When Laura Howe’s 8-year-old golden retriever, Pearl, is in heat, male dogs for miles around her western Montana home might catch a whiff of pheromones on the breeze.

Unlike the vast majority of pet cats and dogs in the United States, Pearl goes through hormonal cycles tied to fertility. She underwent an ovary-sparing hysterectomy, performed by a veterinarian at Washington State University, as a puppy, so she can’t get pregnant, but twice a year, she gets extra-affectionate and starts waggling her behind at the other dogs in the family.

In the United States, Pearl is a rarity. Only about 30 percent of pet dogs in the country are not neutered, according to the American Veterinary Medical Association (AVMA), while the American Pet Products Association’s owner surveys estimate around 20 percent. The percentage of dogs that receive an alternate procedure — such as a vasectomy or hysterectomy, which sterilizes the dog while sparing the hormone-producing reproductive organs — are so few that it doesn’t feature as an option in the pet product association’s polls.

But a long-held orthodoxy that responsible owners must always remove their pets’ reproductive organs may be starting to shift, as a growing body of research finds that neutering can increase the risk of cancer, obesity and joint problems, and as pet owners look to other countries with different ideas.

That’s what led Howe to seek out a veterinarian to do an ovary-sparing spay on her dog seven years ago, shortly after another golden retriever of hers died of cancer. In an online forum for dog owners, Howe came across a study that found that early neutering increased the risk of several cancers — including hemangiosarcoma, which killed her dog — in golden retrievers.

That study, published in 2013, is one of several that have started to chip away at the default of neutering all dogs. Since then, two of the researchers behind it, Benjamin Hart and Lynette Hart, have looked at the risks and benefits of neutering in 34 additional dog breeds.

They found that early neutering increased the risk of joint disorders and cancers in many larger dogs, although the effects varied widely by breed and were often ameliorated by waiting until the dog was older. Smaller dogs saw little increased risk.

“I came out of [veterinary] school with a fairly rigid dogma that everybody gets neutered at six months,” said veterinarian Brennen McKenzie, who works at the Adobe Animal Hospital outside of San Jose. “And I think the research evidence has built to the point where it’s clear that those kinds of generalizations and that kind of rigidity is no longer appropriate.”

The AVMA also says that “there is no single recommendation that would be appropriate for all dogs” when it comes to neutering.

Even as data mounts about the physical effects of the procedure, there is far less information about how it shapes a dog’s internal world.

“We do know that the hormones produced by the gonads aren’t just involved in reproduction, but also in growth, in the musculature and in the brain,” said Alexandra Horowitz, an expert of canine cognition at Barnard College. Estrogen, for instance, is implicated in learning and memory function, and progesterone can play a role in inflammation in the case of injury, she points out.

Neutering almost certainly reduces sex drive, although it does not necessarily eliminate it, and it will often improve behaviors such as mounting or marking.

But there’s little evidence to suggest that it reduces aggression. The Harts’ research has found that only about one-quarter of male dogs neutered for aggression saw an improvement in their behavior. And other studies have found evidence that neutering can increase aggression and fearfulness in some dogs.

In some cases, the decision of whether to spay or neuter a dog may be more about human psychology than it is about dog psychology.

“We are especially puritan about our dogs: often people don’t want their dogs sniffing or mounting each other!” Horowitz said in an email. “The idea of our dogs — family members, even considered ‘children’ — being ‘sexual’ is not easy for Americans.”

Widespread neutering is not the default everywhere. In some parts of Europe, it is considered an unnecessary, or even cruel and abusive, intervention. One survey of German pet owners found only 43 percent of dogs are spayed or neutered. In Sweden, estimates are as low as 10 percent. Norway’s animal welfare prohibits neutering in most cases, although it allows exemptions if the surgery would improve the welfare of the animal. Dogs in these countries are left intact, and it is up to the owners to ensure that they do not mate.

Patrick Pageat, a French veterinarian, said that many people in France regard neutering in the same category as controversial procedures such as declawing a cat or cropping a dog’s ears.

“People [in France] are less and less keen to neuter animals. It is regarded as something close to abuse,” Pageat said. “It’s your duty as a responsible owner to make sure that you can control your dog. Neutering or spaying dogs is not a medical necessity.”

Plus, Pageat added, some European owners have psychological biases of their own, especially when it comes to male dogs with male owners.

