This is a really interesting question, and raises a number of really valid points. To answer it, we need to look in more detail the reasons why dogs behave the way they do, and the hormonal influences on them.

Common misconceptions of neutering

It is widely – and falsely – believed that neutering “calms a dog down”. However, an adult or adolescent dog’s basic temperament is relatively independent of testosterone, and neutering won’t make any significant changes to his intelligence or personality. What it can do is affect his emotional state, and what behavioural options occur to him.

So what does testosterone do?

In behavioural terms, testosterone has two main functions.

Firstly, it is the main trigger for a dog’s sex drive – so any behaviour driven by the need to breed will be at least partly influenced by testosterone levels. This would include roaming in search of bitches in heat, humping and masturbation (although see below!), and, interestingly, urine marking. All of these behaviours are reduced (if not eliminated) by castration.

The second is that testosterone does appear to increase the expression of aggressive behaviour. This might be an evolutionary hang-over from having to fight for mates, but whatever the reason, the threshold for initiating an aggressive act becomes lower the higher the dog’s testosterone level is.

So, an entire dog is more likely to respond to annoyance or frustration by snapping than a neutered one is. The exact mechanism isn’t clear, but testosterone is also a major factor in determining a dog’s levels of self-confidence, so he may be more likely to start a fight because he’s more likely to believe that he can win it!

What does testosterone not do?

It doesn’t initiate any particular behaviour, it just alters the likelihood of that behaviour being expressed. For example, not all “sexual” behaviours are actually linked to sex drive – so humping and mounting can occur in neutered animals as well. These patterns of behaviour are “hard-wired” into a dog, but they can be used to express different things – for example, mounting may be an expression of dominance. Some neutered dogs also continue to hump, possibly because it has become a learned behaviour.

As far as we can tell, testosterone is also NOT involved in the vast majority of “unruly” or “disobedient” behaviours. While adolescent dogs are more easily distracted, and are full of energy, poor behaviour at this age is more often related to poor training.

So, shouldn’t castrating make him less aggressive?

Unfortunately not necessarily, and for a very simple reason…

There’s more than one type of aggression!

Aggression between male dogs is mediated largely by testosterone; however, aggression towards people, female dogs and other animals is far more weakly linked to his hormones. If you think about it, it makes sense: if you’ve got to fight other males to get access to a mate, there’s no point in fighting against the females, or against other species who aren’t competing with you.

Aggression that is directed towards people, other animals, or generalised (anything that moves and looks suspicious) is most likely to be fear-based aggression. And unfortunately, fear-based aggression is often made WORSE (or even triggered) by castration. While it would be a nice story to say that dogs became anxious after castration because of the psychological trauma from the loss of their testicles, this doesn’t actually happen. What does happen is a fairly rapid drop in blood testosterone, the very hormone that was keeping them self-confident and relaxed in the presence of potentially threatening objects. So, we might well imagine that the dog’s thought process of “yes, that person’s a bit scary, but I’m sure I could take them so I don’t need to worry” changes to “oh no, that person’s scary, I need to drive them off before they can hurt me”. Without that confidence-boosting hormone, dogs who are prone to anxiety often get a lot worse.

So can we replace the testosterone?

In theory, yes. However, there aren’t any licensed products in the UK, and you could be storing up problems for yourself. As we’ve seen, testosterone has complex and subtle effects on the brain, it isn’t anything like as straightforward as many people think (and it still isn’t fully understood).

The best way to get on top of any behavioural issue, in an entire or neutered dog, isn’t to mess with their hormones, but to seek a referral to a properly qualified canine behaviourist. If you’ve got a problem, talk to your vet who will be able to arrange a referral for you!

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