Never put your hands between two fighting terriers Jack Russells are often aggressive with other dogs. Same-sex aggression and aggression towards other breeds of dogs is well documented with this terrier. Jack Russells require firm, consistent discipline. They are extremely intelligent, continue to test their limits throughout their life. Jack Russells can become very possessive of their owner or a favorite member of the family or of what they consider to be their personal property if allowed to do so to the point of showing aggressive protective behavior that must be controlled from an early age.
Most behavioral problems are due to a lack of companionship, discipline, activity and exercise.
If you've only seen perfect, well-behaved JR's, they are ones that were lucky enough to be exercised, well socialized, and trained. Nipping and Biting Nipping and biting is how a puppy explores its world and learns its boundaries. When you allow a puppy to nip or bite at you, or your clothes, without an appropriate correction or consequences, you are actually teaching this youngster that it is acceptable behavior. I have found two types of corrections that work well for me. I don't recommend hitting a puppy or holding a puppy's mouth closed for biting, rather, putting your hand OVER their muzzle and just pushing her lips into the teeth, if they exert pressure downward, they only end up biting themselves.