Cockapoo

Royal Veterinary College photo

Cockapoos like Enzo don't get a health benefit simply for being a crossbreed, research suggests.

When the famed naturalist Charles Darwin experimented with plant reproduction and evolution more than a century ago, he made a curious discovery. Cross-pollinated maize plants were taller and more fertile than their self-pollinated counterparts.

The observation gave rise to the concept known as hybrid vigor (technically called heterosis), whereby a richer assortment of genes makes an organism hardier.

But the idea that hybrid vigor makes crossbreed and mixed-breed dogs inherently healthier than purebreds — an enduring belief, including among some veterinarians — is looking questionable, judging from several scientific studies. (Mixed-breed dogs are descended from many breeds, whereas crossbreeds are a blend of two distinct breeds.)

The latest paper, published in August and authored by practitioners at England's Royal Veterinary College (RVC), found limited overall health difference between three popular designer crossbreeds — Labradoodles, cavapoos and cockapoos — and their pedigreed ancestors. And a study published in November 2023 conducted by the Dog Aging Project, an American research initiative based at Texas A&M University and the University of Washington, went further. It suggested that purebred dogs are, on average, less likely to experience common medical conditions than mixed-breed dogs.

Each study had noteworthy caveats. Both, for instance, were based on reports by pet owners, who may be more susceptible to misjudgments than trained veterinary professionals. And the conclusions aren't hard and fast for every situation. For example, the researchers on both teams acknowledged that the offspring of a purebred with genetic predisposition to certain diseases or with extreme physical traits — like squash-faced pugs that struggle to breathe — would be better off if their other parent were of a different breed not prone to disease.

Designer doodle dominance

Designer crossbreeds have grown in popularity ever since the first Labradoodle was created Down Under in 1980. Wally Conran, then working for the Royal Guide Dog Association of Australia, wanted to create a non-shedding dog for a blind woman whose husband was allergic to long-haired canines, Conran told Australia's public broadcaster in a 2019 interview. Conran mixed a Labrador retriever (for its pleasant disposition) with a poodle (for its short, curly coat). A puppy from the litter went to the couple; the rest of the pups needed homes. Conran said he coined the name Labradoodle and asked his employer's public relations department to market the pups as hypoallergenic — though he had no proof that they were.

In retrospect, Conran said, creating the Labradoodle was his "life's regret." He believes the popularity of the cross has given rise to indiscriminate and potentially unhealthy breeding practices. But a doodle craze was unleashed. Today, there seems to be a poodle-cross with everything.

Research about them isn't nearly as abundant.

"There's no study currently out there that's looked into specific crossbreeds and their overall health," said Gina Bryson, a postgraduate researcher at the RVC and an author of the paper published in August. "These breeds are so popular, and yet we know very little about them."

In their study, Bryson and colleagues surveyed the owners of 9,402 dogs in the United Kingdom. The study pool comprised Labradoodles, cavapoos, cockapoos and the breeds from which they descended: Labradors, cavalier King Charles spaniels, cocker spaniels and poodles.

The researchers identified the 57 most common disorders affecting all of the dogs, making for 342 health comparisons when taking into account the three crossbreeds and their progenitor breeds. They found that the likelihood of having one of the medical conditions did not differ significantly between the crossbreed dogs and the progenitor breeds in 296 (or more than 86%) of the health comparisons. For the remaining health comparisons, the results were almost evenly mixed: The oodle blends had greater likelihood in 24 of the comparisons and lower likelihood in 22 of the comparisons.

Where was the hybrid vigor observed by Darwin all those years ago? One likely explanation is that the level of diversity in the gene pools of dogs is immensely different compared with maize and plants like it.

Since many species of crops and livestock are often inbred, boosting their genetic diversity through crossbreeding can lead to healthier stock and higher production yields. But hybrid vigor doesn't necessarily work the same in dogs because dogs as a species are already very genetically diverse, explained Dr. Dan O'Neill, associate professor of companion animal epidemiology at the RVC and one of the study's authors.

In fact, some dog breeds are so genetically different from one another that each could almost be considered a species in its own right, he suggested: "In effect, we're crossing between species, not between breeds, and producing a new type of dog that can be very different to both parent breeds."

Another factor differentiating crops and livestock from dogs: Breeders and farmers don't necessarily have the same goals in mind, O'Neill noted. Breeders may select traits that they perceive make the dog's appearance more appealing, sometimes at the expense of the animal's health. Such a narrow focus on looks has resulted in the creation of breeds with extreme physical characteristics such as brachycephaly (flat faces) that make them more prone to health problems.

With their high numbers, French bulldogs have skewed the statistics for all purebreds. "Because French bulldogs have become hugely popular, the health of purebred dogs at a population level is way, way, way worse than it was 10 years ago," said O'Neill.

What about mutts?

The Dog Aging Project study took a broader look at the issue. It compared the health of 25 purebred breeds to mixed-breed dogs (sometimes known as mutts).

Using owner-reported data for 27,541 dogs in the United States, the researchers identified 53 common medical conditions. Among purebreds, 22.3% were free of all the medical conditions, according to their owners. By comparison, 20.7% of mixed-breed dogs were free of the conditions, a difference the researchers found to be statistically significant.

