Testosterone is the primary hormone found in male cats, or toms. This hormone greatly affects the tom's behaviour, such as becoming aroused when scenting a female cat's pheromones. Neutering the tom can not only prevent unwanted kittens, but also change undesirable habits.
One theory is that a kitten inside its mother's womb becomes male when a surge of testosterone floods its brain. This programs the kitten to behave in a masculine way and grow masculine traits, such as getting jowls, according to the "Cat Owner's Home Veterinary Handbook."
More testosterone is produced as a result of longer daylight hours in spring and summer, triggering the tom's sex drive. This also triggers more production of sperm.
Testosterone also makes a tom mark his territory by spraying bad-smelling urine to drive away other males and advertise his availability to females. After neutering, 90 per cent of toms stop spraying.
Vasectomies in a tom cat will make the tom infertile but will not stop behaviours like growing jowls, mating, fighting other toms or spraying. This is because the testicles are still able to produce testosterone.