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My home typically contains two or more dogs, and research has shown that having more than one dog is typical for nearly a third of dog-owning households in North America. In a multiple-dog home, one of the most disturbing situations is when there are aggressive incidents between the dogs. These are not only disturbing for the peace and of the humans living there, but it can be quite dangerous for the dogs and for people who try to intervene and break up the fight. A scientific report published in the Journal of the American Veterinary Medical Association looked at this issue, specifically assessing the characteristics of the dogs involved and what can be done to help eliminate the problem of fighting among dogs living together.

Researchers Kathryn Wrubel, Alice Moon-Fanelli, Louise Maranda, and Nicholas Dodman recruited 38 pairs of dogs that came to the Clinic at the Tufts University Cummings School of Veterinary Medicine in Grafton, Massachusetts, specifically because they were involved in aggressive incidents with their housemates. The research team then conducted in-depth interviews and administered questionnaires to determine the characteristics of dogs that had been involved in such situations. Later, they would prescribe a treatment method for the problem.

The first thing that might be surprising to most people is that female dogs are more often involved in such fights than are males. Only 32 percent of the aggressive incidents involved conflict between two males, while in the remaining 68 percent, females were active participants. This is consistent with previous research showing that when females get into an aggressive situation, injuries are apt to be more severe and the fights tend to be longer and more furious.

If we look at the overall characteristics of the dogs involved, we find that the instigator of the is usually the dog that has been most recently brought into the household (70 percent). Further, in 74 percent of the cases, it is the younger dog that starts the fight. These fights are often a surprise to the owners, since 39 percent claim that the dogs usually get along with one another most of the time. The conflicts can be quite intense; 50 percent required veterinary care for the dogs and 10 percent of them required medical for owners who tried to intervene. The owners placed themselves in jeopardy because 54 percent of them felt that the fight would not stop unless they separated the dogs, and only 8 percent successfully separated the dogs using learned obedience commands.

What tends to trigger a fight among housemates? The actions of the owner, such as paying attention to one dog rather than the other, are a trigger for 46 percent of the pairs. Simple excitement, usually involving the owner's arrival or other activities, was involved in 31 percent. Conflict over food was involved in 46 percent of the pairs, while found items or toys are triggers in 26 percent.

There appear to be a number of risk factors that the study isolated for one or both of the dogs. Among pairs of dogs involved in aggressive incidents, 41 percent had at least one member who had lived in multiple households. When at least one of the dogs in the pair was 12 weeks of age or older when adopted, the rate of conflict was 39 percent; dogs adopted from a shelter were involved in 33 percent of the cases, and dogs from pet shops in 16 percent.

There is some evidence that dogs involved in aggressive situations with the dogs they live with do have a tendency to show aggression in other situations. For example, 40 percent have shown aggression to other dogs, 27 percent have shown aggression toward humans living in the household, and 27 percent toward human strangers. Most distressingly, 20 percent have shown aggression toward their owners.

Aggression may not be their only problem since 50 percent of the pairs of dogs involved in conflicts had at least one member with noticeable , and 30 percent had phobias, fearfulness, or other forms of .

The good news is that aggression between housemates does appear to be treatable using behavioral techniques that owners can institute. The first is the technique that Nicholas Dodman calls "nothing-in-life-is-free." This simply requires the dogs to respond to some simple learned command (such as "sit," "down," "come") before they get any resource that they want including their meal, a treat, petting, attention, etc. The second involves "supporting" one of the dogs, meaning that the chosen dog gets everything first (food, treats, attention etc.).

Here, the problem is which dog to select, and a pragmatic way of doing this is to choose the dog that is larger, stronger, healthier, more active, etc. An alternate method—which seems to fit with human notions of priority, deference, and respect—is to select the "senior" dog, in which the "senior" dog is the one that was in the household first, and has lived with the owner the longest.

Both methods work, but not instantaneously; on average the data shows that noticeable improvement does not occur until more than five weeks after the process starts. The "nothing-in-life-is-free" technique produced improvement in 89 percent of pairs, while the "senior support technique" produced improvement in 67 percent. The researchers suggest that these techniques work for two reasons: First, the dogs must act in a controlled manner, this takes some of the excitement and arousal out the situation. Second, events occur in a predictable order, the dogs learn that each of them will eventually get what they want and no conflict is needed.

It is important to note that the of the dogs not only makes a difference in the likelihood of conflict but also in the likelihood of improvement with behavioral treatment. As we noted at the beginning of this article, female dogs are more likely to engage in conflict with housemates, and their fights are apt to be more serious. This is consistent with the fact that the improvement with behavioral treatment is found to be less pronounced, although still significant, in female pairs. In male-male pairs, conflict was reduced in 72 percent of cases, while for male-female pairs, the reduction was 75 percent. In female-female pairs, the reduction success rate was only 57 percent, which, while not as large as in the other pairings, is still a reasonable improvement rate and well worth the effort.

For more about canine aggression, click here.

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