Why Do Dogs Have Dewclaws?

DEWCLAWS ARE SHORT claws or nails on the side of the foot that 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. In several breeds, however, such as the Great Pyrenees and the Briard, 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, but they are an interesting bit of evidence of the distant evolutionary past of the species. Some 40 million years ago, there was a tree-climbing catlike animal known as Miacis. Obviously, if you climb trees, having five toes is an advantage. However, Miacis eventually evolved into the ground-dwelling species Cynodictis. From this point on, successive generations of the animals that would become our dogs began to become specialized as social hunters. Because they were hunting 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. To obtain the 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 mode of walking gives good balance and stability, it is a relatively slow process. What evolution did to dogs was to rock their legs forward so that their heels would no longer touch the ground. In so doing, they became a “digitigrade” species, meaning that they walk on their digits. This manner of walking, along with longer and stronger forelegs, gives them additional speed. Human beings depend on 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 some limited manipulation of things with their paws, so hooves would not be an advantage to them or to those of us who keep our dogs in our homes and would like our wood floors to stay intact.