With two animals underground (dog and quarry), it is essential that a flow of air be maintained to avoid asphyxiation. The tighter a dog is in the pipe, the more the air flow will be constricted. In addition, a small dog has better maneuverability and can more easily avoid being bitten. Because of this, small dogs often receive less injury underground than larger dogs, which are more likely to find themselves jammed in a den pipe, face to face with the quarry, and unable to move forward or backward.