There was, however, a caveat: Breeds with genetic disorders or extreme physical traits that are predisposed to certain health conditions had a higher likelihood of developing health problems.

The risks of disease are even greater for dog breeds with small gene pools and high levels of inbreeding. A separate study from April 2023 that was based on data from the Finnish Kennel Club found that these breeds often have shorter lifespans. This may help explain the recent findings in the Dog Aging Project's November 2023 paper, said Dr. Kate Creevy, the project's chief veterinary officer and a paper coauthor.

"For dog breeds that have such a small population size, it's not surprising that once a disease gets in the breed, there's no way for it to get out," said Creevy, who is a professor of small animal internal medicine at Texas A&M University.

The Dog Aging Project research published last year was the second such study the group had done comparing purebreds and mixed breeds. In the earlier foray into the subject, researchers analyzed data on the lifespan of 20,970 dogs seen at three independent veterinary clinics in the United States. They found no significant difference between the two categories in the dogs' longevity. However, purebreds with greater population sizes or lower rates of inbreeding had median survival times three to six months longer than breeds with smaller populations and more inbreeding.

Could that last finding and others like it — that some purebred dogs are more prone to disease and have shorter lifespans — support the idea that hybrid vigor is a reality for at least some types of dogs?

Breeding dogs to be dogs

RVC researcher O'Neill has pondered the lifespan question. A study published in 2013 that he coauthored had appeared to support the notion of some hybrid vigor effect in mixed-breed dogs, finding that among 5,095 pets in England, crossbreeds, on average, lived 1.2 years longer than purebreds.

"On paper, that's alluring," O'Neill said. "That seems to say hybrid vigor is really powerful … but it's not the fact they're hybrid that's necessarily making them live longer."

Instead, O'Neill suggests "regression to the mean" — or return to normal — could be at play, whereby a cross of two relatively "healthy" dogs, such as a poodle and a cocker spaniel, creates a puppy with traits that reflect an average of its healthier parents.

If the RVC researchers who authored the 2024 paper had focused more on breeds with significant inbreeding or extreme morphologies, like flat faces or excessive skin folds, the results might have differed, O'Neill maintained.

Creevy, who was not involved in the RVC's study, noted that the researchers may not have seen the hybrid vigor effect in doodles because their parent breeds do not "have the problem of a very small gene pool or a very extreme phenotype in the first place." (A phenotype is an organism's observable traits, as opposed to its genotype, or genetic makeup.)

A study now underway at the RVC is comparing the health of pugs, a purebred brachycephalic breed, versus the health of two longer-nosed breeds, Jack Russell terriers and beagles, versus the health of two designer crosses: Jack Russell terriers with pugs (jugs) and beagles with pugs (puggles).

"We're now crossing a really sick dog and a very healthy dog, and we're trying to see what we get in the middle," O'Neill explained.

The researchers hypothesize that the healthiest dogs will be the purebreds with longer noses (Jack Russells and beagles) and that the squashed-nosed purebreds (pugs) will exhibit the most health challenges. The designer crosses, they anticipate, will land somewhere in between.

Generally, O'Neill believes that the fewer extreme physical traits a dog has, the healthier it will be and the longer it will live.

"That's not hybrid vigor," O'Neill said. "That's just regression to being a dog, as opposed to the direction of much of dog breeding for the last 150 years, which has often prevented dogs from being dogs."

A way forward for dog owners, veterinarians, researchers

One possible shortcoming of the RVC and Dog Aging Project studies is that they explored a broad spectrum of disorders — from hip dysplasia to chocolate ingestion — to offer a comprehensive view of canine health. But when assessing the existence of hybrid vigor, only genetic disorders should be considered, says Dr. Mark Rishniw, director of clinical research at the Veterinary Information Network, an online community for the profession and parent of the VIN News Service.

"My concern with these studies is that they're essentially conflating general well-being with hybrid vigor," Rishniw said.

Health issues such as Giardia infections or dog bites demonstrate nothing about the health of purebreds versus mixed or crossbreeds, Rishniw maintains. "The risk is identical," he said.

That view is not universally held. Anita Oberbauer, an animal scientist and executive associate dean at the University of California, Davis, College of Agricultural and Environmental Sciences, believes that all health conditions have "a little bit of a genetic component."

For their part, O'Neill said the RVC researchers adopted a broader approach to physical health, recognizing that no condition is solely influenced by genetic inheritance.

"Every disorder is both acquired and inherited, but the relative contribution of the ‘acquiredness' and the ‘inheritedness' varies across those disorders," O'Neill said.

The RVC and Dog Aging Project acknowledge that a limitation of their respective studies is that they rely on owner-reported data. While most dog owners have less health literacy than veterinarians, surveys of owners tend to be more comprehensive, in part because many minor issues like coughs, bumps and bruises are not treated in clinics.

In future studies, RVC researchers hope to compare veterinary and owner-reported data for doodles, other designer crossbreeds, mixed breeds and purebreds.

By recognizing and trying to fill research gaps, veterinarians can help owners make informed health decisions based on their dogs' genetic background and physical traits.

"I want people to have happy, healthy dogs that work well for their families," Creevy said. "I think that the way to achieve that is to understand the nuances and not to make simple assumptions that all mixed-breed dogs are healthier than purebreds."