Large dogs such as golden retrievers may benefit from delayed spaying and neutering.Victoria Neer / Getty

In the 1970s, a time when tens of millions of unwanted dogs were being euthanized in the United States annually, an orthodoxy began to take hold: Spay and neuter early. Spay and neuter everything. It’s what vets were taught. It’s what responsible pet owners were told to do.

A growing body of research, however, suggests that spaying and neutering—especially in some large breeds when very young—are linked to certain disorders later in life. “As time has gone on, vets are starting to question the wisdom,” says Missy Simpson, a veterinary epidemiologist with the Morris Animal Foundation, which recently published a study that found higher rates of obesity and orthopedic injury in golden retrievers that had been fixed. Other studies have linked early spaying and neutering to certain cancers, joint disorders, and urinary incontinence—though the risks tend to vary by sex, breed, and living circumstances. As such, the American Veterinary Medical Association (AVMA) now says in a guide for veterinarians, “There is no single recommendation that would be appropriate for all dogs.”

And yet anyone adopting from a shelter is unlikely to be told of these risks—or even to be given a choice. Today, according to the AVMA, 31 states and the District of Columbia require sterilization or a promise of such before pets can be adopted out of shelters. The American Society for the Prevention of Cruelty to Animals (ASPCA) also advocates early spaying or neutering of all companion animals at two months or two pounds in weight. Its information page for pet owners touts the very real benefits of the procedures—behavioral changes, fewer uterine infections, a decreased risk of certain cancers—but with nary a mention of possible downsides.

For animal-welfare groups trying to manage unwanted populations, this strategy makes a kind of sense. “We’re trying to look at the big picture,” says Lori Bierbrier, the medical director of the ASPCA. “One of the ways to manage that population is not to have animals going out and having puppies and kittens all the time.” For dogs that already have an owner, she says, whether to spay or neuter is that owner’s individual decision. But that also makes talking about the research reevaluating the risks of spaying and neutering tricky. How do you balance raising concerns about risks for individual dogs with the welfare of dogs as a whole?

“Oh my gosh, we got pushback,” says Benjamin Hart, a professor emeritus at the University of California at Davis School of Veterinary Medicine. In 2013, a team led by Hart and his wife and collaborator, Lynette Hart, published a study that found higher rates of joint disorders in golden retrievers spayed or neutered before one year of age and of certain cancers in female golden retrievers that were spayed early. It immediately caused an uproar. “This is irresponsible,” Hart recalls critics saying. “You’re looking at just one breed. You can’t generalize.”

So they started looking at other breeds. The Harts have since published two follow-up papers, on Labrador retrievers and German shepherds, also finding an elevated risk of joint disorders but not of cancers after early spaying and neutering. And they have just finished another study, on 35 different dog breeds as well as mixed breeds. The risks of cancers and joint disorders appear to vary significantly by breed and sex, Hart says, with small dogs generally less affected by early neutering.

The takeaway, Hart says, is that when to spay or neuter should be a case-by-case decision, even for dogs adopted out of shelters. Simpson, of the Morris Animal Foundation, says that vets have already, based on recent research, started recommending delaying spaying and neutering for owners of large breeds. Puppies in shelters, though, might not get the same individual attention.

The risk of obesity, Simpson adds, is often the major concern for vets making spaying or neutering recommendations. Somewhere between a quarter to a third of pets in the United States are now obese. The link between obesity and spaying or neutering has to do with hormones. Removing a dog’s testicles or ovaries disrupts its hormonal balance, and this makes it both hungrier and slows its metabolism to require fewer calories. Yet animal-welfare groups that promote spaying and neutering are often quick to “debunk” the idea that fixing a dog could make it gain weight. The ASPCA’s website says, “Lack of exercise and overfeeding will cause your pet to pack on the extra pounds—not neutering.” This is technically true, but it elides a very real biological connection that owners might find useful to know.

When I brought this up with Bierbrier, she said the ASPCA staff would have to look into updating the website. She added that the ASPCA’s spay-and-neuter clinic does tell owners taking dogs home after the surgeries that their pets will require less food.

Elsewhere in the world, spaying and neutering is not necessarily seen as the “responsible” thing to do. It is heavily discouraged in parts of Europe, such as Norway. Those countries also have very few stray dogs and a far less casual relationship with dog ownership.

Dogs that have not been fixed are, to put it one way, less convenient pets. Intact male dogs will want to roam in search of a mate; female dogs will go into heat and have bloody discharge. The campaign to spay and neuter dogs has also changed their very relationship to us as pets.

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