Zeus, the tallest dog in the world, passed away recently at the ripe age of 5. Here's why experts think big dogs have shorter lifespans than smaller ones.
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Sad news for dog lovers: Zeus, the title holder of World's Tallest Dog in the Guinness Book of World Records, passed away from old age on September 3. The "gentle giant," as his Michigan owner lovingly described him to ABC News, was just shy of his sixth birthday.

At 44 inches tall—or a massive 7-foot-4 standing on his hind legs—Zeus was larger than your average pooch. Still, it makes you wonder why big dogs generally have shorter lifespans than smaller ones. A healthy toy poodle, for example, could live up to 18 years, while a Great Dane like Zeus has an estimated lifespan of 7 to 10 years.

A recent study in The American Naturalist used data from the Veterinary Medical DataBase to examine more than 74 dog breeds and determine when and why they died. Though researchers aren't exactly sure why, they found large dogs tend to age faster. They speculate it could be tied to the strain that size puts on their bodies. Many of these dogs weigh anywhere between 100 and 200 pounds, says Jeff Werber, a licensed veterinarian and founder of Century Veterinary Group in Los Angeles.

"The demands on the body are greater for key organs like the heart and lungs," Dr. Werber says.

Genetics could also play a role for these giant breeds, including the Irish Wolfhound, Newfoundland, Mastiffs, and Great Pyrenees. Dogs prone to shorter life spans can pass on that trait, Dr. Werber says. With continued breeding, the offspring become more and more likely to develop a short longevity too.

The owners that gravitate toward giant breeds seem to accept that fate from the get-go. "People who get these breeds know it's part of the plan," he says. "Having that bond for even a short amount of time is worth it because they fall in love with the personality of the dog."

No matter the size of the dog, though, the loss of any furry friend is heartbreaking, so keep Zeus and his owners in your thoughts.