For most animals, us included, the chance of dying in any given year keeps climbing with age; for people it roughly doubles every eight years after thirty. In 2018 a team went through the records of more than three thousand naked mole-rats kept in one colony since the early 1980s and found that theirs doesn’t climb. Once grown, a mole-rat of twenty-five was about as likely to die on a given day as one of two: something like one in ten thousand. They still die, of accident and illness and of fights over who gets to be queen, but not, so far as the numbers could show, of having been alive a long time. I find I can’t quite picture it. Every birthday of ours is also a small entry in a ledger that only runs one way. Theirs, as far as anyone can tell, is just a day.