Territorial behavior, anxiety and other medical issues lead dogs to bite children, a new study shows.

To see if there were any common links among dogs who had bitten a child within a particular four-year period, researchers examined 111 cases of dog bites by 103 dogs, all referred to the same veterinary behavior clinic in Philadelphia. They found several distinctive behavior patterns that related to the dogs' territorial behaviors, and suggested that these were the main causes of aggression in dogs:

Young children (under 6 years) were more likely to be bitten when a dog felt the kids were threatening to take the dogs' food or toys.Older children were bitten when the dog felt the kids were encroaching on its territory.Children familiar to the dog were more likely to be bitten while the dog was guarding its food.Unfamiliar children were more likely to be bitten while the dog was protecting its territory.These behaviors were seen in many different breeds of dogs. (A total of 41 breeds were represented in the study.)

Three quarters of the biter dogs studied exhibited anxiety, either by being left by their owners or being exposed to some loud noise, such as a thunderstorm or fireworks. Young children in particular tend to be noisy and make unpredictable movements, which could frighten an already anxious dog and cause them to bite the child, the researchers said.

Half of the dogs also had medical conditions, such as eye problems, liver and kidney disease, and diseases that affected their bones and skin. Study leader Illana Reisner of the University of Pennsylvania and her colleagues suggest that pain from these conditions could have pushed the dogs over the edge, causing them to bite.

The study's results are detailed in a recent issue of the journal Injury Prevention.

Video: Extraordinary Dogs10 Amazing Things You Didn't Know about AnimalsDogs Get Found, Cats Stay Lost