Urban dung beetles forge fresh paths

Abstract

Dung beetles know how to make the best of any situation: they can turn a pile of dung into a feast, a nursery and even a lookout post. One species of dung beetle in South Africa, Scarabaeus satyrus, caught the eye of Marie Dacke and her research team at Lund University in Sweden. The beetles are nocturnal, but somehow maintain relatively straight paths when rolling balls of dung – even on the darkest nights when there's no visible moon. Previously, the researchers found that when the moon isn't visible, dung beetles use light from stars as guides to roll away from the central dung pile in straight paths. In a follow-up study this year, the researchers sought to measure whether light pollution, which may obscure guiding light from stars, changes the paths of rolling dung beetles.Usually, dung beetles obtain a lump of dung from a central dung pile and then roll it away to find their own place where they can consume it in peace and lay eggs on it. However, before they begin their journey away from the source of their lump of dung, they typically stand atop the ball and turn in a full circle, taking a mental snapshot of the view of the sky at the starting point of their journey. They then roll their dung trophy away along a straight path, comparing their course with their mental snapshot. The research team wanted to measure how the beetles navigate away from the central dung pile when starlight is shrouded by light pollution.To investigate whether light pollution interferes with their mental snapshots, the researchers looked to see if beetles in a light-polluted city were more disoriented than beetles living in the surrounding countryside, where the stars are clearly visible. Travelling to Johannesburg, South Africa, Dacke and her colleagues found that the beetles in the city forged relatively straight paths. However, when she compared how well the city and rural beetles were dispersed around the dung piles, the city beetles weren't as evenly distributed, possibly leading to conflict over particularly imposing balls of dung. Instead, the urban beetles tended to drift toward the brightest light near the horizon. However, the beetles were still steering their balls of dung along straight paths, suggested that the beetles were not disoriented, and the shared directional preference suggested that the beetles were still using something to guide their way.To test whether the urban beetles were guided to their shared destination by the city's artificial lights when starlight wasn't visible, they placed a flood light near the dung pile, and most of the beetles rolled their dung trophies toward the bright light. In other words, they were guided by the earthly light when it was brighter than celestial light. The researchers then observed the beetles as they encountered a dung pile surrounded by dark walls where no earthly lights were directly visible and the stars were masked by light pollution. This time the beetles rolled their balls of dung along curvier paths, as if they were disoriented when neither starlight nor building lights could be used to guide their way.When dung beetles can't use the moon as a guide, they look to the stars. When they can't see the stars, they rely on earthly lights as landmarks to make their way away from the dung pile. Even when they can't tell where to go and where they've been, dung beetles keep on rolling their eponymous balls, even if their lack of direction sends them on a wobbly course that could send them into the path of another, with potentially disastrous consequences.