Dung beetles guided by Milky Way

Dung beetles manage to maintain straight roll paths even on moonless nights

They may be down in the dirt but it seems dung beetles also have their eyes on the stars.

Scientists have shown how the insects will use the Milky Way to orientate themselves as they roll their balls of muck along the ground.

Humans, birds and seals are all known to navigate by the stars. But this could be the first example of an insect doing so.

The study by Marie Dacke is reported in the journal Current Biology.

"The dung beetles are not necessarily rolling with the Milky Way or 90 degrees to it; they can go at any angle to this band of light in the sky. They use it as a reference," the Lund University, Sweden, researcher told BBC News.

Dung beetles like to run in straight lines. When they find a pile of droppings, they shape a small ball and start pushing it away to a safe distance where they can eat it, usually underground.

Getting a good bearing is important because unless the insect rolls a direct course, it risks turning back towards the dung pile where another beetle will almost certainly try to steal its prized ball.

Dr Dacke had previously shown that dung beetles were able to keep a straight line by taking cues from the Sun, the Moon, and even the pattern of polarised light formed around these light sources.

But it was the animals' capacity to maintain course even on clear Moonless nights that intrigued the researcher.

So the native South African took the insects (Scarabaeus satyrus) into the Johannesburg planetarium where she could control the type of star fields a beetle might see overhead.

Importantly, she put the beetles in a container with blackened walls to be sure the animals were not using information from landmarks on the horizon, which in the wild might be trees, for example.

The beetles performed best when confronted with a perfect starry sky projected on to the planetarium dome, but coped just as well when shown only the diffuse bar of light that is the plane of our Milky Way Galaxy.

Dr Dacke thinks it is the bar more than the points of light that is important.

"These beetles have compound eyes," she told the BBC. "It's known that crabs, which also have compound eyes, can see a few of the brightest stars in the sky. Maybe the beetles can do this as well, but we don't know that yet; it's something we're looking at. However, when we show them just the bright stars in the sky, they get lost. So it's not them that the beetles are using to orientate themselves."

And indeed, in the field, Dr Dacke has seen beetles run in to trouble when the Milky Way briefly lies flat on the horizon at particular times of the year.

The question is how many other animals might use similar night-time navigation.

It has been suggested some frogs and even spiders are using stars for orientation. The Lund researcher is sure there will be many more creatures out there doing it; scientists just need to go look.

"I think night-flying moths and night-flying locusts could benefit from using a star compass similar to the one that the dung beetles are using," she said.

But for the time being, Dr Dacke is concentrating on the dung beetle. She is investigating the strange dance the creature does on top of its ball of muck. The hypothesis is that this behaviour marks the moment the beetle takes its bearings.

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