This article relies largely or entirely on a single source. (August 2019)
The term soft mouth is used by breeders and users of hunting dogs to refer to a behavioral tendency to pick up, hold, and carry quarry gently. It is not a preferred characteristic of terriers and ratters, who are expected to roughly shake and mangle pests such as rats and snakes in order to kill them quickly and efficiently. It is desirable in gundogs such as retrievers and spaniels which are expected to produce quarry intact and in good condition, and is a notoriously difficult behavior to teach to a dog without an inborn temperament to do so. As a result, breeders and users of gun dogs arrived on this term to describe a characteristic important to this enterprise. Dog breeds known for producing more soft-mouthed individuals include Golden Retrievers, Cocker Spaniels, Poodles, and Gordon Setters, but in any litter some individuals may be much less soft-mouthed than others, as there may be relatively soft-mouthed individuals in a litter of even classically hard-mouthed breeds such as Jack Russell Terriers or Mastiffs.