This is a really common question, and (as usual…) the answer isn’t straightforward. In general, however, neutering will have no effect on your dog’s personality, but it may influence his mood and make some behaviours more or less likely.

What are the effects of neutering?

Castration (surgical neutering for male dogs) involves the removal of both testicles. This obviously eliminates the dog’s fertility (as he can no longer make sperm), but it also stops him from making the male hormone, testosterone. Testosterone has a wide range of effects, including producing of secondary sexual characteristics during puberty (such as a deeper bark, increased muscle mass, bigger bones, and larger head - just like in humans!). However, the effects of testosterone on behaviour are much more subtle.

As a puppy, before and just after birth, testosterone levels are quite high, “pre-programming” the brain for certain characteristics, before dropping off to virtually zero throughout puppyhood. However, this process is poorly understood, and there isn’t much we can do about it anyway! At puberty (usually about 6 months of age), however, his testosterone levels jump dramatically. This is when the “testosterone-mediated” behaviours begin.

So, what behaviours are associated with testosterone?

Testosterone-mediated behaviours are common to all male mammals (although of course their exact expression will vary between species); and all of these have been observed in dogs to at least some extent. The most important ones are:

• Increased aggression. Testosterone in the bloodstream reduces the dog’s aggressive threshold (the level of concern before he starts acting aggressively towards other dogs or people). In addition, once he becomes aggressive, it’ll take him longer to calm down than it would for a bitch, or a neutered dog. Note, however, that this does NOT relate to “fear-based” aggression (see below).
• Increased self-confidence. Testosterone leads to an increased sense of self-confidence; this is usually expressed as:
  • Increased risk taking - which is one of the reasons that entire male dogs are more likely to be involved in road traffic accidents.
  • Reduced fear response. This is important, because it suggests that entire dogs are LESS likely to undergo fear-based aggression than their neutered counterparts.
• Territorial Behaviour. In dogs, we see this primarily as urine-marking. When your puppy starts cocking a leg to urinate, it’s roughly the equivalent of a teenager locking himself in his room and listening to loud music - it’s a way of saying “I’m here, this is my patch”.
• Massively increased libido, or sex drive. This is characteristic of all adolescent males, but in our dogs it tends to be particularly unwanted! We mainly see this expressed as:
  • Roaming - wandering off looking for bitches in heat to mate with. This is potentially dangerous, especially if there are busy roads between him and his intended. It can also be a problem there are other, bigger and nastier, dogs who are also intent on courting her!
  • Sexual behaviours (especially if you don’t let him go out to find a bitch). This category includes humping, mounting, and masturbation (although fortunately for your soft furnishings, it is fairly uncommon for a dog to masturbate to ejaculation). Of course, these are perfectly natural and harmless behaviours for the dog, but they are unwanted and embarrassing for many owners, and occasionally messy.

So, surely by castrating him we’ll stop him from doing all this?

Not necessarily! If you have him castrated before puberty, it is likely that these unwanted behaviours won’t develop in the first place, it is true. However, if you leave it until later, the situation is a lot more complex.

The trouble is that testosterone does not cause behaviours in adolescent or adult dogs - it simply makes them more likely to occur. The reason is that the dog is a sentient creature who is quite capable of learning - so if he starts humping things and realises it’s fun, he may well continue to do so even when his testosterone levels have dropped away to pretty much zero.

So, what are the effects of castration on a dog’s behaviour?

Despite what I’ve said above, the most reliable effects of castration relate to its effect on libido. A neutered dog will have a much, much lower sex drive than when he was entire. Yes, there are some very highly sexed neutered dogs out there, but imagine what they’d be like if they hadn’t been castrated! So, humping, mounting, masturbation and roaming will almost certainly be significantly, and possibly dramatically, reduced.

The effects on aggression, however, are much more nuanced. If the dog is becoming aggressive (especially towards other male dogs), this is likely to be reduced (as a greater stimulus is needed to trigger his aggressive instincts). HOWEVER, the loss of self-confidence that (understandably! - Ed.) comes with castration may make him more prone to fear-based aggression.

Once activated, his territorial instincts will be slightly blunted, but still present - he’ll probably still cock his leg to mark his patch, but perhaps not quite so often as before.

Finally, however, his basic personality will not be affected. If he is a happy, outgoing dog, he’s likely to stay that way. Castration may make him slightly lazier, but don’t expect it to have any effect on adolescent enthusiasm or rowdy behaviour either!

If your dog has a behavioural issue, don’t rely on castration as a “quick fix”. While it can help in some situations, it can make other problems much worse. In every case, a proper workup by a qualified canine behaviourist is likely to be far more useful than jumping in with the scalpel. Your vets can refer you to a behaviourist if needed; and if they think castration may help, there is now a reversible implant that has all the same effects (lasting for 6 or 12 months) so you can “try it out” without the need for surgery.