Zeus’s story is heartbreaking for any animal lover. It’s also too common among big dogs. Larger breeds tend to die younger; Great Danes live just 8–10 years on average, according to the American Kennel Club. A lot of factors influence a dog’s lifespan, but body size is a big one. In a 2019 study of dogs that visited U.S. veterinary hospitals, body size was a better predictor of a short life among pet dogs than any other statistic, and researchers have since confirmed that the link between size and lifespan holds up regardless of breed.
Body size vs. lifespan
This relationship between body size and dog lifespan seems stranger when you consider that, looking across species, the opposite is true.
“Typically larger animal [species] live longer, smaller animals don’t live as long, but then we see that opposite with dog breeds,” says Bobbie Ditzler, a veterinarian on the Dog Aging Project at the University of Washington. This means a larger body size alone doesn’t reduce an animal’s lifespan— otherwise, 90-foot blue whales would live like mayflies.
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Veterinarian Silvan Urfer, who also works on the Dog Aging Project, says several competing theories explain big dogs’ short lives. Urfer rules out an easy explanation based on inbreeding. “All modern dog breeds are already very inbred, and there is no reason to think the large ones are somehow more inbred than the small ones,” he says. He also points out that big dogs of a mixed-breed background live, at most, just a few months more than purebreds.
One theory centers on growth. Despite their giant size, Great Danes begin as surprisingly small puppies. “Dogs are not that different in size as newborns,” says Urfer. Between all dog breeds, he says, there is at most a twofold difference in size. But a fully grown Newfoundland lives 9‑10 years and weighs up to 150 pounds. A Chihuahua lives 14‑16 years and weighs six pounds – not much more than a bag of flour. That leaves big dogs with a huge amount of growing to get through.
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