Cells acquire mutations throughout life, a process that is known to give rise to cancer and has been proposed to contribute to ageing. There is little knowledge, however, about the rate at which mutations accumulate in species other than humans, and whether this rate is influenced by biological traits such as lifespan or body size. In this week’s issue, Alex Cagan, Adrian Baez-Ortega and colleagues address these questions. The researchers studied the speed at which mutations accumulate during life in 16 mammalian species and found that the number of mutations increases by a roughly constant amount each year. They also observed that the molecular processes causing mutations are broadly similar across species. Crucially, the team identified a strong anticorrelation between lifespan and mutation rate: longer-lived species accrue mutations at a slower pace than shorter-lived ones, such that different species have roughly the same number of mutations by the end of their respective lifespans.