Coded for a Journey You've Never Taken

Every autumn, tens of millions of monarch butterflies lift off from meadows across North America and travel up to 3,000 miles to a cluster of oyamel fir forests in the mountains of Michoacán, Mexico — a place not one of them has ever been. This is the Methuselah generation. Named by researchers for its extraordinary lifespan — six to eight months rather than the usual few weeks — this generation carries something their short-lived predecessors did not: a built-in compass.…