little digging (ha) shows that hyper-retractable claws are a feature only shared with a couple of other animals: viverrids (which I would describe as, generally, sort of a cat-lemur-ferret crossover-looking animal)(I am not a biologist), martens, and some badass frogs. As I thought, dogs' little nails are stuck out permanently, for the most part; some foxes (fennec fox, gray and red foxes*) have semi-retractable claws for running and even climbing trees. (Neat!) And wolverines have partly-retractable claws, too.
These paws were made for huntin'
Cats' retractable claws, like other animals' digit plates (and most other things in nature), evolved for a specific function. Horses have hooves because they don't need to do much slashing and disembowling (typically); their appendages are pretty much for running and
standing and laying down and trying to get back up again. (Horses represent basically an end-member for digit development: an animal can have their whole foot on the ground, just the front bit (for us, standing on the balls of our feet; dogs today), or stand on our tippy-toes. Horses and other ungulates are on their tippy-toe all the time.) Humans and other primates' digits are tipped with smaller fingernails that are best suited for detailed tasks like digging out termites and playing scrabble on our phones.
In short, cats' claws evolved for their prey. They're great for slicing and tearing and disembowling. They can be popped out for dinner and tucked cozily away for naps.
But why not other mammals? Why don't dogs (except those foxes) have retractable claws? They hunt, too–shouldn't they want to keep their claws sharp?
It may all go back to running–or, as the papers put it, locomotion. Cheetahs' semi-retractable claws provide a little extra traction as they zip along, and dogs do more sustained running than cats, who sprint around after their prey (or a laser pointer) and then nap. Cats' claws are more central to their hunting** ("prey acquisition") and eating than running or digging; if they run less, they can afford a little wear-and-tear on their little foot-weapons. Dogs, on the other hand, use their jaws/mouths more for hunting and eating, and paws are more for running, so they use their claws-out mode more often, and they don't need to be as sharp for running as cats' claws need to be for hunting.
So that's what I'm taking away from my afternoon of casual research: cats need to keep their claws sharp for hunting, but not so much digging or running, so they can be sheathed away. Like everything in the natural world, no single thing is usually the cause of another, and lord knows evolutionary biology is not simple. But it does satisfy my "why don't cats have hooves?" question.
Machairodontinae apparently means "dagger tooth," from the Greek word for "sword."
This extinct "marsupial lion" also had retractable claws and is pretty neat.
A group of foxes is called a skulk.
Not so neat: Housecats kill some huge number (many millions) of birds and rodents every year, affecting local ecosystems and whatnot. Keep your cats inside! (Or take them outside in a controlled way, like on a leash. Quarantine is a great time to develop eccentric habits and not have anyone question them. I'm no longer the only person in my neighborhood who takes their cat on a walk!)
*This dissertation is available on ProQuest: Comparative osteology, myology, and locomotor specializations of the fore and hind limbs of the North American foxes Vulpes vulpes and Urocyon cinereoargenteus
Feeney, Susan. University of Massachusetts Amherst, ProQuest Dissertations Publishing, 1999.
*Kitchener, A. C., Van Valkenburgh, B., and Yamaguchi, N. (2010). “Felid form and function,” in Biology and Conservation of Wild Felids, eds D. W. Macdonald and A. Loveridge (Oxford: Oxford University Press), 83–106.