My dog Zeus (intact) has recently became aggressive with our other dog Bingo (neutered). I think there is a dog around the block that is in heat but I'm not sure. Bingo is a 12 years old standard poodle that weighs 75 lbs and Zeus is 3 1/2 Colorado Bulldog that weighs 120 lbs. Zeus has been in the family since he was 8 weeks old. His behavior is so off. We have never experienced this aggression until now. Do I take him to get neutered and hope his aggression stops?

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This answer is part of Pet's Spring Cleaning Campaign. This question is old, but this answer will still help people with the same problem.

Many people think that neutering will reduce aggression in male dogs, but in fact the opposite is true.

Neutering reduces behaviors like urine marking and roaming in search of a mate, but it increases aggression - especially towards strangers - in many dogs and makes them harder to train.

Please have a look at this very similar post that summarizes a few scientific studies about aggression in dogs (that were not neutered with the goal to reduce aggression, so the bias of the owners is relatively low). The gist of it is

Among the findings, neutered dogs were more aggressive, fearful, excitable, and less trainable than intact dogs.

If there's a female in heat nearby, the pheromones can cause or increase excitement in intact males, which may express itself as aggression. But usually that would only be directed towards another intact male.

Another possibility is that your dog is in pain, which causes him to keep others away. If he shies away from your touch or becomes lethargic, please get him checked by a vet.

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