Dung beetles navigate using the Milky Way

Ever look up at the stars and wonder if some bug-eyed creature is doing the same? It turns out at least one does: the dung beetle uses the glow of the Milky Way to navigate.

Once a beetle (Scarabaeus satyrus) has constructed its dung ball, it moves off in a straight line in order to escape from rival beetles as quickly as possible, lest they try and steal its carefully crafted ball. This behaviour doesn't sound complicated, but several years ago, Marie Dacke of Lund University in Sweden and colleagues showed that polarised light from the moon is important for dung beetles to keep to a straight line.

Then the researchers were surprised to find the insects were able to stay on course even on a moonless night. "We thought there was something wrong in our set-up," Dacke says.

The team allowed the beetles to crawl around the floor of a plain-walled cylindrical drum with an open top, meaning they could only use the night sky to orientate themselves. The researchers timed how long it took the beetles to reach the edge of the drum from the centre, and found that under a full moon, the insects took around 20 seconds on average; on a starry but moonless night, they took around 40 seconds.

But when beetles had a cardboard cap placed on them to prevent them from seeing the sky, they needed over two minutes, suggesting the stars were playing a role.

Planetarium clincher

To test this, the team moved the experiment to a planetarium. By switching stars on and off, Dacke discovered that the glowing strip of the whole Milky Way was what guided the beetles' movement. "Before it was assumed insects could not use the stars because their eyes don't have the resolution to see them," she says. Navigating using the whole of the Milky Way does away with the need to see individual stars.

Dacke says the results suggest moths, locusts and other insects might navigate by the Milky Way, too. Her team is now looking at whether the beetles prefer to navigate by the moon or the Milky Way when both are on view.

Journal reference: Current Biology, doi.org/kbm

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