“When you begin to speak about male dogs, you get to a point where you don’t understand if you are talking about neutering the male dog or the male owner,” he said.

With restrictions lifting and schedules changing, both pets and humans are experiencing separation anxiety. (Monica Rodman, Sarah Hashemi/The Washington Post)

In the United States, in contrast, neutering your dog has long been a badge of responsible pet ownership. In many places, the sight of a pet dog with testes intact will earn more than a few askance glances at the dog park — if the dog is even allowed in. Many boarding centers, dog parks and apartment buildings require pets to be neutered. At least 31 states and the District of Columbia require that shelters “fix” dogs before adopting them out, the AVMA says.

In Montgomery County, Md., the owner of Lucky, a pit bull terrier, ended up battling in court with the county animal rescue in 2019 over whether the dog could keep his testicles. (The parties settled on a vasectomy.) “My experience is that castration changes the dog’s personality,” the owner argued. “I like Lucky’s personality as is.”

Katie Herzog, a freelance journalist in Seattle, has detailed her indecision about whether to neuter her Goldendoodle, Moose, and her experience constantly fielding a question at the dog park whenever his testicles peeked out from behind his butt fur: “When is he getting fixed?”

The strong social pressure in favor of neutering in the United States is in part a result of the work of humane societies and activists who have fought since the 1970s to reduce the population of unwanted dogs. That campaign has been a huge success. One study found that about 13.5 million cats and dogs were killed in 1973. Today, that number is about 1.5 million, according to the American Society for the Prevention of Cruelty to Animals. Although there is some debate about how much of a role widespread desexing played, experts say it probably saved lives.

Lori Gruen, a philosopher specializing in animal ethics at Wesleyan University in Connecticut, said that she spent part of her early career working in shelters where she saw perfectly healthy dogs destroyed and the toll it took on employees.

“The overpopulation issue sounds abstract,” she said. “But these are dogs whose lives end and the people who have to bring those dogs’ lives to an end often can’t get certain dogs out of their minds.”

Many, if not most, dog owners in the United States never confront the question of whether to spay or neuter their pets. Neutering is standard practice in the rescue organizations and shelters, where about 36 percent of owners obtain their dogs, according to data published by the Humane Society. An additional 19 percent of owners buy their dogs from breeders, who are also likely to require that the animals be fixed.

But for those who do have to decide, making the call can feel agonizing.

Leaving a dog intact comes with dangers, especially for female dogs who are at risk for pyometra, an infection of the uterus that can be life-threatening. This risk can be ameliorated if the uterus is removed, like the procedure Pearl received, but it can be hard to find a veterinarian to perform that surgery, which is not widely taught in veterinary schools.

Female dogs that are never spayed or that are spayed after their first heat are also at higher risk for mammary cancer. Dogs of both sexes that are not diligently monitored may roam, courting accidents as they cross busy streets in a frenzied pursuit of a mating partner.

And despite the cancer risks to some dog breeds, studies have found that dogs that are neutered live longer. Some of those findings could be attributable to population bias in the studies. Dog owners who get their dogs sterilized may be more willing to take their pets to the vet for other things, but there could also be a biological basis for desexed dogs living longer, McKenzie said.

“There’s very good evidence that neutering increases life expectancy in almost every species that’s been looked at,” McKenzie said, referencing studies in sheep, invertebrates and even humans. “There’s even some evidence that human eunuchs live longer than intact males.”

In the end, it’s possible that there’s no one right decision.

McKenzie says that fact should help owners relax. Neutering “is not the primary risk factor for most diseases and it’s not the only risk factor for any disease,” he said. “I think we can be a little less stressed about making the quote unquote right decision because it’s all about shifting risk profiles. … We can never perfectly predict what will happen to an individual pet.”

One argument that does not gain much traction with many experts is that dogs should be left intact because it’s more “natural.”

“The idea that there’s something natural about dogs in the first instance is somewhat of a problem,” Gruen said. Dogs have been profoundly shaped by humans and live among them.

“There’s all sorts of control that we impose on the dogs that we live with. Most people don’t let their dogs dig holes. Most people tell their dogs what to eat. Most people tell their dogs not to have sex,” she said. “There are all these limitations we put on dogs to fit our world.”

Gruen said her dogs are desexed. But she does allow them to dig an ever-expanding pit in the backyard. She feels there are only so many limitations the dog spirit can bear.