4 August 2011

Castration ~ Effects on male dog health and behaviour

Owners of entire or ‘intact’ male dogs that exhibit any number of behaviour problems are frequently told by dog-owning friends, dog trainers, breeders and vets to have their dogs castrated.  Castration is the removal of the testicles (testes), rendering reproduction impossible.  Neutering is also a term commonly used for the castration of male dogs.

Castration is considered to be a routine surgery, but the procedure is not without risk.  As with other surgeries, there is the risk of reaction to anaesthesia, excessive bleeding, bruising and infection.  Occasionally I hear the sad story of an unfortunate dog who, unbeknown to his owners and vet prior to castration surgery, had a blood-clotting disorder and died as a result of the procedure.  On the whole though, for healthy dogs, the prognosis of castration surgery itself is very good, however, medically, castration has only a handful of plus-points.  Removal of the testes eliminates the very small risk (<1%) of death from testicular cancer, and reduces the risk of perianal fissures and non-cancerous prostate disorders.  That’s it – no other known health benefits.

On the medical, negative side, castration carries a number of heath risks.  More and more vets and dog trainers are recommending that male dogs, regardless of whether a behaviour or health related problem actually exists, are castrated at around 6 months of age.  This in itself carries a serious risk to long-term health because if removal of the testes is done before 1 year of age, this significantly increases the risk of osteosarcoma (bone cancer) – a common cancer in medium-large breed dogs with a poor prognosis.  Regardless of the age at which castration is carried out, removal of the testes quadruples the small risk (<0.6%) of prostate cancer, triples the risk of hypothyroidism (under-active thyroid function) and obesity, doubles the small risk (<1%) of urinary tract cancers, increases the risk of cardiac haemangiosarcoma (cancer of the heart) by a factor of 1.6, and increases the risk of orthopaedic (bone) disorders, adverse reactions to vaccinations, and progressive geriatric cognitive impairment (senility).

So, with the number of health cons associated with castration (including the actual surgery risks) far outweighing and exceeding the health pros, castration as a preventative for future health problems is a highly questionable practice, particularly so for immature male dogs.  Obviously if a dog actually has a testes-related medical problem, castration may be the best option, but it shouldn’t be viewed as some kind of preventative ‘cure all’ for otherwise healthy dogs, because it’s not.

The testes themselves are glands, and part of the endocrine system.  The endocrine system is a collection of glands and organs situated throughout the body that produce and secrete various hormones.  The secreted hormones are carried in the bloodstream to target organs and cells, where they exert an effect of some kind.  Some hormones affect and regulate just a single organ, while others affect many cells throughout the body.  Broadly speaking, hormones regulate cell metabolism, change or maintain enzyme activity in receptor cells, and control growth, development, metabolic rate, sexual rhythms and reproduction.  So by using hormones as chemical messengers, the endocrine system regulates numerous bodily functions.

A dog’s testicles perform two major functions – the production of sperm, and the production of the hormone, testosterone.  The role of sperm is clear to all who know how puppies are made, but the role of testosterone is less so, and often misunderstood too. 

For a male dog, the story of testosterone starts in the womb when his brain is organised by a surge of testosterone just prior to and after birth.  This neural organisation becomes most evident at puberty, when at between 6 and 8 months another surge of testosterone occurs.  Under the influence of this surge, the dog starts to ‘fill-out’ as his muscles develop and define, and the emergence of ‘sexually dimorphic’ behaviours associated with maleness are seen.  Sexually dimorphic behaviours are urine marking, roaming and inter-male aggression due to the scents/appearances of in-season bitches, and actual mating.  The appearance and expression of the these behaviours is variable, with maternal stress appearing to influence foetal changes associated with androgenisation – how ‘male’ a male dog may turn out to be. 

