Does neutering have an effect on the dog’s aggressive behavior?

While a few years ago, it was common practice to neuter dogs “prophylactically” – among other things to prevent aggression – today, the subject of neutering is very controversial.

On the one hand, neutering is still used to treat and prevent behavior problems – including aggression. But on the other hand, there is evidence that neutered dogs exhibit higher levels of aggression than non-neutered quadrupeds.

A scientific consideration of the connection is more complex because there are different forms of aggression. So one would assume that – if at all – sexually motivated aggression decreases by neutering. However, other forms of aggression, such as territorial aggression or resource defense, are probably not affected.

Is there a connection between neutering and aggressive behavior of a dog

A team of researchers investigated to what extent aggressive behavior towards known persons, strangers or other dogs differs depending on the age at neutering.

All questions about aggressive behavior were evaluated, and mean values were formed. Then, dogs with no aggression were compared to dogs with moderate to severe aggression. There was also a comparative analysis of data from intact dogs with dogs neutered at six months or less, 7 to 12 months, 11 to 18 months, and more than 18 months.

The results indicate no association between neutering per se nor between the age of neutering and aggressive behavior toward familiar humans or other dogs.

However, the likelihood of moderate or severe aggression toward strangers was slightly increased in neutered dogs. This study pertained exclusively to dogs neutered between 7 and 12 months of age – these dogs showed a 26% higher likelihood of exhibiting aggression toward strangers.

However, this study made no distinction between males and females.¹

Does castration reduce aggression in male dogs?

Since testosterone in male animals is primarily produced in the testes, one could speculate that castration would be a suitable remedy for increased aggressiveness. But, in fact, different studies show that male mice, for example, show less aggressive behavior than uncastrated conspecifics.

It is also generally known that castrated horses and bulls are more sociable and manageable. In dogs, however, the link between castration and aggressiveness is not as clear. While some studies find a lower aggression score in neutered dogs, data from other studies show the opposite results.

In one survey study, 60% of dog owners reported reduced aggression toward fellow sexes after their male dogs were neutered. But unfortunately, there was no change in the remaining respondents.

Similar results were obtained in another study: in 60% of the males, aggressive behavior toward other males decreased after neutering. However, territorial and fear-motivated aggression remained unchanged.

In another questionnaire study, data were collected from 6235 male dogs. The results of this survey show a correlation between the age of neutering and aggressiveness or fearfulness: the earlier the dog was neutered, the higher the probability of problem behavior in fear and aggression. Also, neutered male dogs appear more prone to resource aggression toward conspecifics and humans.

Early neutering can have unwanted consequences

Other studies also show that early neutering, in particular, can cause unwanted behavioral changes. For example, in the 2014 questionnaire study, Vizsla owners were asked about the health of their dogs. It showed that animals neutered at less than six months of age were at increased risk for behavioral problems compared to un-neutered conspecifics.

Neutered female dogs show increased reactivity and fearfulness.

Another study documented the reactions of female dogs to the approach of strangers and conspecifics. Bitches spayed between 5 and 10 months of age reacted significantly more aggressively than their intact littermates in these situations five months later. A study with Labrador retrievers also suggests that spayed females exhibit higher anxiety.

Neutering is not a suitable treatment for aggression in dogs

When various factors that may trigger aggressive behavior are considered, there appears to be a complex relationship between spaying and aggressive behavior. The studies indicate that neutering is not generally appropriate to treat or prevent aggression problems. Given the growing evidence of the negative effects of neutering, this procedure should also be reconsidered to stop unwanted reproduction. Therefore, neutering should never be a blanket decision but should always be a case-by-case decision.²

1. Aggression toward Familiar People, Strangers, and Conspecifics in Gonadectomized and Intact Dogs
2. Castration of adult male dogs: effects on roaming, aggression, urine marking, and mounting