A dung beetle steers by the Milky Way

On moonless nights, a dung beetle rolls its ball in a straight line by keeping a heading against the band of the Milky Way. Hide the sky and the path curls.

After a beetle cuts a ball of dung it leaves in a straight line, which is the shortest way to get the prize away from competitors. Marie Dacke and colleagues showed that the cue, when the Moon is down, is the Milky Way. In the field, under a moonless sky, beetles with a view of the stars rolled straight. Beetles whose view was blocked wandered. In a planetarium, a projection of the full starry sky worked, and so did a projection of the Milky Way's band alone. Individual stars, without the band, were not enough. The experiment is the clean kind: same beetle, same ball, sky on, sky off.

The Moon, when it is up, is a brighter compass and they use it. The galaxy is the backup, and the backup is extraordinary only because of what is doing the looking. The eyes do not resolve a star map. They resolve a stripe of light against a darker sky, and a stripe is a direction. A landmark that vast does not shift as you walk a few metres. For an animal the size of a thumb, that is as good as a fixed star, and a great deal easier to see.