Dung beetles use stars to orient themselves
After locating a suitable dung pile, the ball-rolling dung beetle cuts off a piece of dung, shapes it into a ball and rolls it away to a distant location for burial and consumption. While rolling, the beetles move away from the dung pile in a straight line; a remarkable feat given that they do this facing backwards with their head pointing towards the ground.
Rolling along a straight path is crucial for dung beetles because it guarantees they will not return to the dung pile, where they risk being attacked by other beetles which, rather than making their own ball, would prefer to capitalise on the work of others.
Although their eyes are too weak to distinguish individual constellations, dung beetles use the gradient of light to dark provided by the Milky Way to ensure they keep rolling their balls in a straight line and do not circle back to competitors at the dung pile.
Before rolling a new ball away, the beetles perform a characteristic ‘dance’ in which they climb on top of the ball and rotate about their vertical axis. This dance behaviour can also be observed during the beetles’ straight line departure from the dung pile.
“The dung beetles don’t care which direction they’re going; they just need to get away from the bun fight at the poo pile,” said Professor Marcus Byrne from Wits University, who co-authored the results of the study with Dr Emily Baird, a biologist at Lund University in Sweden.
Byrne and the team previously showed that dung beetles use the sun, the moon and polarised light for orientation. In their experiments, they gave the beetles ‘caps’ which blocked light from reaching their eyes. The experiments showed how the beetles climb on top of their dung balls to perform the orientation dance during which they locate light sources to use for orientation.
Further experiments conducted under the simulated night sky of the Wits Planetarium revealed that the beetles also use the Milky Way – giving new meaning to dancing with the stars!
“We were sitting out in Vryburg [conducting experiments in the North-West province of South Africa] and the Milky Way was this massive light source. We thought they have to be able to use this – they just have to!” said Byrne.
Not all light sources are equally useful landmarks for a dung beetle. A moth keeping a constant angle between itself and a candle flame will move in a circle around the flame. However, a celestial body is too far away to change position relative to a dung beetle as it rolls its ball, with the result that the beetle keeps travelling in a straight line.
The scientists suspect that the beetles have a hierarchy of preference when it comes to available light sources: if the moon and the Milky Way are visible at the same time, the beetles probably use one rather than the other. A few other animals have been shown to use stars for orientation but the dung beetle is the first animal proven to use the galaxy.
In an earlier study, the scientists discovered that the beetles climb on top of their moist balls whenever their front legs and heads overheat. This fact was discovered by accident while scientists watched for the orientation dance. Further experiments found this midday phenomenon only happened while the beetles were crossing hot ground, when they climbed their balls seven times as often as those on cooler ground.
To show it was the beetles’ hot legs that made them climb the ball, the researchers applied some cool silicone ‘boots’ to their front legs as alternative protection from the heat. “To our great surprise, this actually worked and beetles with boots on climbed their balls less often,” said Dr Jochen Smolka from Lund University, who collaborated on the research.
This discovery also marked the first example of an insect using a mobile thermal refuge in this way. It demonstrated the remarkably sophisticated strategies that insects and other cold-blooded creatures employ to maintain their body temperatures.
* Results of the orientation study were published in the journal PLoS One.