Large migrations are some of nature's greatest spectacles. Wildebeest and zebra chase the rains through the Mara ecosystem every year, monarch butterflies trace a path from Mexico to Canada and back, and tiny songbirds fly nonstop for days at a time. And now scientists are starting to figure out how they know where to go, and when.

Some of these animals, they’ve found, have their migration pathways written into their genes. A songbird hatched in a laboratory, having seen nothing of the natural world, still attempts to begin migration at the right time of year and in the right cardinal direction.

But large mammals like bighorn sheep and moose are a different story. Wildlife researchers have long suspected that they