Dung beetles rely on the Milky Way for direction at night, but new research shows how light pollution is causing them to lose their bearings. (Photo by Taylor Weidman/LightRocket via Getty Images)
Unless you’re a Viking, or a member of a similarly ambitious, seafaring nation back when maritime navigation consisted of little more than your eyes and a good sense of direction, you’ve probably got bigger things on your mind than how to get around at night using only the stars above you as a map.
But then again, you’re not a dung beetle. New research shows that dung beetles are struggling to see that map, leaving them very much in the dark.
New research, published in the scientific journal Current Biology in July, shows that light pollution is tampering with dung beetles’ ability to move around in the dark, with potentially devastating consequences for other creatures who move around at night.
The research was conducted by James J Forster from the University of Wurzburg, Germany; Claudia Tocco from Wits University; Jochen Smolka and Lana Khaldy from Lund University, Sweden; Emily Baird from Stockholm University and others.
Researchers discovered that dung beetles use the Milky Way to navigate in 2017. Now, the researchers set out to track dung beetles in the Johannesburg city centre and in rural Limpopo, to find out how they react to different kinds of night skies.
But first, they needed to understand how dung beetles use the night sky to find their way.
Before a dung beetle rolls his ball of dung away, it jumps on top of it and does a quick turn, called an “orientation dance”. This is how the beetle looks for markers in the sky or elsewhere to help it find its way.
The primary marker for dung beetles is the Milky Way, as the 2017 research showed. Once the beetle has its bearings, it takes its dung ball and moves off in a straight line. By observing this behaviour at light-polluted, urban sites, and dark-sky sites in rural areas, the researchers were able to tell whether light pollution affected the beetles’ movements.
They also observed how the beetles respond to skyglow – a phenomenon where the sky is lighter due to artificial lighting in the distance.
When the beetles lost sight of the Milky Way, one of two things happened: either there was another clear light source such as a floodlight or light from a nearby building that they gravitated towards. Or, the skyglow clouded the sky so much that there was no clear light source, either celestial or terrestrial, and the beetles struggled to move in a straight line as they normally would.
At first, the research appeared to show no difference in how the city dung beetles and rural ones orientate themselves.
“It appears that, despite the loss of natural celestial cues to skyglow, light pollution did not hinder orientation in these animals,” the researchers wrote.
This might seem odd, but it shows that the dung beetles adapted: in the absence of the Milky Way, the beetles relied on different light cues. They looked towards the artificial sources of light for direction, like the lights from buildings.
But when the sky was heavily obscured, the beetles were completely disoriented, and couldn’t move in a straight line – an energetically costly situation.
When there was a clear light source, such as a floodlight, the beetles tended to gravitate towards it. By gravitating towards the light source, they also gravitated towards each other, and this is the problem: the very reason why individual dung beetles roll their dung balls away from where they’ve been collected is to avoid having to share with other creatures. Gravitating towards a single light source defeats this purpose and increases the risk of dung ball-hijacking.
Here’s the concern for the researchers: if this is happening to dung beetles, could it be happening to other animals who rely on the night sky to orientate themselves? If it is, this could have wide-reaching implications for other types of creatures, including some migratory birds, and even seals.
From an evolutionary perspective, the reasons why some animals move around at night are quite self-evident: to avoid predators and other threats which are present during the day. Simply put, if the light produced by humans is making these night creatures night-blind, this could have serious consequences for their survival, and the ecosystems that support them.