Neutering is the removal of a male dog's testicles in a surgical procedure also known as castration. It's the primary means of ensuring the dog can't reproduce. Besides controlling the dog population, other benefits include the reduction of the hormone testosterone and with it associated bad behaviors, and the lesser likelihood of certain cancers.
Testosterone and Neutering
Testosterone is the male sex hormone produced by the testes in large amounts and by the adrenal gland in considerably lesser amounts. Neutering, or the removal of the testes, reduces testosterone production. Increased testosterone levels are responsible for many bad behaviors, which are seen less frequently in the neutered male. Dogs as young as 8 weeks of age can be safely neutered. Early neutering makes for a faster recovery from surgery and reduces the likelihood that bad behaviors related to high levels of testosterone will become habitual.
Straying Versus Staying
Neutering restricts the production of testosterone, which is linked to urine marking. Urine marking is the method by which unneutered dogs advertise their presence to receptive females. Marking can occur both inside and outside your home. At least as worrisome is the likelihood that an intact dog will roam in search of a female companion. Countless male dogs stray from home and are killed on roadways while seeking a mate in heat.
No Good Fight
According to author James O'Heare of the Association of Animal Behavior Professionals, testosterone spikes and affects the male dog intensively prior to birth and at puberty, and because of this, behaviors associated with high levels of testosterone don't normally disappear entirely. Aggression and reaction to stimuli tend to diminish in neutered dogs. High testosterone levels in your dog may make him a target for other intact males. Fighting over receptive females will be significantly reduced or may disappear entirely once the male dog is neutered, but not all urges to fight are sexually motivated, and the urge to fight may remain. Similarly, not all urges to mount are sexually motivated. Mounting is often seen in play.
The reduction of testosterone by neutering may protect the male dog from testicular and prostate cancers. Additionally, high testosterone can cause enlargement of the prostate, which may make it difficult for your dog to urinate. The medical benefits of neutering should always be discussed with your dog's veterinarian.