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I have 3 dogs: a female Ellie (7 yrs), male Blake (2 yrs), and male Bosco (9 months). Blake was neutered a few months ago and since then has gone after Bosco completely unprovoked. Blake is half the size of Bosco but their fights are bad. Bosco hates the confrontation and tries to get away but Blake just keeps going after him. Luckily, neither of them had sustained any serious injuries.

Blake was very territorial over my sectional. It was his couch and his thing to hump. I got rid of it hoping that would help and it didn’t. Blake spent some time away at my Mom's and was perfect. I tried to reintroduce them in my backyard and he immediately went after Bosco. There weren’t any issues prior to Blake being neutered. I am having to keep them completely separated.

Will neutering Bosco help or is this behavior continue? The thought of having to rehome one of them is unbearable. I’ve had them both since they were 8 weeks old.

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I'm really sorry to hear that. Usually, we expect neutering to make a dog calmer and less aggressive. But unfortunately, several studies found that neutering increases territorial aggression (for example against strangers and delivery workers approaching or just passing by the home) and anxiety in dogs.

This article has a summary of 2 studies;

The number of dogs tested in these two studies is quite large. The Duffy and Serpell study tested two different samples, one of 1,552 dogs and the other of 3,593 dogs. The Farhoody study tested 10,839 dogs, thus the combined studies provide data on 15,984 dogs in total, making this an amazingly powerful data set.

There are too many measures for me to report in detail, however, the main results were the same across all three samples of dogs. Given that one of the accepted behavioral reasons for spaying and neutering is to reduce aggression, the distressing results of these studies are that spayed and neutered dogs actually show considerably more aggression. Depending upon the specific form of aggression (owner directed, stranger directed, etc.) the size of these effects is quite large, varying from a low of around a 20 percent increase to more than double the level of aggression in the neutered dogs as measured by the C-BARQ scoring scale.

Farhoody summarizes her findings saying, "Our data showed that the behavior of neutered dogs was significantly different from that of intact dogs in ways that contradict the prevailing view. Among the findings, neutered dogs were more aggressive, fearful, excitable, and less trainable than intact dogs."

The article references a study (from 2006) and a masters thesis (from 2010) as sources, but I couldn't find any of them online. However, the team that worked on the study published a new study in 2018: Aggression toward Familiar People, Strangers, and Conspecifics in Gonadectomized and Intact Dogs

So, the "problem" is Blake and neutering Bosco won't change Blake's behavior. The best advice I can give you is to seek the help of a professional dog trainer. Not one of those puppy schools, but a trainer who specializes in dog psychology and training of problematic dogs. Even though Blake is the "problem" at the moment, you might be the solution. The dog trainer should teach you how to recognize signs of aggression in him (there are a lot of those signs and they start way before any growling or attacking) and how to redirect his attention away from the aggressive behavior.

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