- Two large sample studies have suggested that spaying and neutering may actually cause an increase in canine aggression.
- According to one study, the earlier the age of neutering, the worse the effects are on the dog's subsequent behavior.
- Aside from aggression, neutering dogs can result in fearful behavior and over-excitability.
A short while ago a woman dropped by our dog club on one of the evenings that we hold beginners' dog obedience classes. She wanted to ask me a question about some advice her son had received from a veterinarian. He had just purchased a male Boxer puppy, and the vet advised him to have the dog neutered as early as possible "in order to avoid any aggressive or excitement-based behavior problems."
The vet went on to say that the earlier the neutering occurred, the less likely such problems were to occur. I was unhappy to hear that veterinarians were still suggesting that course of action. Two large sample studies have suggested that spaying and neutering may actually cause an increase in canine aggression. The information that I shared left her puzzled and insecure about what her son was supposed to do next, so I offered her some additional suggestions and the name of a good puppy socialization class.
I was reminded of this incident just recently when I encountered a new piece of research from a team headed by Paul McGreevy of the Sydney School of Veterinary Science at the University of Sydney. This large-scale data collection involved 9,938 dogs. The study focused on the effects of neutering on male dogs, and the final samples contained 6,546 neutered males and 3,392 intact dogs.
In many countries, the vast majority of male dogs are castrated routinely to prevent the overpopulation that has crowded many shelters and forced them to euthanize countless dogs. However, neutering male dogs has also become a routine suggestion of many veterinarians when their clients tell them that their dog has shown aggression — especially toward family members. So this recent study sought to see if there were any behavioral benefits or problems associated with neutering and to see if the age at which the dog was neutered made any difference.
The research team used data involving the C-BARQ survey instrument, which was originally developed by James Serpell of the University of Pennsylvania. It is a 100-item, behaviorally validated questionnaire that uses the dog owner's observations of a dog's behavior to provide assessments of a wide variety of canine behaviors.
The wonderful thing about this data collection is that beginning in 2006, the C-BARQ was placed online. Its presence there was advertised via an article in the newsmagazine of the School of Veterinary Medicine at the University of Pennsylvania and by notices sent to Philadelphia-Area Veterinary Clinics. Notices were also sent to the top 10 USA breed clubs based on American Kennel Club registrations.
Since that time, information about the survey has been disseminated around the world. Dog owners who are interested can simply go to the website and fill out the questionnaire to have information on their own dog entered into the data bank. This means the number of entries continues to grow over time, allowing studies of canine behavior to become ever more precise.
For this study, the investigators accepted data on dogs who had been neutered for the following reasons: It was required by the breeder or dog shelter; for purposes of birth control; for prevention of health problems; or for the correction of health problems. Since the ultimate focus of the study was dog behavior, they did not want to contaminate the sample by using dogs who had been targeted for castration because of behavior problems.
For that reason, they excluded dogs who were neutered for the following reasons: to correct a behavior problem; to prevent a behavior problem; because it was recommended by the veterinarian; and those who were neutered for unknown reasons. Because data was taken on dogs who had been castrated at ages up to 10 years, this meant that the researchers could also look at the effects of early versus late neutering.
As in previous studies, the new data clearly shows that the positive behavioral effects that were expected from neutering did not occur, and if anything, the behaviors of neutered male dogs tended to be considerably less desirable. Of the 100 behaviors assessed, 40 showed statistically significant differences between the castrated and intact dogs. Only four of these behaviors showed a more positive outcome as a result of neutering. Neutered dogs were less likely to leave urine marks indoors, or to howl when left alone. Neutered dogs, when off-leash, were also more likely to return when called and tended to reliably fetch tossed items. That's it for the positive effects of neutering. The other 36 behaviors were all more negative in neutered male dogs.
The most serious effects of neutering were those that ran counter to the expectation that castration would reduce aggression. Neutered dogs were more likely to show aggression when delivery workers approached the home, when strangers walked past their home, when joggers, cyclists, and rollerbladers passed by, when they were approached directly by an unfamiliar female dog, when an unfamiliar person approached the owner or another family member or even just visited the home, and when small animals such as cats or squirrels entered the yard. Further, the earlier the age of neutering, the worse these effects were.
Neutered dogs also showed many more fear-related behaviors. These included: Responses to loud noises; when first exposed to unfamiliar situations; when approached directly by an unfamiliar child; when barked at, or growled at, by an unfamiliar dog or even when approached by another dog of similar or larger size; when encountering strange or unfamiliar objects on or near the sidewalk; when encountering windblown objects; when examined by a veterinarian; or when having their nails clipped. Once again, the younger the dog when neutered, the greater these fear-related effects appeared to be.
There were other problems that appeared more frequently in the neutered dogs, such as eating droppings or feces (its own or from other animals); rolling in droppings or other smelly substances; stealing food; barking persistently when alarmed or excited; or licking themselves in an obsessive manner.
The pattern of these results is quite clear: Neutering male dogs causes an increase in aggressive behavior, fearful behavior, over-excitability, and a variety of other miscellaneous, undesirable behaviors. What is also clear is that early neutering produces an even greater negative effect on behaviors.
Although these results are obviously important to the average pet dog owner, the researchers also look at their results from a more societal level and conclude:
The beneficial effects of gonadectomy [neutering] are underpinned by the need to reduce the number of unwanted companion animals. Thousands of dogs are euthanized in shelters and pounds annually in many developed countries. However, shelters are inundated by dogs that are most commonly surrendered because they display undesirable behaviors. So the current findings present the paradox that castration may reduce the numbers of unwanted dogs but may also increase the likelihood of problem behaviors that reduce the appeal of the castrated dogs and make them more vulnerable to being surrendered."
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McGreevy PD, Wilson B, Starling MJ, Serpell JA (2018) Behavioural risks in male dogs with minimal lifetime exposure to gonadal hormones may complicate population-control benefits of desexing. PLoS ONE 13(5): e0196284. https://doi.org/10.1371/journal.pone.0196284