Most dog owners have heard the advice: spay or neuter your pet, and do it early. But a growing body of research is challenging that conventional wisdom. A study published in July 2025 in the peer-reviewed journal Biology of Sex Differences offers some of the most striking evidence yet. It found that dogs with longer lifetime gonad exposure were significantly more likely to be robust — not frail — in old age.
What is frailty in dogs?
Frailty isn’t just old age; it’s a clinical condition. Think of it as a cluster of declines that make an individual more vulnerable to bad health outcomes. In dogs as in people, frailty involves reduced strength and stamina, poorer cognition, weakened senses, and reduced ability to bounce back from stress.
Researchers measure frailty using a “frailty index” — essentially a checklist of health deficits. The more deficits an animal accumulates, the higher its frailty score. Dogs with high frailty scores have shorter survival times. Dogs with low scores are considered “robust.”
The study: “centenarian dogs” as a research model
The research team, led by David J. Waters at the Gerald P. Murphy Cancer Foundation, tapped into a remarkable database: the Exceptional Aging in Rottweilers Study (EARS). Since 2003, more than 400 exceptionally long-lived Rottweilers have enrolled in the study. These dogs live at least 30% longer than their breed average. They’re the canine equivalent of human centenarians.
Waters and his colleagues assessed frailty in 222 of these dogs — 135 females and 87 males — all at least 13 years old. They used a validated 34-item clinical frailty index. Variables included appetite, strength, sensory function, cognition, continence, pain, mobility, and several disease conditions.
The key variable? Lifetime gonad exposure — how long each dog had its sex organs (ovaries or testicles) intact before being spayed or neutered.
“Bort” (pictured) is one of the oldest male Rottweilers living in North America. Investigators studied dogs like Bort to discover the linkage between lifelong gonad function, late-life frailty, and overall mortality risk. Image courtesy of the Center for Exceptional Longevity Studies, Gerald P. Murphy Cancer Foundation.
The findings: gonad exposure strongly predicts late-life robustness
The researchers divided dogs into three groups based on duration of gonad exposure: short (less than two years), middle, and long. Then they looked at which dogs were in the “robust” category — the one-third of dogs with the fewest health deficits.
The results were striking. In male dogs, those with the longest gonad exposure were more than 13 times more likely to be robust compared to males neutered before age two. Even after controlling for other variables (including age at frailty scoring, overweight versus not overweight, and birth cohort), the association remained strong: males with the longest gonad exposure were nearly 11 times more likely to be robust.
Female dogs showed the same pattern with somewhat smaller effect sizes. Females with the longest gonad exposure were about three times more likely to be robust compared to females spayed before age two.
“This research in companion dogs supports the notion that gonadal hormones exert an important impact on the retention of late-life robustness in both males and females,” the authors wrote.
Each additional year of gonad exposure increased the likelihood of late-life robustness by 14%.
Why might gonad exposure matter so much?
Sex hormones don’t just regulate reproduction. They’re part of a complex system called the hypothalamic-pituitary-gonadal (HPG) axis, which interacts with the nervous system, the gastrointestinal tract, the thyroid, and the pancreas. Estrogens help build and repair skeletal muscle. Testosterone promotes muscle mass over the lifespan.
When the gonads are removed early, the feedback loop breaks. The brain keeps producing luteinizing hormone (LH) — a pituitary hormone that normally signals the gonads — but there are no gonads to respond. Chronically elevated LH may drive a range of negative health effects. Researchers have linked this hormonal disruption to joint disorders, certain cancers, urinary incontinence, obesity, and immune problems — a cluster sometimes called “spay-neuter syndrome.”
The new study adds frailty to that list.
Sex differences explained by gonad exposure
In humans, women tend to be more frail than men. That’s a consistent finding in the literature. But why? It could be due to biological or behavioral factors, or some combination.
Dogs offer a rare opportunity to untangle those threads. They don’t smoke, seek medical care on their own, or fill out self-report surveys. Using dogs as a model lets researchers strip away many of the sociobehavioral variables that confound human studies.
When Waters and his team controlled for lifetime gonad exposure and age at frailty scoring, the male-female difference in frailty disappeared entirely. Males and females had similar frailty scores to begin with. After adjustment, neither sex was significantly more likely to be robust than the other.
This finding suggests that the well-known sex difference in human frailty may be driven — at least in part — by differences in lifetime gonad hormone exposure. Women enter natural menopause; men experience more gradual hormonal decline. If gonadal hormones truly buffer against frailty, that biological difference could explain much of the gap. For further insight, see this article on frailty and menopause.
What this means for dog owners
The study doesn’t argue that dogs shouldn’t be sterilized. Population control remains a real and serious concern. But the findings add to a growing body of evidence suggesting that early gonadectomy — especially before two years of age — carries long-term health costs.
Many veterinarians have already shifted their thinking. The question is no longer simply whether to spay or neuter. It’s when — and whether hormone-sparing alternatives like vasectomy or hysterectomy deserve more attention.
If you’re considering sterilization for your intact dog, you can find a veterinarian who offers hormone-sparing sterilization in our Veterinarian Directory. If your dog has already had a gonadectomy, talk with your veterinarian about whether hormone restoration may be beneficial for your dog.
See our other articles on hormone-sparing sterilization in pets.
