Current advice re desexng is that the dog should be fully grown as the sex hormones play a role in growth and joint health. For all but giant breeds this is about 12 months old. Giant breeds eg great danes newfoundlands etc, it is 18- 24 months old . Hormones play a role in the growth of some cancers and the prevention of others, however it is important that dogs are not allowed to breed indiscrimately due to the vast numbers of unwanted pets.
MALES
From a behaviour perspective testosterone does not cause aggression- BUT it does INCREASE whatever your dog will do when challenged . Some people argue that testosterone increases confidence and therefore reduces aggression . In an non aggressive dog ,yes - BUT most dogs will respond to a challenge with AGGRESSION. Overconfidence will also lead to faulty risk assesssment and may cause a male not to back down from a challenge . Testosterone makes an animal less cooperative . An intact male will be more determined in a fight because more testosterone is secreted when challenged and this will increase whatever is the animal's response to being challenged . If that's aggression you are in trouble ! It also increases energy , motivation,and dopamine release ( released in anticipation of the reward , not the reward itself ) which will make your dog fight harder and inflict more damage . In addition I find in my practice the dogs who persistently hump are usually intact males, intact males are more likely to aggress to owners when removed from something they want, and are harder to train because they are more interested in the environment( rival / mate ) and less interested in their owners .
FEMALES
Neutering a female makes little difference to aggression in dog s who live together because hormones and neural transmitters are produced in other organs besides the ovaries ( eg gut, adrenals ) . Oxytocin ( along with other hormones eg estrogen progesterone and testosterone ) not only works to increase mother/ baby bond and bonds with partners but also to increase aggression towards those "not us "- eg a new dog who doesn't belong, or a rival for resources. Therefore fights between females are usually much more aggressive, and in the case of dogs who live together much harder to resolve ,than fights between neutered males ad male / female pairs .
If your male dog is aggressive to other dogs I find desexing will usually help and it is irresponsible to breed indiscrimately .
