Dew-claws are the remnants of the first toes of the remote ancestors of the dogs. When the members of the dog family began to specialize as runners, during the course of evolution, their legs became longer and their feet narrowed from five toes to four. The first toes vanished altogether from the hind legs of wild dogs, but those on the front legs survived as vestiges which no longer touched the ground.
This design gives wolves an impressive turn of speed, 35--40 m.p.h. having been recorded on several occasions over distances of up to a quarter of a mile. Single bounds of sixteen feet have been measured. Endurance over long distances is also remarkable. Huskies, the breed closest to the wolf ancestor, have been known to draw a sledge for over 500 miles in a total time of only eighty hours.
Becoming specialized in running meant sacrifices in other directions.
The ability of dogs to climb and jump worsened as their running improved. But their increased speed and greater stamina on the chase became immensely efficient and successful enough to enable the wild dogs to survive worldwide, from the hot tropics to the frozen wastelands.
So dew-claws should be on their way out, a casualty of the coming-of-age of canines as track athletes. But if this is so, then it seems strange that many breeds of domestic dogs appear to be reversing the trend. One would imagine that modern dogs, being even further removed from the ancient canine ancestor than are wolves or Dingoes, would have lost all their dew- claws, the 'thumbs' of the front feet following the 'big toes' of the hind feet into oblivion.
Instead the reverse is the case. Many breeds of modern dogs have all four dew-claws present. The hind ones are never as solid or as well attached as the front ones, usually consisting of no more than a free bone and a claw loosely connected to the foot by a small flap of skin, but even so they represent a slight turnabout in dog evolution. Breeds with these hind-foot dew-claws, however vestigial they may be, are closer in that respect at least to the ancient canine ancestor than either the Dingo or the wolf. Why has this return towards a primeval condition taken place?
The answer lies in the process known as neoteny -- the survival of infantile characteristics in adult animals. This is what has happened to dogs during the 10,000 years of their controlled breeding by man. They have, in effect, become juvenile wolves. They can breed, but they retain many of their young behaviour patterns, such as playfulness and obedience to a pseudo-parent -- the human owner. They also retain a number of juvenile anatomical features, such as the floppy ears seen in so many breeds today.
Retaining the extra dew-claws is part of this process. We may have bred a number of increasingly extreme features into the different modern breeds, but in other ways they are more primitive than the highly specialized wolf from which they were all derived. In other words, when we set about converting the wolf into the dog we turned the clock backwards as well as forwards.
It is interesting that dog-breeders intuitively feel that there is something wrong with dew-claws and advise that they should be removed when puppies are three to six days old. They recognize it as an 'unspecializing trend' and correct it. The excuse is given that if these vestigial claws are allowed to remain, they may become caught in undergrowth and torn. Bearing in mind that they are on the insides of the legs and above ground, this is a fairly unlikely accident and a trivial excuse, but the unconscious urge to 'refine' the dog's legs is strong enough to overlook this. (Except in certain specific breeds, such as the Briard and the Pyrenean Mountain Dog, where hind-leg dew-claws have to be retained to comply with breed standards.)
Kathy Davison lectures on Animal Health at College
Her Blog offers Free Dog Health Advice.