Dung Beetles Navigate by the Milky Way, But City Halos Draw Them Toward Streetlights

Since 2013, the nocturnal dung beetles of South Africa have been known to navigate by the glow of the galaxy. Yet light pollution is gradually erasing this celestial reference. An experiment conducted on a rooftop in Johannesburg shows how these insects react when the starry sky disappears.

A 2013 study showed that Scarabaeus satyrus orients to the Milky Way’s light gradient

The starting point goes back to January 2013. In Current Biology, the team led by Marie Dacke, from Lund University, and her colleagues at the University of the Witwatersrand described a first for an insect. Thus, Scarabaeus satyrus maintains a straight trajectory by relying on the light gradient of the Milky Way, without distinguishing the stars one by one.

That straight line serves a precise function. Indeed, the insect must move away quickly from the dung pile to prevent rivals from stealing its cache. Consequently, any disruption of its celestial compass costs energy, time, and additional confrontations.

In July 2021, dung beetles placed under the Johannesburg sky lost their natural celestial compass

On July 29, 2021, a new study published in Current Biology tested this vulnerability. Under the direction of James Foster, then a researcher at Lund and now at the University of Würzburg, the experiments compared two sites. The first was on the roof of a central Johannesburg building, the second in the rural Limpopo province, under a sky that remained dark.

The result is clear. In Limpopo, dung beetles oriented normally using celestial cues. In contrast, in the city they could no longer exploit their stellar compass and headed toward streetlights and illuminated façades.

The urban halo with no visible source completely disorients the insects, while lamps draw them together

The researchers distinguish two situations. Dung beetles that could directly see a lamp remained oriented, but in what was deemed an abnormal way. However, those that only perceived the veiled sky, with no building or lamppost in view, advanced randomly.

Moreover, several individuals often converged toward the same source. Yet in nature, these scarabs disperse in all directions to avoid each other. Consequently, the authors fear increased competition and unnecessary battles over the balls. According to the team, urban peripheral populations, deprived of stars as well as nearby lamps, would be the most exposed.

Falchi’s measurements in 2016 and Kyba’s in 2023 describe a night sky brightening very quickly

The issue extends beyond dung beetles. In 2016, the global atlas published by Fabio Falchi in Science Advances estimated that more than 80% of the world’s population lives under a sky polluted by light. Then, the Milky Way remained hidden to more than a third of humanity, including about 60% of Europeans and nearly 80% of North Americans.

Furthermore, in January 2023, Christopher Kyba‘s team leveraged in Science more than 50,000 naked-eye observations from the Globe at Night program. Between 2011 and 2022, sky brightness would have risen by about 9.6% per year, compared with nearly 2% according to satellites. However, some 68% of these observations came from Europe and North America.

Meanwhile, Marie Dacke suggests that a similar effect could push moths toward lamps, though the study did not test this. Thus, the dung beetle serves as an early warning model for nocturnal wildlife. If the sky continues to brighten at this rate, the galactic compass of these insects could become unusable well beyond the big cities.