What does neutering actually change?

It removes the source of the sex hormones, and most of the effects — good and bad — follow from that rather than from the surgery itself.

What the procedures are. In males, castration removes the testicles. In females, spaying removes the ovaries, and usually the uterus as well. Both are performed under general anaesthetic and are routine.

What changes reliably:

Reproduction stops.

Seasons stop in females — no bleeding, no attracting males, no phantom pregnancies.

Certain diseases are prevented or made far less likely. Pyometra, a serious uterine infection common in unspayed older bitches, becomes impossible. Mammary tumour risk falls substantially when spaying happens early. Testicular cancer becomes impossible; some prostate conditions become less likely.

Roaming and mate-seeking behaviour reduces in males, with the associated road-traffic and fighting risk.

Metabolic rate falls. This is real, predictable, and the main reason neutered animals gain weight — the food requirement drops by a meaningful margin and portions frequently do not. Weight gain is a management consequence, not an inevitable one.

What changes less than people expect:

General temperament. Neutering does not make a dog calm, obedient or less energetic. Behaviours driven by fear, excitement, learning or lack of training are unaffected — only hormone-driven behaviours change.

Aggression. The evidence is mixed and depends heavily on the type of aggression. Some studies find no reduction, and some find increases in fear-based reactivity in certain cases.

The genuine trade-offs now discussed more openly:

Timing matters, particularly in larger dogs. Sex hormones influence growth plate closure, and early neutering in some large breeds is associated with increased rates of certain joint disorders and cancers.

Urinary incontinence in some spayed bitches.

Coat changes in some breeds.

Discuss timing with your vet for your specific animal — the blanket recommendations of twenty years ago have been refined, and breed and size now factor into the advice.