To aid in their fierce competition over fresh feces—a source of food for both adults and their brood—African ball-rolling dung beetles, Scarabaeus satyrus, look up to the heavenly light of the stars, scientists report.
Dung beetles, it was previously known, can use light polarization patterns from the Sun and the Moon as guides. That enables them to efficiently flee in straight lines from the mad scramble at dung heaps, where other beetles may try to steal a hard-won ball. Researchers from Sweden and South Africa led by Marie Dacke of Lund University, Sweden, noticed, however, that even on moonless nights, beetles still hew to straight paths.
To test whether the beetles navigate via the stars like birds, seals, and humans—the only animals proven to do so—the researchers fitted either black or transparent caps over the beetles’ eyes. They then videotaped the insects rolling dung balls outward from the center of a closed arena. Those beetles prevented from seeing a moonless sky meandered considerably, creating paths about three times as long as beetles wearing clear caps, which made beelines by starlight alone. In a second test, overcast conditions or dark caps resulted in far longer times needed for beetles to reach the edge of a wooden platform from the center.
Finally, the researchers took the experiment indoors to the Johannesburg planetarium. When just the eighteen brightest stars were simulated in the sky, the beetles could not roll straight, probably because their tiny compound eyes are not sensitive enough to discriminate those lights. But under the glow of a typically star-studded night, including the bright stellar band known as the Milky Way, or under the Milky Way alone, the beetles swiftly traversed the arena.
“We have discovered that nocturnal dung beetles are able to use the broad stripe of light in the Milky Way as a compass cue,” said coauthor Eric J. Warrant, also at Lund University. The lowly dung beetle now stands as the first convincing case of insects using the stars and “the first animal we know of that is able to use the Milky Way for orientation.” (Current Biology)