Recent studies are challenging the widely-held belief that crossbreed and mixed-breed dogs enjoy superior health and longevity due to a phenomenon known as “hybrid vigor.” This idea, rooted in the concept of “heterosis” first observed by Charles Darwin in plants, suggests that crossing genetically distinct organisms results in stronger, more resilient offspring. However, new evidence suggests that the health benefits of hybrid vigor may not apply to dogs as previously thought.
In an August study conducted by the Royal Veterinary College (RVC) in the UK, researchers examined the health of three popular designer crossbreeds—Labradoodles, Cavapoos, and Cockapoos—compared to their purebred ancestors. The study found no significant health advantages for the crossbreeds over their purebred counterparts. Similarly, a study published in November 2023 by the Dog Aging Project, an American initiative, indicated that purebred dogs are less likely to suffer from common health issues than mixed-breeds, on average.
Both studies, however, have limitations. Owner-reported data was used, which can sometimes be less reliable than professional veterinary assessments. Furthermore, the researchers acknowledge that certain purebred dogs, particularly those with inherited genetic conditions or extreme physical traits, may benefit from crossbreeding to reduce the likelihood of disease.
The rise in designer crossbreeds, such as Labradoodles, dates back to the 1980s when Australian breeder Wally Conran created the first Labradoodle, mixing a Labrador Retriever with a Poodle to meet the needs of an allergic individual. The success of the Labradoodle has led to the proliferation of other poodle-cross breeds. Despite their popularity, the long-term health impacts of these mixed-breed dogs remain under-researched.
In their study, RVC researchers surveyed over 9,000 dog owners in the UK, comparing the health of the three crossbreeds and their purebred progenitors across 57 common medical conditions. The results revealed no significant health differences in 86% of the comparisons. For the remaining conditions, the odds of illness were nearly evenly split between crossbreeds and purebreds, suggesting that hybrid vigor may not be as pronounced in dogs as it is in plants or livestock.
One reason for this, according to Dr. Dan O’Neill, a lead researcher on the RVC study, is that dogs are already genetically diverse. Unlike crops or livestock, where crossbreeding can introduce much-needed genetic variation, dogs as a species have a broad genetic pool. In fact, O’Neill compares breeding dogs to crossing species, rather than merely breeds, which could result in offspring with unpredictable traits.
The issue is further complicated by the fact that dog breeding practices often prioritize appearance over health, leading to the creation of breeds with extreme physical characteristics. For example, brachycephalic dogs like pugs and French Bulldogs are prone to respiratory issues, and some breeds with small gene pools, such as certain terriers, are more susceptible to inherited health disorders.
The Dog Aging Project study took a broader look, comparing the health of 25 purebred breeds with mixed-breed dogs in the United States. They found that 22.3% of purebreds were free from common medical conditions, compared to 20.7% of mixed-breeds. The study also found that purebred dogs with larger populations and less inbreeding had longer lifespans. However, breeds with small gene pools, such as French Bulldogs, showed increased health risks, suggesting that hybrid vigor may have some merit in breeds with high levels of inbreeding or extreme physical traits.
Dr. Kate Creevy, chief veterinary officer for the Dog Aging Project, pointed out that mixed-breed dogs with healthier genetic backgrounds tend to live longer, while purebreds with problematic traits face higher health risks. The key to dog health, O’Neill suggests, is minimizing extreme physical traits and focusing on breed characteristics that support overall wellness.
While both studies offer important insights, some experts argue that they conflate overall health with hybrid vigor. Dr. Mark Rishniw, director of clinical research at the Veterinary Information Network, emphasizes that the genetic factors behind specific health conditions are more critical than general well-being. Diseases like Giardia or dog bites, for instance, are not influenced by breed type and do not offer insight into the broader question of hybrid vigor.
Looking ahead, researchers at the RVC are planning further studies to explore the health of designer breeds with extreme traits, such as pugs, alongside healthier breeds. They hope to provide more nuanced insights into the health impacts of crossbreeding, with a focus on genetic disorders rather than general well-being.
Ultimately, the goal of this research is to help dog owners make informed decisions about their pets’ health, taking into account genetic factors, physical traits, and breeding practices. While hybrid vigor may not be the clear-cut health advantage it was once thought to be, understanding the complexities of dog genetics will be key to improving the health and longevity of all dogs.
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