All wild canids in the genus Canis normally have 5 digits on the forefeet and 4 digits on the hindfeet. The missing digit on the rear is the equivalent of the big toe, the medial (innermost) digit (digit 1 for anatomists trained in North America and the United Kingdom). The loss of this first digit seems to accompany the evolution of the 'up on the toes' (digitigrade) stance of canids. The flatter footed (plantigrade) carnivores have 5 toes on each foot.
Although natural selection dumped the 1st digit from the hind foot fairly far back in the evolution of canids (and also, independently in cats) the thumb has persisted on the forefoot in digitigrade carnivores. There may well be a reason it is retained.
In my PhD work I found that really fast, slender legged dogs hyperextend the forefeet down to the stop pad when at a full gallop. They also roll slightly medially onto the inner surface of the carpus (pastern) when running. Examination of tracks in firm moist soil and examination of the feet of dogs running in grass show that the first digit of the forefoot (the thumb) comes into contact with the ground during high speed locomotion. In fast galloping dogs with small tight thumbs the claws wear with racing so that they do not need periodic trimming.
I have had dogs who had had thumbs removed get injuries in the thumb stump region unless they were run with their pasterns taped. This does not always happen but it occurs, in my experience, about as often as thumb claw injuries in dogs in which the thumbs are not removed.
(2) In tight footed breeds that gallop or dig the athletic animal has a use for the thumb and it should not be removed as a preventive measure. Pastern injuries are as probably as dew claw injuries in the working animal. Dogs that repeated tear their thumb claws need the thumb removed completely or they they need the claw removed but the little digital pad that accompanies the thumb claw brought into place over the end of the remaining thumb bones. To leave the thumb bones under the skin without protection on the projecting end is to set up a situation where abrasive injuries can occur when that part of the foot comes into contact with the ground during the gallop. These injuries are not really nasty but they can lame a dog on the day of a competitive event and cause the owner to have to forfit a tie, etc.
End of Dew Claw Monologue...
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