A Great Dane may tower over a Chihuahua, but the small dog will probably live much longer. A new study in the journal Science explains why, at the level of DNA.DNA is not just a fixed instruction manual. Chemical marks on top of it control which genes are active.

This layer, called the epigenome, changes with age. Scientists can read these changes like a biological clock.The new study found that large dogs lose certain protective marks faster. The marks normally keep “jumping genes” — rogue DNA segments that can copy and move around — under control.

Giant breeds lost about 35 per cent more of this protection each year than small breeds, the researchers found.When the jumping genes break loose, they can damage DNA and cause the instability linked to ageing and cancer. Male dogs showed faster changes than females too, a surprise that researchers are still studying.The work used the Dog Aging Project, which tracks tens of thousands of dogs. Because dogs share our homes, food and habits, their ageing could reveal clues about our own.