The humble dung beetle makes its living rolling big balls of excrement to feed its offspring and itself. But this lowly occupation doesn't mean the insect doesn't have its eye on the skies—even when the sun goes down.
Recent research has shown that African ball-rolling dung beetles (Scarabaeus satyrus) use strong light cues from the sun and moon to keep traveling in a straight path. But researchers observing these beetles noticed something curious. "Even on clear, moonless nights, many dung beetles still manage to orientate along straight paths," Marie Dacke, of Lund University in Sweden, said in a prepared statement. What else could the animals be using for guidance? The stars, of course.
"We were sitting out in Vryburg [in South Africa] and the Milky Way was this massive light source," said Marcus Byrne, of Wits University and co-author on the new study, in a prepared statement. "We thought, they have to be able to use this—they just have to!" Their findings were published online January 24 in Current Biology.
Humans, of course, have long used stars for navigation. And some bird and seal species are thought to do so as well. The use of the Milky Way in particular has been suggested for some insects, spiders and vertebrates, but it has yet to be shown quite as convincingly as the researchers were able to demonstrate for these beetles, which have also recently been shown to engage in complex "dances" to orient themselves on top of their excrement orbs.
To see if the starry sky was really serving as such a lofty guide for these little bugs, researchers designed specially crafted cardboard caps for their subjects.
On a starry, moonless evening, researchers released capped-beetles with their dung balls from a central spot. The area was a flat sandy surface surrounded by a one-meter high, circular wall. As a test, other beetles were left uncapped and a third group received transparent plastic caps. The cap-less beetles and those with clear caps had standard, relatively straight paths. But those with the obstructed views meandered far afield and had much longer, inefficient trails.
To make sure the stars were the only guiding landmarks, the researchers designed a second outdoor experiment in an elevated, enclosed arena from which no trees, structures or other objects were visible to the beetles. In this test, beetles were timed to see how long it took them to reach a circular black cloth wall from a central release point in the open-air enclosure. In this case, researchers could hear a "thump" when beetles reached the wall and fell into a slightly lower trough below it. Rolling speeds remained constant for capped bugs as well as those tested in complete darkness. So those sticking to a straight path would reach the wall sooner, whereas those that wandered would take longer.
A moon meant beetles took the most direct route, reaching the edge in an average of just 21 seconds. With a full, starry sky (sans moon) as the main feature to go by, beetles took about 40 seconds. But with a star-obscuring cap, it took the bugs some 125 seconds to roll their balls to the edge, suggesting they took a far less direct route than bugs that had the moon or stars to navigate by. Overcast conditions had a similar effect, resulting in an average time of 117 seconds to reach the edge of the enclosure.
So are these beetles relying on the star scape itself or the swath of the galaxy visible from Earth as the Milky Way stripe? "The vast majority of these stars should be too dim for the tiny compound eyes of the beetle to discriminate," the researchers noted in their paper. To control the star conditions, the researchers transported their beetle arena to the Johannesburg planetarium, which can project some 4,000 stars and the Milky Way on the domed ceiling.
A close approximation of the night sky, including individual stars and the Milky Way led to the more exacting navigation (about 43 seconds to reach the edge of the enclosure). But if shown the Milky Way—as a diffuse stripe of light, similar to how it might appear to the beetles outside—the beetles still made good time, reaching the outside wall in just 53 seconds. If the researchers took the Milky Way out of the picture, and gave the beetles 18 bright "stars" to navigate by, they didn't stick to their straight-and-narrow paths so well. It took these bugs some 83 seconds to reach the edge. "This clearly shows that the beetles do not orientate to a single bright 'lodestar,' but rather to the band of light that represents the Milky Way," the researchers noted.
"This finding represents the first convincing demonstration for the use of the starry sky for orientation in insects and provides the first documented use of the Milky Way for orientation in the animal kingdom," the researchers wrote.
But don't think that these bugs are going to start creating cross-continental shipping routes or long-distance precision migrations. "The dung beetles don't care which direction they're going in; they just need to get away from the bun fight at the poo pile," Byrne said.