A Google search shows many veterinary and animal-welfare websites claim neutering prolongs the lifespan of male and female dogs. Certainly, many studies^1-10 have reported a consistent association between canines being gonadectomized and longer average lifespan—several months to two years longer in females, and none to over a year longer in males.

However, the methodology of these studies should be carefully assessed before accepting that the association between being gonadectomized and lifespan reflects a causal relationship (i.e. that neutering really does prolong lifespan).

Methodology

These studies^1-10 are retrospective analyses of populations consisting of many breeds with widely varying average lifespans, and utilized “dichotomous binning,”¹¹ in which dogs were categorized as neutered or intact depending on their neuter status assessed when they died. So, they took no account of the age at which dogs were neutered—of how long the dogs had been exposed to gonadal hormones—which was likely the major determinant of any effects of gonadectomy on dogs’ physiology, health or behavior that might result in any changes in their lifespan.

Retrospective epidemiological studies to identify and quantify associations present in population data are often fraught with confounding factors. This article explains multiple artefacts that affected the association between neuter status and lifespan in these studies, such that the studies’ results do not reliably indicate how much—or even whether—neutering influences average lifespan.

Five confounding artefacts

• Forward causation: If the biological effect of neutering causally alters—whether increasing or decreasing—average lifespan, that can be called forward causation. However, in these studies,^1-10 several factors other than neutering also acted to cause an association between neuter status at death and lifespan, which was not a biological effect of neutering.

None of these factors are mutually exclusive, and their effects on the association between neuter status and lifespan are indistinguishable from, and will sum with, any effect of forward causation, confounding interpretation of lifespan differences between neutered and intact dogs as evidence of forward causation.

• Reverse causation: The longer an intact dog lives, the more opportunity there is for it to be neutered and reasons for neutering increase with age. Given a longer life makes being neutered more likely, all else being equal, there will be an association between being neutered and longer average lifespan, but the direction of causality is the reverse of that where neutering causally
  affects lifespan.
• Census effect: All gonadectomized animals were intact and alive until the surgery. So, dogs identified at death as intact could have died at any age, but dogs identified at death as neutered could have died only after the age at which they were neutered. All else being equal, therefore, the neutered population is, on average, older than the population of intact animals.
• Incorrect classification of intact dogs as neutered: Some studies, instead of calculating average lifespan from age-at-death data,^1-4,6 estimated average lifespan from the distribution of mortality by age, either by calculating the proportion of age-specific mortality in each year-of-age bin^7,10 (or wider age bins)^5,7 or from Kaplan-Meier survival curves.^8,9

However, because neuter status was categorized only at death, many intact dogs were incorrectly categorized as neutered when at ages younger than the age at which they were actually neutered. This incorrect categorization reduced for intact dogs, and increased for neutered dogs, the apparent number of surviving dogs at each age, thereby artefactually increasing and decreasing the apparent mortality of intact and neutered dogs, respectively.

One study,⁹ after analyzing the association of neutering and lifespan across all dogs in their study population, repeated their analysis including only dogs five years or older. Doing so effectively eliminated the apparent survival advantage of neutering for males and reduced it for females.

• Spurious correlations: As all veterinarians know, correlation does not necessarily imply causation. Spurious correlation refers to a third factor correlated to, and influencing, both neuter status and lifespan.

Neutering is perceived to be part of responsible ownership in some of the countries where studies of its effects on lifespan have been done, and so it may be a proxy for better husbandry and medical care.¹² If owners of neutered dogs provide better care (i.e. diet, vaccinations, parasiticide usage, pet insurance, medical care when ill, etc.) than owners of intact dogs,^13,14 and that better care increases lifespan, that would result in neutered dogs having a longer average lifespan, although that longer lifespan would not be a biological effect of neutering.

