On certain nights, behind some of your favorite restaurants, roving groups of dog owners set their posse of pooches loose on urban rodents. The sound of a rat screaming in the jaws of a terrier is the same sound that a stuffed squeaky toy makes. It seems so obvious. Of course the toys sound that way, because that sound awakens something deep in a docile dog’s neurons that says: Shake it. Shake it till it’s dead. It’s Saturday night. Flavia and Jigs, a duo of mother-daughter border terriers, are shaking a rat. It is already dead. On one side of...