I’ve written extensively over the years about the risks and benefits of neutering. It’s a complicated subject that tends to draw a lot of passion even in the absence of robust scientific evidence. Variable such as sex, breed, age at neutering, and the particular procedure performed all influence the risk, and there are few conditions for which we can make absolute predictions in an individual dog or cat. The best we can say is that a few conditions are clearly much more common in intact animals (e.g. uterine infections, breast cancer), some are more common in neutered animals (e.g. obesity), and many, many conditions are influence subtly by neutering and also by many other variables.
The overall outcome for any individual is affected by so many factors that it is effectively unpredictable. Strident claims that neuteringmustbe done or should notbe done or that the age at which it is done is crucial conflict with the much more nuanced and complex reality evidence in the scientific research on the subject. A balanced, rational approach is to recognize that some risks increase and others decrease with neutering and that the best we can do is make tentative recommendations for individual pets based on the population literature. As the evidence changes, new research can shift our estimate of the risks and benefits, though radical revisions in practice are rarely justified. A new piece of evidence recently published strengthens one claim that has been tentatively made based on previous research; neutered dogs appear to live longer on average than intact dogs.
A number of studies in dogs (as well as other species, including humans) have found greater longevity in neutered animals than in those left intact.1-9Not all studies agree, and there are significant differences in longevity between breeds. There are also potential differences in other factors, such as the quality of husbandry and medical care received by intact versus neutered animals, especially in countries in which neutering is the norm. Generalizations about the effect of neutering on lifespan may not apply to every individual, but overall the pattern in the research literature has been for neutered animals to live longer than intact animals.
A recent retrospective study looking at records from a chain of hospitals across the U.S. evaluated lifespan in a cohort of over 2 million dogs seen at these clinics during a two-year period.
Silvan R. Urfer, Mansen Wang, Mingyin Yang, Elizabeth M. Lund, and Sandra L. Lefebvre (2019) Risk Factors Associated with Lifespan in Pet Dogs Evaluated in Primary Care Veterinary Hospitals. Journal of the American Animal Hospital Association: May/June 2019, Vol. 55, No. 3, pp. 130-137.
The results showed statistically significant differences in lifespan associated with neuter status and breed size. The figures below illustrate the key findings.
These data agree with the majority of the results of previous studies, which suggests that the relationship between neutering a longevity is real. This does not, of course, explain what this relationship is. Differences in breed, husbandry and medical care, specific causes of death, and many other factors are involved in determining lifespan. However, it is at least clear that broad claims neutering shortens lifespan are not consistent with the evidence.
This study also provides a good example of a problem common in the interpretation of scientific literature, which is the difference between statistical and real-world significance. It is possible to find differences between groups which are statistically significant, especially if you include a very large number of subjects, but these differences can be effectively meaningless in the real world. In this study, for example, the difference between the median lifespan of neutered and intact male dogs was 0.06 years (about three weeks). This is clearly not a meaningful difference in the life of an actual male dog.
The difference between spayed and intact females, in contrast, was 0.58 years (about 7 ½ months). This is a more significant difference in terms of real life, though still small as a portion of the overall lifetime of a dog.
Breed size had a much larger effect, with giant breed dogs (over 90 lbs adult weight) living almost four years less than small-breed dogs (under 20 lbs adult weight). This too is consistent with previous research though again it doesn’t explain the cause for this difference.
This study strengthens the claim that neutered dogs live longer than those that are not neutered. Longevity is affected by breed, sex, care, and many other factors, and the reason for this difference is unclear. The magnitude of the difference in lifespan between intact and neutered dogs varies among studies, and the difference is not always large enough to be of any real-world significance. However, these results clearly contradict broad claims about the overall negative effect of neutering on health and longevity in dogs.
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