• The dog populations studied^1-10 consisted of many different dog breeds, including mixed-breed: Mixed-breeds have a longer average lifespan than the average for pure breeds.^{2,6,8-10,15-18} Mixed-breeds are also more often neutered than purebred dogs,^13,14,19,20 resulting in more mixed-breed (longer lived) ending up in the neutered-dog population than the intact-dog population, making a spurious association between being neutered and a longer average lifespan.

Similarly, smaller breeds have substantially longer lifespans, on average, than larger breeds.^{2,6,8-10,15,16,18,21,22} The proportion of dogs neutered varies widely between breeds,^14,23 although little information is available regarding the proportions of each breed neutered in any country.

If, on average, the proportion of each breed neutered covaried with breed size, perhaps because of owner socioeconomic, household or population demographic factors,^13,14 or variation across breeds in management requirements at home or disease profiles–and small breeds may be more likely to be neutered simply because they live longer–that would result in a positive or negative association between being neutered and lifespan. Again, the association would not be a biological effect of neutering.

In the one study of a heterogeneous population that also separately analyzed individual breeds within that population,⁷ neutering was a significant factor affecting longevity in “more than half” of the 25 breeds assessed. Three other studies^24-26 investigated one breed only and used dichotomous binning of neuter status at time of death.

In necropsied golden retrievers from a U.S. referral hospital population, being gonadectomized was associated with a longer lifespan in females, but not males.²⁴ In the vizslas in the U.S., being gonadectomized was not associated with lifespan in data for males and females combined.²⁵ In Swiss Bernese mountain dogs, being gonadectomized was not associated with a significant survival difference in males or females.²⁶

The association between being gonadectomized and longer lifespan in individual breeds appears less consistent than in studies of heterogeneous breed populations.

Conclusion

Some of the discussed artefacts definitely, and others probably or possibly, increase the association between being neutered and longer lifespan. These artefacts each sum with any genuine effect (whether positive or negative) of neutering on lifespan.

The magnitude and direction of any genuine effect of neutering on lifespan is unknown, and the magnitudes of each of the artefacts are unknown.

Clearly, however, studies utilizing dichotomous binning of neuter status at time of death, particularly in a heterogeneous population of dogs, do not provide reliable evidence of the effect of neutering on lifespan—it is not even clear from their results whether neutering increases or decreases lifespan in either females or males.

The two studies that accounted for both age at neutering and breed heterogeneity (See “Breed-specific studies that accounted for age at neutering”) found being neutered was associated with substantially shorter
average lifespan.

To answer the question in the title of this article: The available evidence does not show that gonadectomy increases dogs’ average lifespan–in fact, gonadectomy may possibly shorten it.

BREED-SPECIFIC STUDIES THAT ACCOUNTED FOR AGE AT NEUTERING

Two studies27,28 investigated one breed only and accounted for age at neutering—effectively assessing the ‘dose-response’ relationship between duration of exposure to gonadal hormones and longevity. One studied only female Rottweilers,27 the other studied boxers, but only up to 10 years old, and combined both sexes in their analysis.28 Both studies found being gonadectomized was associated with substantially lower survival. Both studies directly compared the results obtained accounting for age at neutering with the results from the same data analyzed using dichotomous binning of neuter status at time of death. In both studies, the association between neuter status and survival reversed from negative when accounting for age at neutering to positive when dichotomous binning was used. The results of these two studies27,28 are not themselves free of artefact, and the extent to which findings in Rottweilers or boxers can be extrapolated to other breeds is unknown. Clearly, however, these two studies produced results contrary to the very consistent results reported when dichotomous binning is used in heterogeneous populations of dogs.

Martin L. Whitehead, BSc, PhD, BVSc, CertSAM, MRCVS, is a member of the Evidence-Based Veterinary Medical Association (EBVMA), with different members writing this column. While all articles are reviewed for content, the opinions and conclusions of the author(s) do not necessarily reflect the views of the EBVMA or Veterinary Practice News. For information about the association or to join, visit https://www.ebvma.org.

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