Re: Why do dogs have dewclaws? and why are they only in the front?

First of all, let's explain what a dewclaw is, for the sake of onlookers.
A dewclaw is a claw that doesn't hit the ground with the rest of the 
claws when an animal walks. From fossil evidence, some dinosaurs had
dewclaws. Modern cats, wolves, and tigers have them also, in addition to
the dogs that you referred to. And, while they are more commonly found on 
the front legs of dogs than on the back, some dogs do have them in both 
places. Sometimes the dewclaws are firmly attached by bone to the rest of 
the skeleton; other times they are "floppy", and simply dangle from a 
connective tissue attachment. As you may know, they are often removed 
surgically, especially from hunting dogs, who may snag them on brush. A 
ripped dewclaw will bleed profusely.

Now to the second part of the question--what is the function of dewclaws? 
It is commonly agreed that they have NO function. They are vestigial 
remnants of a fifth digit that was reduced in the process of evolution. 
While the general pattern in mammals is to have five digits at the end of a 
limb, fossil evidence shows a loss of digits, especially in animals that 
maintain high speeds over long distances, as do dogs. (Horses, for example, 
walk on just one digit, the third--the hoof is the remnant of one 
"fingernail".) Members of the dog family (canids) similarly have small 
feet, with usually four digits in contact with the ground. The small size 
and weight of their limbs require less energy to move, allowing them to run 
more efficiently.

For more information on this topic see the entry under "Dog Family" at this 
website:
      http://www.iversonsoftware.com/business/mammal/d.html