Why big dogs seem to grow old so much faster

Anyone who has loved a large dog may have noticed the unfairness of it: Great Danes, Irish wolfhounds and other big breeds often seem to race through life, while much smaller dogs can remain sprightly well into their teens.

Scientists have long known that larger dogs generally live shorter lives than smaller ones. A new study, however, may help explain what is happening inside their bodies as they age.

According to research recently published in Science, larger dogs appear to show faster age-related changes in their DNA than smaller dogs.

The study from Arizona State University involved 894 dogs representing around 120 breeds and formed part of the long-running Dog Aging Project. Researchers were not looking for changes to the DNA sequence itself, but at chemical markers attached to DNA that help regulate how genes behave.

These markers naturally change with age, which means scientists can use them to estimate how biologically “old” an animal appears to be.

A biological clock that runs faster

Notably, the researchers found that these age-related DNA changes accumulated more quickly in larger dogs. In simple terms, two dogs may be the same age in years, but the cells of the larger dog may show signs of having aged more rapidly.

The researchers also found that dogs whose biological age appeared older than their actual age were more likely to die during the following two years. This suggests the measurements were picking up real differences in the ageing process, rather than simply producing an abstract laboratory number.

One particularly interesting clue involved sections of DNA known as transposable elements – stretches of genetic material that can move or copy themselves within the genome. Cells normally keep them under tight control, but that control can weaken with age.

According to Arizona State University, larger dogs showed stronger age-related changes around these regions than smaller dogs, suggesting that this loss of control may be one piece of the puzzle.

Researchers stress, however, that this does not mean these DNA changes are definitively causing large dogs to age faster. They may instead be one sign of a broader ageing process.

Bigger isn’t always longer-lived

The pattern in dogs is especially interesting because it seems to go against what we see when comparing different animal species. Large mammals such as elephants and whales often live much longer than tiny animals such as mice.

But within the same species, larger individuals can sometimes age faster.

Dogs are a particularly useful example because centuries of selective breeding have created an enormous range of body sizes, from tiny Chihuahuas to giant breeds weighing many times more. Some very large breeds have average lifespans of around 10 years or less, while smaller dogs can often live well into their teens.

The new study suggests that this difference is reflected not only in lifespan statistics, but also in the ageing of their cells.

The researchers also found that male dogs showed somewhat faster molecular ageing than females, although the biological patterns linked to sex were different from those associated with body size.

The study does not provide a single, definitive reason why large dogs tend to die younger. Ageing is influenced by many factors, including genetics, body size, growth rate and disease risk; what this research does offer is another clue to how those factors may play out at the cellular level.

For dog owners, the broader takeaway is simple: larger breeds appear to age faster not just on the calendar, but within their bodies, too.

Subscribe to our newsletter and get news delivered to your mailbox.