A guard dog, watch dog, or sentry dog is a dog employed to guard against, or watch for, unwanted or unexpected animals or people.

Both guard dogs and watch dogs bark to alert their owners of an intruder's presence. The barking is also an attempt at scaring away the intruder. The watch dog's function ends here, while a guard dog might be trained to restrain or attack the intruder. For example, livestock guardian dogs are often large enough and strong enough to attack and drive away predators such as wolves. In cases where intruders are more likely to be human, guard dogs are sometimes trained simply to restrain an intruder with his owner's guidance, as in Schutzhund or K9 Pro Sports training. Occasionally, guard dogs are trained to attack human intruder(s), though this practice might be locally illegal. Some breeds (such as Keeshonden) are excellent watchdogs but not so excellent guard dogs, as they bark loudly to alert their masters of intruders but are not given to attack behavior.

Breeds

Many of the now prominent guardian breeds such as Rottweilers started as farm dog types but then developed over many years into guard breeds. Some breeds, such as the Weimaraner and Rhodesian Ridgeback, were originally bred for hunting, but their large intimidating look and territorial instincts have helped them evolve into guard dogs in today's society. Others like Dobermans were specifically bred as guard dogs. Many of the below breeds have a greater amount of molossoid or mastiff DNA. This is confirmed by a DNA study done on >270 pure breed dogs. The study suggests that in the distant past thirteen ancient breeds broke off early on after which a group of mastiff style dogs were developed"Genetic Structure of the Purebred Domestic Dog" Science, 21 May 2004: Vol. 304. no. 5674, pp. 1160 - 1164. http://intl.sciencemag.org/cgi/content/full/304/5674/1160?ck=nck (Subscription required) Note: pay special attention to Figure 3 of the study.. These dogs are grouped with the mastiffs Great Dane and Irish wolfhound. The Irish wolfhound (a traditional hunting guardian breed) has evidence that its population was nearly wiped out 200 years ago and its existing members descend from a very small group of dogs and thus it is a rebuilt breed, but evidently out of somewhat different building blocks"Genome sequence, comparative analysis and haplotype structure of the domestic dog" Nature 438, 803-819 (8 December 2005) | doi:10.1038/nature04338 http://www.nature.com/nature/journal/v438/n7069/full/nature04338.html. Some people think that the Irish wolfhound contributed to the Great Dane.