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Knowing the age dogs start fighting is important regardless whether you are a dog owner or a professional working with dogs. 

Many dogs get along well with other dogs for most of their puppyhood, and then as they mature, things start changing. 

While every dog is different, studies have revealed that dog fights tend to occur the most at a certain age. 

"Fights" Among Puppies 

Puppies (under the age of 6 months) are often reported to "fight." In many cases, this isn't  real "fighting," just really rough playing. 

However, there can be cases, where the puppies really fight over things like toys and food, but luckily, in most cases, these little squabbles are more noise than anything. 

More serious issues may be seen in cases of "puppy sibling rivalry" where two puppies close in age get into squabbles, often with one puppy repeatedly bullying another puppy. 

We also need to consider that, in certain cases, there may be a genetic predisposition to dog aggression.

There have been reports of puppies attacking other puppies from a young age (even as young as 6 weeks) and requiring to be separated. 

However, not all is doom and gloom in these cases. These puppies in the rights hands, may grow up pretty tolerant, doing decently with other dogs. 

Can Puppies Be Born "Dog Aggressive?"

This opens up the famous nature versus nurture debate, basically, the role of genetics and the environment in which dogs are raised. 

In other words, can puppies be born to be naturally dog aggressive, or are they made this way?

The answer is that, yes, it is possible for some puppies to be born dog aggressive. 

There have been instances of puppies manifesting seriously aggressive behavior towards other dogs at the age of 6 weeks.

Even in expert hands, these dogs may remain unpredictable and volatile when around other dogs, requiring constant management.

These dogs may be mentally challenged, up to the point of being the canine version of sociopaths. 

Patricia McConnell, expert in dog behavior, refers to them as "Jeffry Dalmer Dogs" in her book" The Other End of the Leash."

Did you know? Stress during a mother's dog pregnancy can impact the puppies and subsequently impacts their future development. This is discussed in depth here: how mother dog's stress impact puppies. 

Often these dogs are believed to be the instigators, when in reality they are defending themselves.

Often these dogs are believed to be the instigators, when in reality they are defending themselves.

The Impact of Negative Experiences 

Dogs can really become dog aggressive and start fighting at any age if they endure trauma or negative experiences. 

All it takes is feeling threatened enough. If another dog bullies them or attacks them, they may learn that "offense is the best defense." 

Often these dogs are believed to be the instigators, when in reality they are defending themselves and objecting to becoming the victim of "domestic abuse," so to say. 

The Onset of Medical Conditions

Medical conditions need to be factored in as well, when it comes to dogs starting fights with other dogs out of the blue. 

If your dog has been rather peaceful for most of his life, and now he is attacking, consider having him see a vet and have full physical, blood panels and thyroid tests done (CBC, serum chemistry profile, total thyroxine).

If you have a dog being attacked, do the same, because sometimes dogs who are being attacked out of the blue may be ill.

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The Age Many Dogs Start Fighting With Other Dogs 

Many cases of intrahousehold interdog aggression, that is, fights among dogs sharing the household, occur when the dogs reach social maturity. 

In general, dogs reach social maturity between the ages of  24 to 36 months. 

This is a time when the dog's brain neurochemistry changes. 

It is also a time when dogs sharing the household start fighting over toys and attention, points out board-certified veterinary behaviorist Dr. Meredith Stepita. 

"If the behavior started at a young age, there could be a stronger genetic predisposition to engage in the behavior, compared to another animal for which the behavior started at social maturity. "~Melissa J. Bain, board-certified veterinary behaviorist. 

The Impact of Aging 

As dogs age, they may become more intolerant of other dogs, especially young, boisterous puppies. 

This may lead to growling and other distance-increasing signals. 

In many cases, the fact that older dogs suffer from joint issues, makes it more difficult removing themselves from obnoxious puppies pestering. 

With no other option, they may decide to therefore use "aggressive displays" to ask for space and being left alone. 

Puppies may not take these requests seriously, or their drive to play may be too strong. 

This may lead to more and more intense distance-increasing behaviors being displayed, potentially even leading to snarling, snapping and biting. 

It goes without saying the importance of seeing the vet. When a problematic behavior takes place in an older dog, there is a higher likelihood for there to be an underlying medical condition, further points out Dr. Bain. 

How are Fighting Dogs Separated? 

There are several methods employed. You want to avoid as much as possible direct physical interventions such as grabbing the dogs by the collar or pulling them apart as this puts you at risk for a redirected bite.

Safer options include clashing two pot lids together, blowing a small maritime air horn, or ringing a doorbell to distract the dogs from each other.

How is Intrahousehold Interdog Aggression Treated?

When dogs start fighting, it's important to seek intervention as soon as possible. Without intervention, there are risks for things to escalate. 

It's always best to nip issues in the bud. More and more vets who treat bite injuries are starting to emphasize the importance of seeking professional help, considering that, that first bite is a red flag for future similar or worse incidents to occur. 

A good place to start is to rule out medical problems and work with a board-certified veterinary behaviorist or Certified Applied Animal Behaviorist.

Once medical issues have been ruled out, these professionals can devise a plan often involving management and behavior modification using desensitization and counterconditioning. 

Medications may be prescribed, but it's only a part of the treatment plan. 

No punishment or other aversive techniques should be used. According to studies, the use of harsh, confrontational methods only exacerbate aggression.

 If you happen to punish a dog for growling at another dog, you risk him going for a direct bite next time as you have suppressed his early warning signals. 

References:

American College of Veterinary Behaviorists, Behavior Tips: Choosing the right dog for the right family, by Meredith Stepita, DACVB

Psychology Today, Risk Analysis of Behavior Problems: Animal Factors, 2, Melissa J. Bain, DVM, DACVB, MS, DACAW.

Feltes, E., Stull, J. Herron, M. and Haug, L. Characteristics of intrahousehold interdog aggression and dog pair factors associated with poor outcome. American Veterinary Medical Association website, accessed 6/2020.

Dodman, N. (2011) Interdog household aggression: 38 cases. Journal of the American Veterinary Medical Association 238:6, 731-40.

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