Miniature Schnauzer - chewing on paws

Question
My father has a miniature schnauzer who is constantly chewing on his paws.  They are actually discolored (browinish color) I assume from being chewed on.  Any idea why he is doing this?  So far his vet doesen't seem to have an answer.

Thanks!!!!

Answer
This is most commonly due to allergies of some sort, but I have a retired agility dog who licks his front feet because of severe arthritis in his toes.

Yes, the discoloration is from the dog's saliva.

Allergy testing is expensive, but it is really the only way to determine what the dog might be allergic to.

Karen