Why Do Smaller Dog Breeds Tend to Live Longer?

Why is it more common to encounter senior Chihuahuas than elderly Rottweilers? The reality is that smaller dogs, barring illness or accidents, generally outlive their larger counterparts. This phenomenon has intrigued veterinarians for decades, yet a conclusive explanation remains elusive.

In the animal kingdom, larger species typically enjoy longer lives than smaller ones — for instance, elephants can reach their 70s, while mice may only survive a year or two. However, this trend reverses within a single species. Despite their diverse sizes, all dogs belong to the same species. On average, giant breeds live about seven to eight years, whereas smaller breeds can often reach 14 or 15 years, assuming no other factors intervene.

"Dog Years" Aren't Really a Thing

The widely accepted notion that one human year equals seven dog years lacks backing from contemporary veterinary science, despite its simplicity and enduring popularity. In reality, the aging process is more nuanced. A dog's first year equates to about 15 human years, the second year adds nine more, and subsequent aging rates depend heavily on breed and size. Small dogs are typically deemed senior at eight years, whereas larger breeds reach old age by five or six.

"The reasons behind smaller dogs outliving larger ones remain unclear, but this disparity in life expectancy likely stems from multiple factors rather than a single cause," explains Dr. Sandra Lefebvre, a veterinarian and epidemiologist who co-authored a 2019 study on factors influencing pet dogs' lifespans.

Life Span Can Depend on Breed

A dog's size isn't the sole determinant of its lifespan; genetics and breeding also significantly influence longevity. Certain breeds are more prone to life-threatening conditions, which can shorten their lives. The aging process varies not only between dogs and humans but also among different dog breeds.

Breeding appears to be a significant factor in reducing a dog's lifespan. A study published on April 28, 2022, in the journal Nature, which analyzed an extensive database of UK veterinary records, found that breed had a greater impact on longevity than size, at least among purebred dogs. For instance, small Jack Russell terriers and medium-sized border collies, despite their size difference, have similar lifespans of 12.7 and 12.1 years, respectively. However, flat-faced breeds like pugs and French bulldogs face shorter lifespans due to respiratory issues, health complications, and birthing difficulties. French bulldogs, for example, have an average lifespan of just 4.5 years, with other flat-faced breeds averaging no more than 7.8 years.

The Speed of Aging Is Key to Canine Life Span

One explanation for the longer average lifespan of small dogs lies in the time it takes for them to reach full maturity and the size difference between their puppy and adult forms. Rapid growth to a large size seems to negatively impact a dog's overall lifespan.

"Large dogs grow and age at a faster rate compared to smaller breeds," explains Lefebvre. "For example, a Great Dane reaches full size in 18 to 24 months, growing up to 32 inches (81 centimeters) tall and weighing 175 pounds (79 kilograms). This rapid growth is incredibly physiologically demanding!"

Cancer is more prevalent in large breeds than in small ones, a trend researchers attribute to their rapid growth during the first year of life. This swift development may increase the likelihood of abnormal cell growth or accelerate the onset of age-related diseases. For instance, a Chihuahua reaches full size in 10 to 12 months, typically growing to just 8 inches (15 centimeters) and 6 pounds (2.7 kilograms), a process far less taxing than the growth of a Great Dane.

Size also correlates with obesity, which shortens a dog's lifespan, much like in humans. "Health issues such as diabetes or cardiovascular disease, often linked to inactivity caused by obesity, are likely contributors," explains Lefebvre.

Lifestyle and access to veterinary care also play a role in a dog's longevity. Smaller breeds, like toy dogs, often receive more attention and care due to their convenient size, potentially extending their lives.