Why Does the Dog Have a Thumb It Never Uses? It Used to Reach the Ground
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
The small claw set high on a dog's leg is a reduced digit that no longer touches the ground in most gaits, and whether it does anything at all is genuinely argued about.
What it is
The structure is the first digit of the foot, corresponding to a thumb or a big toe, retained in reduced form and positioned further up the leg than the other digits so that it does not contact the ground when the animal is standing or walking normally. Front ones are usually attached by bone and joint like any other digit, with tendons and muscles operating them. Rear ones, where present, are frequently attached by skin and connective tissue alone, with no bony connection, which is why they are loose and why they differ so much between individuals.
What it might still do
The front ones are not simply vestigial and several functions are documented:
- •Gripping a held object, such as a bone being chewed
- •Contacting the ground at speed, when the leg flexes under load
- •Stabilising the wrist joint during a fast turn
- •Providing extra grip when climbing out of water or up a bank
- •Carrying tendon attachments that act on the whole limb
- •Rear ones, by contrast, do very little in most breeds
The argument about removal
Removing them from puppies has been routine in some breeds and is now contested, which is worth stating carefully. The case for removal is that loose rear ones catch and tear, which is painful and requires treatment, and that working dogs in rough cover are particularly exposed. The case against is that front ones are functional, that removal is a painful procedure performed without anaesthesia on very young animals in some practice, and that the risk of injury is low enough not to justify routine amputation of a working digit. Veterinary opinion has moved substantially against routine removal of front ones, and several countries now restrict the practice.
The problems they cause
The injuries associated with them are real and recognising them explains why the removal argument exists at all. A loose rear one hanging by skin can be caught in undergrowth, in a grooming brush or in fencing and torn, which bleeds freely and needs veterinary attention. The claw on any of them grows in a curve and does not wear down against the ground as the working claws do, so it continues growing, curls round and can grow into the pad if it is not trimmed, which is a common and entirely preventable problem in older and less active dogs. Owners frequently miss it precisely because the claw sits where it is not seen.
Which animals have them
The pattern across species is informative about how limbs reduce. Dogs generally have front ones and variable rear ones. Cats have front ones that are genuinely used in climbing and in holding prey. Horses have lost the digit entirely and carry only a horny remnant higher on the leg. Deer and cattle retain two reduced digits behind the main pair, which do contact soft ground and spread weight. Several breeds of dog have been selected for double rear ones, which is required by the breed standard in a small number of cases and is precisely the kind of feature that is otherwise reducing away.
The takeaway
The claw is the first digit, held clear of the ground, attached by bone and muscle at the front and frequently by skin alone at the rear. Front ones grip held objects, contact the ground at speed and stabilise the wrist in a turn, so they are not simply vestigial. Veterinary opinion has moved against removing them routinely, while loose rear ones do catch and tear.