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Animal Behavior

Spayed and Neutered Dogs Show More Signs of Aging

Desexing dogs doesn't control aggression but exposes them to physiological risks.

Key points

  • Evidence from large-scale studies show that desexing dogs increases the probability of aggressive behaviors.
  • Data indicates that spaying and neutering can increase the risk of physical problems, including cancers.
  • New research demonstrates that spayed and neutered dogs are more likely to show increased signs of aging.
Mikhail Nilov / Pexels
Mikhail Nilov / Pexels

In North America, the spaying and neutering of pet dogs has almost taken on a moral or religious tone. Some of the websites maintained by humane societies and veterinary groups even talk about "The Responsibility of Spaying and Neutering Dogs" and refer to the behavioral benefits of these surgical procedures with statements like, "Your dog should be spayed or neutered because sex hormones lead to unnecessary stress and aggression among dogs."

Not a Universal Practice

Apparently the message that spaying and neutering are desirable or perhaps even necessary practices for pet dogs has gotten through to the general population, since in North America between 70 and 80 percent of dogs are spayed or neutered. Because of the often repeated belief that neutered dogs are less likely to be aggressive, many dog parks, apartments, and dog boarding kennels, require that pets that use their facilities be surgically desexed. The picture in Europe, however, is quite different. A Swedish study found that 99% of the dogs in their sample were not neutered. A Hungarian study showed 57% intact dogs, and a British survey found 46% intact dogs. In fact it is against the law to neuter dogs in Norway, unless there is a specific medical reason.

Unwanted Physical Consequences of Spaying and Neutering

North Americans tend to focus on the purported behavioral benefits of spaying or neutering while ignoring the scientific data suggesting that there are physical downsides related to these procedures. Michelle Kutzler of Oregon State University has pointed out that spaying and neutering is associated with an increased risk of several long-term health problems, including urinary incontinence, bladder stones, hypothyroidism, diabetes mellitus, hip dysplasia, cruciate ligament rupture, obesity, as well as several forms of cancer.

Does the Behavior Change?

What is even worse is the fact that the scientific data suggests that the behavior changes that are being sought by spaying and neutering dogs (reduced aggression and lower excitability) are actually not obtained according to two major studies. The number of dogs tested in these two studies is quite large. Deborah Duffy and James Serpell, at the University of Pennsylvania, tested two different samples, one of 1,552 dogs and the other of 3,593 dogs. Parvene Farhoody, at Hunter College in New York, tested 10,839 dogs. Thus, the combined studies provide data on 15,984 dogs in total, making this an amazingly powerful data set. The main findings were consistent across all three samples of dogs. Given that one of the accepted behavioral reasons for spaying and neutering is to reduce aggression, the distressing results of these investigations is that spayed and neutered dogs actually show considerably more aggression, as well as increased likelihood of some ancillary unwanted behaviors. Thus the data indicates that spaying and neutering actually increases the severity of the problem behaviors that they are intended to solve.

Spaying, Neutering, and Signs of Aging

A new study by a team of investigators headed by David Vajányi at the University of Veterinary Medicine and Pharmacy in Košice, Slovakia provides additional arguments against the advisability of spaying and neutering dogs based on both behavioral and physiological changes caused by these procedures. This new research suggests that dogs that have been surgically desexed actually show accelerated signs of aging.

This current study had a moderate sample size of 785 dogs. It looked at a number of different factors associated with signs of aging and found that whether dogs had been spayed or neutered was one of the most significant determinants as to their frequency. For example, they observed that regardless of sexual status, the weight of dogs increased with age. Desexing itself causes a significant rise in a dog's weight, and the expected age-related weight gain was almost twice as much in the neutered dog than as the gain observed in intact dogs.

They summarize their results saying:

"When considering neutering status, we observed that neutered animals had more obvious manifestations of ageing than intact males and females; with statistical significance for the manifestations such as weight increase, more frequent urination, and deterioration of vision. Neutered dogs, especially dogs of larger breeds, also had more difficulty with movement, which is probably related to their frequent high weight."

As for cognitive changes that are associated with age and sexual status, the researchers suggest "that the presence of circulating testosterone in sexually intact dogs can slow down the progression of cognitive impairment in dogs that have already shown signs of moderate impairment. Similarly, oestrogens may also have a protective role".

While the desexing of dogs is a reasonable procedure if your sole aim is population control; for other purposes, whether a dog should be spayed or neutered should be carefully thought through. If you are considering these surgical procedures as a means of behavioral control (reducing aggression or over-excitement) it is probably not an advisable course of action. The data suggests that spaying and neutering just doesn't reduce canine aggression and may even make the behavior problems worse. Furthermore the negative physiological consequences, including increased signs of aging, provide a definite risk for your pet.

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Vajányi D, Skurková L, Peťková B, Kottferová L, Kasičová Z, Simanová V, Kottferová J. (2024). Ageing canine companions: Most common manifestations and the impact of selected factors, Applied Animal Behaviour Science, 271, 106164,

Kutzler M. A. (2020). Possible Relationship between Long-Term Adverse Health Effects of Gonad-Removing Surgical Sterilization and Luteinizing Hormone in Dogs. Animals, 10(4), 599.

Duffy DL, Serpell JA.(2006, November). Non-reproductive effects of spaying and neutering on behavior in dogs. Proceedings of the Third International Symposium on Non-Surgical Contraceptive Methods for Pet Population Control. Alexandria, Virginia.

Farhoody P. (2010) Behavioral and Physical Effects of Spaying and Neutering Domestic Dogs (Canis familiaris). Master's thesis submitted to and accepted by Hunter College.

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