Dewclaws are evidence of the evolutionary history of dogs
Dogs do not create museums or libraries to preserve the history of the evolution of their species. Dogs simply store their wisdom in their genes. But sometimes we can read bits of this history by looking at a dog's behavior or his physiology.
Take the case of dewclaws. For those of you who are unfamiliar with the term, dewclaws are short claws or nails on the side of the foot which don’t touch the ground. Most dogs have dewclaws only on their front paws, and it is rare to find them on their back paws. However, in several breeds, such as the Great Pyrenees and Briards, rear dewclaws are common. The Great Pyrenees even has a double dewclaw, an inherited trait called polydactyly, so that there are two bony digits instead of one.
For most dogs, the dewclaws are nonfunctional. However, they really are an interesting bit of evidence of the distant evolutionary past of the species. Some 40 million years ago, there was a tree-climbing, cat-like animal known as miacis which was an early ancestor of our modern dogs. Obviously, if you climb trees, having five toes is an advantage. However, miacis eventually evolved into the ground-dwelling species cynodictus.
From this point on, successive generations of the animals that would become our dogs began to become specialized as social hunters. As hunters of fast-moving prey, speed became an important factor. Today’s dogs are a cursorial species, which means that evolution has adapted them to be swift runners and they could be the fastest land animals on the planet (click here to see more about that). To obtain this added speed required a change in canine physiology.
Animals, like humans and bears, are plantigrade species, which means that they place the full length of their foot on the ground during each stride and then move with a rolling action that goes from heel to toe. While this gives good balance and stability, this is a slow process.
What evolution did to dogs was to rock their legs forward so that their heel would no longer touch the ground. In so doing they became a digitigradespecies, meaning that they walk on their digits. This, along with longer and stronger forelegs gives them additional speed.
Human beings depend upon their ability to manipulate things, so the structure that became the dewclaw in dogs became our thumb. The dog has four digits that make contact with the ground and the dewclaw is simply a vestigial structure that has been left over by evolution. Because of these physical changes, the sole of the dog’s foot never touches the ground and the dewclaw is too short to be of any functional value.
Evolution has an additional trick to further increase the speed of an animal. It involves reconstructing species so that they walk on their tiptoes, which have often developed into hooves. This is what we have in deer and horses. Dogs still do require some limited ability to manipulate objects in their world with their paws, so hooves would not be an advantage to them (nor to those of us who keep our dogs in our homes and would like to have our wood floors stay intact).
Dewclaws, both front and rear, are often a cause a bit of worry in dog owners who are afraid the nail will catch on something during a run through a forest or over rough terrain. If this happens, it can be torn off and cause serious injury. However, some dewclaws are held in tightly against the leg, and with regular nail trimming are unlikely to catch on anything. Others can be loose and floppy, presenting a clear hazard, especially for dogs who like to romp outdoors where roots, trees, and other hazards are common. For that reason, some breeders will have them removed before the puppy is adopted out, although the majority of dogs are still left with their dewclaws intact.
There is an interesting bit of folklore that keeps some people from removing the dewclaws of their dogs. In the southern states in America, there is a common belief that dogs that are born with dewclaws on their hind feet (which is somewhat rare) have a natural immunity to the venomous effects of snake bites as long as the dewclaws remain intact. Once, when I was in South Carolina, an old man brought out a favorite hound of his and showed me the dewclaws on her back legs. He explained to me, “She’s been snakebit more’en one time, but she’s still here ‘cause them dewclaws sucked up the poison.”
Author: Stanley Coren, Ph.D., FRSC