Dog Behavior Problems: Aggression Between Household Dogs Part 1: Assessment

By Ellen Lindell, VMD, DACVB; Debra Horwitz, DVM, DACVB & Gary Landsberg, DVM, DACVB, DECAWBM

Why would dogs fight with a familiar dog living in the same home?

Dogs are complex social animals. Though their wolf ancestors lived in social groups with other wolves, dogs live in social groups that include other dogs and humans. Accordingly, our social dogs have developed communication skills easily recognized by other dogs and by the humans sharing their lives. Dogs routinely use body language to diffuse tension and avoid physical conflict, though some dogs and dog breeds are more adept at communication.

In addition to having a range of communication skills, individual dogs have unique personalities that are partly determined by their genetic makeup. People try to choose a dog whose personality and breed suit their own personalities and lifestyles.

Unfortunately, when a second (or third) dog is introduced to a household, that dog’s personality may not always suit the personality of the resident dog. Sometimes, the result is that the two dogs live side by side and rarely interact, but they do not necessarily fight. Other times, the dogs may exhibit more obvious and severe aggression, particularly if subtle communications have been repeatedly ignored. Unlike their wolf ancestors, who could leave a group where they were not welcome, dogs are bound by their home’s walls, and aggressive conflicts can quickly escalate.

"Unlike their wolf ancestors, who could leave a group where they were not welcome, dogs are bound by their home’s walls, and aggressive conflicts can quickly escalate."

Why did my dogs suddenly start fighting after all this time?

Sometimes, it does seem that fighting begins out of the blue—and that may be the case. Let’s explore some reasons for this.

The first consideration relates to the nature of canine communication. Dogs often signal subtly that, unless you are a trained behaviorist, it may be overlooked until it becomes overtly aggressive (see the handout “Canine Communication – Interpreting Dog Language” for more information). The physical attributes of some breeds render subtle signaling more difficult—large prominent eyes appear to be staring, and short faces make snarling nearly impossible.

If subtle signals are not appreciated, early interventions that might have prevented escalation are not implemented (see the handout “Dog Behavior Problems – Aggression Between Household Dogs Part 2: Treatment” for more information). For example, maybe one of your dogs started to avoid a particular area of the home or changed their favorite resting place and did so to avoid a conflict. Maybe one of your dogs began to approach and stare at your other dog—if either dog then walked away, there would be no growl.

Within any relationship, communication is critical. If the signal recipient does not behave as expected, or if the sender’s message is perceived as a threat, there can be frustration, which can progress to physically overt aggression, such as growling, snapping, or biting.

Sometimes, there is no subtle communication, and physical aggression does begin relatively suddenly. Aggression between household dogs often begins when the younger dog begins to mature socially, somewhere between one and three years of age. From an evolutionary standpoint, a young maturing dog can represent a social threat and reduce an older dog’s access to valuable resources. In the home, even with plenty of resources, some older dogs exhibit an aggressive response in anticipation of this perceived threat.

Alternatively, as dogs mature socially, their personality traits become more evident. Many common behavioral concerns, including fear, anxiety, and frustration-related behaviors, become more evident at social maturity and can underly an overly aggressive response. The younger dog may be initiating confrontations to gain access to resources, though that is just one possible trigger.

Regardless of which dog appears to be the aggressor, both dogs should undergo a thorough behavioral assessment to determine whether any concurrent behavioral conditions may affect the nature of the dogs’ interactions.

"Regardless of which dog appears to be the aggressor, both dogs should undergo a thorough behavioral assessment to determine whether any concurrent behavioral conditions may affect the nature of the dogs’ interactions."

An underlying medical condition in either dog can change the nature of the interactions. Pain, inflamed skin, loss of mobility, loss of ability to digest food, and loss of vision or hearing are among the many physical changes that can create an aggressive response in either the dog with the medical condition or the housemate. If you suddenly notice your dogs are growling, snapping, or biting, the first step is to have them both examined by your veterinarian.

A change in the environment that may seem unrelated to the dogs could trigger a sudden onset of aggression. The change may be related to reduced access to a favorite resting place, leading to competition to establish a new cozy spot. If one of your dogs is excited when people or dogs pass by the house, then access to a new window could trigger an aggressive, redirected response toward the other dog. If one of your dogs suffers from fear of noises or separation-related distress, increased exposure to the noise (e.g., thunder) or a change in your routine could increase anxiety and lead to aggression.

Try to think of any changes that have occurred. Helping your dog tolerate triggers will be necessary for successfully mending the relationship between the dogs.

Could my dogs be fighting because of a dominance struggle?

The concept of dominance in a relationship is complicated and is not often a helpful construct when trying to keep harmony in the home. A dominance hierarchy serves to create predictability and prevent the need for physical confrontation. In most cases, a hierarchy is organized quietly, without physical fighting. Many household dogs do not establish a clear hierarchy. If your dogs are fighting, a veterinary behaviorist will be able to determine whether the aggression relates to a failed attempt to form and maintain a dominance hierarchy and will be able to treat the behavior accordingly. In most cases, another reason for the aggression will be discovered.

What are the common triggers for a fight?

There are some common triggers for fights among dogs in the home. Dogs often fight over access to a resource that is perceived to be valuable. This could be a favorite bone or toy, a special person, a resting space, or even a dog food bowl. Aggressive behavior in these contexts is referred to as resource-based, possessive, or food-based aggression (see the handouts “Possessive Aggression in Dogs” and “Food Bowl Aggression in Dogs” for more information).

"Dogs often fight over access to a resource that is perceived to be valuable."

Some dogs fight when moving through or contained within a tight space or during rough play. This aggression may be excitement-related or related to frustration or fear.

Finally, some dogs become frustrated and behave aggressively toward a housemate dog when they cannot access something, such as a person or animal viewed through the window. This is known as redirected aggression.

It is important to try to identify as many triggers as possible. Watch for subtle signs of aggressive communication in these contexts, even if there has never been an overt fight. Some trigger situations must be managed carefully for safety during treatment and possibly long-term. Sometimes, it can be challenging to identify a trigger. Do your best to describe the contexts in which your dogs have exhibited aggression toward each other. A complete medical and behavioral assessment will be critical if no trigger is found.

Are there common medical conditions that can trigger fighting?

When dogs age or become ill, their relationship with other household dogs may change. A dog that is ill may respond differently when faced with certain stimuli. For example, medical conditions that lead to pain and loss of mobility may cause a dog to become aggressive when approached. Aging dogs often experience a change in their hearing and may bark more often, possibly startling the younger dog and triggering an aggressive response. Effectively, the dog’s behavior is no longer predictable to the housemate.

Though many dogs readily adapt to changes in how their companion dog looks, smells, or moves, other dogs may become anxious and respond aggressively.

How do I find out why my dogs have been fighting?

Managing the relationship between two dogs is complicated. Both dogs need complete medical and behavioral assessments to treat underlying conditions. Your veterinarian can begin the work up. Once some basic testing has been done, a veterinary behaviorist or skilled behavior consultant should continue the evaluation.

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