Testosterone and its metabolites interact with various neuropeptides.  In terms of the expression of male characteristics, it’s testosterone’s interaction with another hormone, arginine vasopressin – AVP for short – that is most important.  Like the male behaviours mentioned above, the production of AVP is also sexually dimorphic, with male animals producing more AVP than female animals.  In the presence of testosterone, AVP is the driver for urine marking and inter-male aggression – behaviour that is aimed at repelling or removing sexual competition – whereas testosterone is the driver for roaming and for mating.

On the behaviour side, castration only affects sexually dimorphic behaviours, so to recap, that’s urine marking, roaming and inter-male aggression due to the scents/appearances of in-season bitches, and actual mating. 

Castration does not directly affect mounting, humping or pelvic thrusting behaviour, which can continue excessively and at pre-castration levels indefinitely.

Mating aside, which can actually take up to 2.5 years to cease, castration is most effective in decreasing roaming behaviour, which rapidly declines in 44% of dogs and gradually declines in another 50%.  Urine marking rapidly declines in 30% of dogs and gradually declines in another 20%.

Inter-male aggression rapidly declines in 38% of dogs and gradually declines in another 25%.  So for 37% of dogs, castration has no beneficial effect on inter-male aggression at all, and in a (as yet undefined) percentage of those dogs, the incidences of inter-male and other types of aggression (e.g. territorial, defensive, possessive) can either increase, or in some dogs can appear out of the blue, where prior to castration the dog showed no aggression at all.  This is because when the production of testosterone is interfered with, if this also coincides with or causes an individual dog's HPA circuit chemistry to change and emotional tolerance thresholds to become lowered for whatever reason – this could be due to naturally high levels of AVP, low levels of serotonin (there is a close interaction between AVP, testosterone and serotonin in the regulation of agonistic behaviour), stress, pre-frontal deficits, temperament type, reactive coping style (as opposed to adaptive coping style), etc – the regulating effect on the hypothalamus – the aggression 'centre' of the brain – is lost, and this can give rise to all manner of defensive and threat behaviour such as dog-dog aggression regardless of the sex or sexual status of other dogs, territorial aggression, extra-familial aggression and resource guarding. 

For the 63% of dogs for whom castration does work to decrease inter-male aggression, perhaps either the removal of testosterone doesn’t adversely disrupt the rest of the dog’s individual brain chemistry, or naturally low AVP and/or high serotonin levels are present.  For these dogs, the effect on AVP-driven inter-male aggression is very direct – any AVP in that part of the brain simply becomes ‘inactive’ in the absence of testosterone.

Will castration stop my dog from escaping from the garden or running off?  If the reason for escaping or running off is the scent of an in-season bitch, there is a 94% overall chance that these behaviours will decrease as a result of castration.  If your dog is escaping because he is bored, because he is seeking stimulation, because your property fencing has dog-sized holes in it, because he is running off after other dogs because he wants to play, because he is chasing wild-life, because you have not trained him well enough in recall, then no, castration is not the answer.  The solution lies in increasing your dog’s exercise, enriching his environment, giving him more of your attention so that he has no requirement to seek stimulation elsewhere, mending or raising your fences, and training and proofing his recall.  Although castration for roaming behaviour has a high success rate, the removal of testosterone may give rise to other behaviour issues (see Will castration stop my dog from being aggressive towards other dogs?).

Will castration stop my dog from peeing every 5 seconds when we’re out for a walk?  If the reason for your dog’s peeing up every blade of grass is because he is trying to repel sexual competition, there is a 50% overall chance that this behaviour will decrease as a result of castration.  If your dog is constantly marking because he is stressed, anxious, insecure, has a UTI or prostate infection, or because it’s a habit, then no, castration is not the answer.  The solution lies in getting your dog less stressed on a walk, working to keep his attention on you, taking him to the vet to check for health problems, or walking him on the other side of you or further away from the urine marks of other dogs that are constantly coming into his path on lampposts, trees, walls and bushes.

