Some breeds have been developed primarily for guarding livestock and property. Many of these, such as the Rottweiler and German Shepherd dog, are now popular as companion dogs. Yet coupled with their loyalty these dogs are likely to have a latent hint of aggression in their natures. This is sadly not appreciated by many owners, and can become a cause of problems, which may sometimes even feature in the news headlines.

There is a particular risk that children could fall victim to the misplaced aggression of a guard dog. Under no circumstances should a young child be left alone with any dog, since there is inevitably a risk of conflict. Even a very trustworthy dog may turn on a person if it is being hurt or teased. Selective breeding can also greatly heighten the aggressive response in some dogs, with American Pit Bull Terriers having become notorious in this respect.

Aggression resulting from the dog's guarding instincts is manifested in various ways. Typical examples are attacks on postmen or aggression towards an owner when a toy or other item is removed from them. The problem can be exacerbated by a failure to teach the dog to give up items on command. Hormonal influences are important since this type of aggression is usually only apparent once the dog has reached puberty.

There is also a specific condition affecting some bitches called pseudo-pregnancy, which can result in displays of unexpected aggression. This results from an increased level of the hormone progesterone in the blood, which normally occurs as the result of pregnancy, but sometimes takes place without conception. The bitch may display all the typical signs of pregnancy, even to the extent of lactating, but actually has no puppies. Instead, she sees toys and other items, such as shoes and slippers, as her offspring. She will carry them around with her, and is likely to prove aggressive if you try to take them away during this phase. If your bitch is not neutered, you should be aware of the possibility of this behavioural change occurring soon after the time when she would normally have given birth, just over two months after her last heat. This phase should soon pass. There is a strong possibility, however, that phantom pregnancies will recur at successive heats. Spaying (sterlizing) may be the best long-term answer, especially if you have a young family, but hormonal treatment might be used when the symptoms first emerge.

According to a British study published in 1984, at least three-quarters of all dog owners look to their pets to provide some security around the home, but proper training is required to ensure that your dog does not become a dominant tyrant at home. Again, it will be much easier to achieve the desired response with a young puppy. While the sound of the doorbell or knocker can trigger the dog to start barking, this can be followed by aggression towards the caller.

You may want your dog to bark initially, to alert you to the fact that someone is at the door. As soon as you hear the dog barking though, you must tell it to be quiet, and make it sit away from the door. You should follow this by commanding the dog to 'stay' when you actually open the door. Apart from the risk of causing injury to your visitor, the dog might rush out into the road. This can be a particular problem with hounds, which tend to be less territorial than some other breeds.