THE number of animals that navigate by the stars is small. Only some birds, a few seals and, of course, humans have the ability. It had been assumed that other creatures that might do this would need reasonably well-developed brains. That notion has now been proved wrong as the celestial-navigation club welcomes its latest member: the humble dung beetle.

Life in the world of dung beetles is fiercely competitive. After rolling up a ball of highly nutritious dung, the beetle must race off with it or risk having the ball stolen by other beetles. Strength is important, but so too is the route taken. The ideal tactic on the open plains where many dung beetles live is to move in a straight line. This is easy enough during the day, but at night—when the beetles are most active—it is more challenging. Previous work has shown that dung beetles can make use of the moon to help them navigate, yet some still set a straight course on moonless nights.

To find out how, a team working in South Africa led by Eric Warrant and Marie Dacke, of Lund University in Sweden, designed an intriguing experiment. They made caps for 19 dung beetles. Ten wore caps made of cardboard to prevent them seeing the sky and, as a control, nine wore caps made of transparent plastic. The beetles and their dung balls were then released in the centre of a circular arena made of flattened sand and enclosed by a featureless circular wall. As the beetles rolled away under a moonless night sky they were filmed by infra-red cameras.

The team found that the beetles prevented from seeing the sky by their caps had path lengths that averaged 476.7cm, much longer than the average of 143.4cm travelled by the beetles wearing clear hats.

As the beetles might have used other overhead landmarks, like trees, a second arena was built with a high black wall and a small, dry moat around it. This time the beetles—minus their hats—were timed to see how long it took them to roll their balls from the centre to the point where they could be heard falling into the moat.

Under a full moon the beetles took an average of 21.4 seconds to reach the moat. On a moonless, starry night, their speed was somewhat reduced, but not significantly so. However, under overcast conditions, when neither moonlight nor the stars were visible, the beetles took an average of 117.4 seconds.

Curious as to what it was in the sky that the beetles were using to navigate, the team moved their arena inside the Johannesburg planetarium and reran their experiments. As they report in Current Biology, the beetles presented with a full starlit sky, including the Milky Way or just the Milky Way, took statistically the same amount of time to exit the arena (43.3 seconds and 53.3 seconds). Under a sky full of dim stars they were only a little slower (65.2 seconds). This, speculates Dr Warrant, is because they were still able to spot the cluster that forms the Milky Way.

When allowed to see only the 18 brightest stars or immersed in total darkness, the beetles took more than twice as long to exit the arena. The team now wonders how many other animals might be able to use the glowing strip of light created by the Milky Way to guide them.