Will castration stop my dog from peeing inside the house?  Again, if the reason for your dog’s marking behaviour is because he is trying to repel sexual competition, there is a 50% overall chance that this behaviour will decrease as a result of castration.  Contributing factors to repellent urine marking could include owning other male dogs, having other dogs regularly visit your home, or the presence of in-season bitches in your local area.  If your dog is marking objects inside the house because he is stressed, anxious, insecure, has a UTI or prostate infection, because it’s a habit, because he is territory marking, because he is not fully house-trained, because you are not scrupulous in your cleaning, then no, castration is not the answer.  The solution lies in getting your dog less stressed at home, increasing his trust in you, taking him to the vet to check for health problems, maybe limiting his access to certain objects and areas, going back to basics with house-training, and cleaning up properly.  Even for a persistent, repellent urine-marking problem, proper cleaning in itself can solve this if every last trace of urine is removed, so scrupulous cleaning using an enzyme-based cleaner should always be the first approach (along with checking for health problems).

Will castration stop my dog humping?  Although humping behaviour is liberated from the sexual system, it is not sexually dimorphic or driven by testosterone.  Some owners do report a reduction in their dogs’ humping after castration, but this is probably because the removal of testosterone lowers the dog’s sensitivity and excitement to the sexy smells of bitches and so the associated energy and frustration that being unable to fulfil his urge to mate is also removed.  Increasing a dog’s exercise, not allowing him access to humping ‘accessories’ such as cushions, and if his humping is compulsive or triggered by social change or stress, providing an alternative activity (e.g. a stuffed Kong), can all help to reduce and often eliminate humping behaviour.  

Will castration stop my dog from being aggressive towards other dogs? 
If the reason for your dog’s aggression is to remove sexual competition, there is a 63% overall chance that this behaviour will decrease as a result of castration.  If your dog’s aggression towards other dogs is indiscriminate (includes neutered males and entire and spayed females), is towards people (familiar or unfamiliar), other animals, occurs when touched, disturbed, displaced or punished, or is defensive, unpredictable or possessive in nature (toys, food, you, etc) then no, castration is not the answer.  Aggression problems of any kind are often complex and should always be evaluated properly by someone with a sound knowledge of aggressive behaviour before a decision as drastic as putting a dog under the surgical knife is made.  Bearing in mind that it’s AVP along with other factors such as environment, genetics, and other neurohormone activity (e.g. MAOA) in different parts of the brain, not testosterone, that drives aggression, castrating any dog, regardless of whether or not it shows inter-male aggression, can give rise to or increase aggressive behaviour in other areas.

If your dog is displaying inter-male aggression, before you decide to get him castrated, consider giving serotonin therapy a try.  Serotonin therapy is only really effective if a dog is entire, as the therapeutic benefits of serotonergic agents for the treatment of inter-male aggression and fighting appear to be facilitated by the presence of circulating testosterone.  There is a close interaction between AVP, testosterone and serotonin in the regulation of agonistic behaviour.  Providing moderate, daily exercise (e.g. 30-40 minutes of brisk lead walking per day) increases serotonergic activity in the brain, and increasing your dog’s dietary levels of serotonin’s protein amino-acid precursor, tryptophan, either via supplementation (e.g. Canovel Calmdown) or food, can help enormously (see my Mood Food article). 

The effectiveness of increasing dietary tryptophan depends on whether the dog is able to process the tryptophan properly, and how many free serotonin receptors are normally available and their whereabouts within the brain, so if a change isn’t seen within a couple of months, consider the drug therapy option.  In particular, the SSRI drug, fluoxitine, inhibits gender-related aggression by antagonising the action of AVP in the hypothalamus via serotonin receptors.  With regards to serotonin drug therapy, your vet will need to be willing to give this a try and, because fluoxitine is not a veterinary 'first choice’ SSRI drug in the UK, before being prescribed fluoxitine your dog may have to take a course of another SSRI drug such as Clomicalm, which will probably make his behaviour worse, simply because it does not provide the same chemical action within the brain as fluoxitine. 

