A guard dog or watchdog (not to be confused with an attack dog) is a dog used to watch for and guard property against unwanted or unexpected human or animal intruders. The dog is discerning so that it does not annoy or attack the resident humans of the house. History The use of dogs as guardians is well known since ancient times. The Romans used to put mosaics (Cave canem mosaics) at the entrance of the houses to warn visitors and intruders of the presence of dangerous dogs at the property. One of the first dog types used as guardians were the ancestral Mastiff-type landraces of the group known as Livestock guardian dogs which protected livestock against large predators such as wolves, bears and leopards. Orthrus is a famous example of a livestock guardian dog from the Greek mythology known for guarding Geryon's red cattle. Some ancient guard dogs in more urban areas, such as the extinct bandogges, were chained during the day and released at night to protect properties, camps and villages. Barking Both guard dogs and watchdogs bark loudly to alert their owners of an intruder's presence and to scare away the intruder. The watchdog's function ends here; a guard dog is then capable of attacking and/or restraining the intruder. Livestock guardian dogs are often large enough (100-200 lbs.) and strong enough to attack and drive away livestock predators. Some smaller breeds (such as Keeshonden and Tibetan Terriers) are excellent watchdogs, but not guard dogs, because they bark loudly to alert their masters of intruders, but are physically small and not given to assertive behavior. Guard dogs will bark to alert their master and to warn off an approaching animal or human threat prior to their interception of the trespasser. They are different from the smaller watchdogs in that they do not continue barking; they take action. Specifically, livestock guardian dogs such as the Kangal use loud alarm barks as a first line of defense against presumed threats; if these do not deter a perceived foe (either human or animal predators), other displays such as bluffing and charging are employed. For livestock guardians, proactive forms of defense such as bites are only used if all other forms of deterrence fail. The following breeds are the best at watchdog barking: Doberman Pinscher German Shepherd Dog German Spitz (including Keeshond) Miniature Schnauzer Scottish Terrier West Highland White Terrier If the