thumb|A [[Presa Canario undergoing guard dog training, 2010]]
A guard dog or watchdog is a dog used to watch for and guard people or property against unwanted human or animal intruders. A dog trained to attack intruders is known as an attack dog.
History
thumb|Roman mosaic of a large chained dog (with [[Cropping (animal)|cropped ears and docked tail) at the Archaeological Park of Lilybaeum, Marsala, Sicily]]
thumb|A raggedly dressed man being bitten by a house guard dog. Etching by [[Thomas Lord Busby, ca. 1826.]]
Dogs have been used as guardians since ancient times. The ancient Romans placed Cave canem mosaics at the entrance of homes to warn visitors and intruders of the presence of dangerous dogs at the property.
One of the first dog types used as guardians were Mastiff-type landraces of livestock guardian dogs, which protected livestock against large predators such as wolves, bears and leopards. In Greek mythology, Orthrus is an example of a livestock guardian dog known for guarding Geryon's red cattle. In more urban areas, some ancient guard dogs, such as the extinct bandogges, were chained during the day and released at night to protect properties, camps and villages.
Barking
thumb|[[German Shepherd dog guarding property]]
Guard dogs bark loudly to alert their owners of an intruder's presence and to scare away the intruder.
Livestock guardian dogs are often large enough——and strong enough to attack and drive away livestock predators.
Breeds
Many currently prominent guard dogs started as general purpose farm dogs, but gradually developed into guard breeds. However, dog breeds such as the Dobermann and the Dogue Brasileiro were specifically bred to guard. Guard dog breeds tend to be territorial, averse to strangers, dominant, and protective and loyal.
Legality
The laws regarding ownership and usage of guard dogs vary from country to country.
See also
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