Interestingly, a 2004 experiment into AVP in humans saw test subjects given a nasal dose of AVP and when shown pictures of neutral facial expressions, show a significant change in the activity of the corrugator muscle in the brow (which is involved in the expression of anger) suggesting that AVP mediates an aggressive bias in response to neutral and ambiguous facial expressions – no reason why this couldn't be the same for dogs with high AVP, particularly those who show impulsive-type aggression towards other dogs who are seemingly 'minding their own business'.

Basically, changing brain chemistry changes behaviour – it's just a question of hitting on the correct balance and delivery.  If AVP can be suppressed, aggressive behaviour will dramatically decrease, and I'm sure that every year, there are many dogs with seemingly unsolvable aggression problems that are put to sleep unnecessarily because of a lack of this kind of knowledge.

So in conclusion, please think long and hard before getting your dog castrated.  Consider the health risks.  Consider the behavioural implications.  If your dog is just generally a bit growly, hyper or distracted, ramp up your training efforts, increase or change his exercise, alter his diet, and make sure that you are channelling his energy constructively and appropriately. 

Testosterone gets blamed for far too much ‘bad’ behaviour, when in fact its effects are extremely specific.  Really, the only viable reason for castration is to cure an existing, testes-related medical condition, and for persistent, adult roamers – the ones who could escape from Fort Knox in the blink of an eye or who constantly seek out in-season bitches on walks – it may be the best option all round, taking into account the risks of a loose dog getting injured or killed, or causing an accident – of the traffic, and the making of puppies kind.


Stats and figures sourced from:


  1. Long bone growth is increased in dogs that are castrated before long bone growth is completed--depending on the breed that could be 2 yrs of age.

    Another option is to do a vasectomy on your male dog--but not till after bone growth is complete. At least then one does stop the chance of an accidental breeding.

  2. Thanks for your comment Helen, yes vasectomy is an option once a dog has reached adulthood, so that testosterone production continues but without the risk of accidental (fruitful) matings.

  3. Lizi - so lovely to read a counter to all the pressure to castrate. I have no problems with my 19 month old intact male, yet still I keep being asked if I have issues, whether I plan to castrate, whether he is bitching, etc. Yes he marks outside but never inside, no he doesn't hump, neither does he roam - but I have trained him from day 1, and I am on his case if he so much as considers anything inappropriate :-)

  4. Thanks for your comment Susan, it's so difficult when people keep asking why your dog 'hasn't been done yet', isn't it? Good for you for not bowing to the pressure - if your boy has no 'male' issues, in particular roaming with all its potential risks, then there is no good reason to castrate. The only thing I would add for owners who choose to keep their dogs entire is to do a 'plum check' every now and again, to check for lumps!

  5. Oh he is very happy for me to stock check billy and bertie ;-)

  6. Excellent article.

    I have a 5 year old intact male collie. I've had him since he was a pup. I have absolutely no problems with him either. He's never mounted a human or an inanimate object. He is non aggressive. He never urine marked inside my home. He's shown absolutely NO desire to run off. He gets along with every dog we meet, male, female, intact or neutered. He's the best dog I've ever had and he's also the only intact dog I've ever had.

    I am constantly asked when I'm going to neuter him and the answer remains the same: He'll be neutered when and if he has a medical problem and neutering is the cure. Period.

  7. Any thoughts? Our most beautiful gentle giant recently had to have his penis amputated & his only testicle removed, due to a blood clot! Crazy situation. He's thankfully now in good health, but uncharacteristically attacking his sister, any ideas what's going on and how do we sort it?

  8. Bordeaux girl, thanks for your comment. Glad to hear that your boy is now in good health but as to why he's attacking his sister, I can do nothing more than guess that it may be pain-related, or that the removal of testosterone has given rise to hormone-related irritable aggression, causing his emotional and social tolerance levels to drop. It may be worth having his thyroid function tested, and check for any UTI too.


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