After the sun sets over the African savannah, the nocturnal male beetle Scarabaeus satyrus leaves its nest in the ground to go on a hunt for a pile of fresh dung. Once he finds the fuming manure, the beetle franticly sculpts it into a ball and rolls it away as quickly as possible to escape competition from vicious dung stealers. At a safe distance from the dung heap, the beetle buries the ball and, if he is lucky, a female will mate with him and then lay her eggs inside the secluded excrement. Otherwise, he simply enjoys feeding on its nutritious content.
The fastest way to get away from the perilous dung pile is to move in a straight line. This is an easy task for the beetle’s diurnal cousins, which can use the sun and terrestrial landmarks, like trees, to guide their way, but for nocturnal dung beetles staying on their path can be quite tricky. New research in Current Biology now shows that dung beetles can use the Milky Way as a compass. “This is the first animal known that does this” says Eric Warrant, leading author in the study.
Even in a full moon, the sunlight reflected by the moon is a million times weaker than direct sunlight. Despite this, beetles and other nocturnal insects such as ants and moths know their way around astonishingly well, as well as diurnal insects in fact. The secret lies in their super-sensitive compound eyes, which are extremely well adapted to see in dim light. For beetles, as for most insects, the main night guidance cue is undoubtedly the moon.
But what happens in moonless nights? A few years ago, while studying how beetles use the polarised moonlight pattern as compass, Warrant and colleagues at the University of Lund noticed that, even in moonless nights, the beetles could still roll their dung balls in a straight line. The only possible remaining visual cue was the stars. “We found this quite a surprise and set out to test whether the beetles were using the stars to orientate” says Warrant “It turned out they did, which was really remarkable!”
The scientists travelled to South Africa to do field experiments under S. satyrus’s native southern-hemisphere starry sky. They placed the beetles with their dung balls in the centre of a circle enclosed by a black cloth (to hide any terrestrial landmark), and measured how long they took to reach the edge of the circle. Under a full moon, the beetles rolled their dung balls in a straight line and quickly reached the periphery. However, on a cloudy night, or when their eyes were covered by tiny cardboard caps, the beetles took six times longer to reach the edge of the circle- they were lost. What happened in moonless, but starry nights? When the only celestial cue available was the stars, the beetles swiftly rolled their balls in a straight direction.
The beetles’ tiny compound eyes, however, are in theory not sensitive enough to distinguish individual stars, but they might detect the Milky Way or constellations, the scientists thought. They tested this in a planetarium, where they could switch stars on and off and check how the beetles behaved under different starry skies. “We found that nocturnal ball-rolling dung beetles are able to roll dung balls in straight lines […] by using the Milky Way as a compass cue” says Warrant. The authors showed that, rather than using the stars themselves, the beetles use the bright streak of light formed by the Milky Way as guidance cue.
Jochen Zeil, an expert in insect navigation from the Australian National University says “The demonstration that an insect with compound eyes that have very low resolution […] can perceive structure in the night sky is significant and interesting.”
Nocturnal insects have morphological adaptations that increase the sensitivity of their eyes. For instance, nocturnal dung beetles have huge light-sensitive cells in their retinas capable of detecting many more photons than the retinas of their diurnal counterparts.
“The main and fascinating remaining question in the context of the present study will be whether the beetles can perceive individual stars” says Rüdiger Wehner, a neurophysiologist specialised in insect vision from the University of Zurich. Warrant suspects they can: “It is very likely that the beetles can see the brightest stars in the sky […] but exactly how many remains to be determined, but this is something we plan to do.”