A running coyote, painting by Carl Dennis Buell
The ancestor of all dogs climbed trees like a cat.
Or so the experts hypothesize. The raccoon-sized, foxy omnivore Prohesperocyon is as likely a candidate for the ancestor of all dogs, wolves and foxes as any fossil species known. It lived in what is now the American Southwest. It probably had retractile claws. The later, closely related species, Hesperocyon almost certainly had retractile claws. It had only been about ten million years since caniforms and feliforms had diverged, and most feliforms can sheath those daggers.
If you had to sum up the history of the world since Prohesperocyon‘s time in two words, the phrase “grasses won” would be as accurate and all-encompassing as most phrases you might choose. Cold spread, and with the cold came dry. Moist temperate and subtropical forests receded. Dry open woodlands became more common. Grasslands spread out from their riparian Eocene habitats. By the Miocene savannas covered much of the Earth.
As grasslands expanded, animals ventured out into them. It’s harder to hide in the grass than it is in the forest. In the forest, you can hold still and hope the trees hide you from the animal trying to eat you, or from the animal you’re trying to eat. In the grassland, you’d better run for your life.
Prohesperocyon‘s successors ran. It was, to mix a metaphor, an arms race. Grazing animals died if they couldn’t outrun the predators. Predators starved if they couldn’t catch their prey. Natural selection favored the survival of the fastest.
Prohesperocyon‘s successors ran, and natural selection sculpted them for running. Strides grew longer. (As legs grew longer necks grew as well, to accommodate running with the nose near the ground — a boon to scent-hunting.) Dog’s ancestors began running on their toes, adding the length of their foot bones to their effective leg length. Toes became less about grasping surfaces for climbing and more about cushioning the shock of impact while running, with frequent sideline use as digging tools.
Eventually, dogs lost the ability to retract their claws.
The bones on the left are the phalanges — fingerbones — of a cat. On the right, a dog’s equivalent phalanges. The drawing is by Mauricio Anton, from the book Dogs: Their Fossil Relatives and Evolutionary History by Xiaoming Wang and Richard Tedford. I’ve cleaned up the scan a bit and darkened the arrow pointing to a concavity in one side of the cat’s second phalange. When a cat retracts its claws, the first phalange slides back into that concavity, more or less locking in place. (Imagine being able to retract your fingernails up behind the first joint.)
See the dog’s far more symmetrical second phalange? No concavity means no retractile claws. This is why dogs make this noise on hardwood floors:
…and cats don’t, usually.
Non-retractile claws take a beating, from hardwood floors or rocky soil. Retracting one’s claws allows them to stay protected from wear and thus sharper. Still, in the modern hardwood-based environment, they sometimes don’t seem to offer much in the way of strategic advantage:
Non-retractile claws would seem to offer an advantage in the cursorial lifestyle, offering a bit of extra traction like studded tires to help those sudden starts and stops and turns. It’s worth noting that the one cat with a truly cursorial hunting strategy, the cheetah, has claws that — while still technically retractable — are constantly exposed and probably noncoincidentally blunt. Other cats run, but rarely in more than short bursts.
There’s a lifestyle threshold for mammalian predators at around twenty kilograms — 45 pounds or so. Meat-eaters smaller than that threshold weight can sustain themselves on small prey: rodents, small lizards, invertebrates and such. Above 45 pounds, though, and moving fast enough to hunt requires more energy than small prey will provide. It’s not an entirely ironclad rule: smart carnivores will take easy pickings even if they’re the wrong size. But it’s not a bad rule of thumb: coyote-sized and smaller predators eat things smaller than themselves, while predators larger than coyotes eat things bigger than themselves.
That’s rather an abrupt change in strategy, and we see it now in Northern coyotes. Freed in most places from competition with larger wolves, and in some places hybridizing with those wolves, coyotes in the North have gotten their average weight up above that 45-pound threshold, and in places like Yellowstone those larger coyotes are now hunting in packs, taking down elk eleven times their size.
The pack thing is important. When cats evolved past the 45-pound mark, they had the tools to tackle larger prey on their own. Those sharp, protected claws let the cats get a secure hold on their intended meal, allowing the hunter to dispatch its prey with a precisely targeted bite. Tigers, for instance, will sever the spinal cord through the nape of the neck for prey of about human size, changing the angle of attack for buffalo-sized victims and crushing their tracheas. The old-school Smilodons, sabre-tooths, targeted a slashing bite at their prey’s abdomen, then stepped decorously back a bit to let it die of blood loss.
Dogs faced a serious disadvantage in taking down the larger grassland equids and camelids and gomphotheres and such. Those blunt claws were just fine for digging out rabbit warrens or slapping kittens off of coffee tables, but nearly useless for grabbing hold of a bison’s hide.
The solution, as expressed by those Yellowstone coyotes: cooperate in the hunt.
Large dog species, from wolves and coyotes and domestic dogs to African wild dogs, hunt cooperatively. They’ve done so for some time, as witness this rendering by Mark Hellet of a Miocene pack of Epicyon haydeni — members of the extinct Borophagine, or “bone-crushing dog” clade, at almost a meter tall at the shoulder the largest known dog in Earth’s history — seriously ruining a Synthetoceras’ day near Red Rock Canyon State Park in Holocene Kern County. They work sophisticated and flexible strategies, acutely aware of the presence and activity of their packmates, and able to change plans as a group on a moment’s notice merely by communicating with one another.
Kind of like this one other blunt-clawed species of social hunting animals whose progenitors were thrown out of the forest and forced to make a living in the grasslands.
And this, this cascade of consequences of a minor modification of a toe bone, is why dogs were, despite being large, dangerous predators, the first species we humans domesticated, and the one most closely interwoven into the human lifestyle. Losing the ability to retract their claws set the stage for innovation of a social hunting strategy into which we humans could cleverly intrude.
Yes, we domesticated cats as well — one of the smallest species available, and they still haven’t quite made up their minds whether the arrangement is really working for them. Domesticated cats live with us in an arrested foster-maternal roleplay, having avoided the near-paranoiac isolation wild adult cats prefer.
Dogs, on the other hand, live with us without repressing much in the way of their basic nature. It’s as if it’s in their bones.