Dung beetles and the Milky Way

To round off this month’s Naked Genetics, it’s time to explore another quirk of evolution...

You join me at dusk on the plains of the African savannah. Another day of life, heat, eating, drinking. Another performance of the trophic ballet from the bottom to the top of the food chain. All of that life, all of those natural processes will inevitably lead to a lot of… Waste. I don’t know why this line keeps cropping up, it just turns out that most of ecology is stuff dying or pooing. Such is the cosmic marvel of existence. But in this particular instance, a cosmic marvel is marvelling at the cosmos. Bear with me.

Night has drawn in. The temperature has dropped, and now it is time for one of the unsung heroes of the savannah to take to the stage - the dung beetle. Now I could wax lyrical for a while about how fantastic these creatures are. Not only do they remove a significant amount of waste from the areas that they inhabit, but also reduce the number of gastrointestinal parasites and nuisance flies that would otherwise make their home in the dung.

But we’re not here to talk about the job that dung beetles do, we’re here to talk about how they do it. The average night for them goes something like this. Wake up, find a fresh pile of dung, roll it a few metres away from the source, so as to avoid spats with other dung beetles, bury the dung, and then eat it. A good honest day's work. But here’s a question for you. When you have your delicious ball of dung ready, what’s the simplest way of avoiding competition or ending up back where you started? Of course, you should go in a straight line, which is what dung beetles do. A very straight line. Almost too straight. To achieve such consistent straightness, they would need a reference point, an ever present straight line, with which to orientate themselves on their journey. And so, this is what led scientists to believe that the humble dung beetle is using the light of the Milky Way galaxy as its compass through the night.

Now, using the night’s sky as a reference point is not common, but also not unheard of in the animal kingdom. It is believed that species of birds use certain constellations when migrating. Seals use certain lodestars as points of reference if they go far out from the shore. But dung beetles are seemingly the only organisms using the Milky Way itself.

But how to test this theory? Well obviously, you ship off some dung beetles to a planetarium in Johannesburg, and fiddle about with the lights. By turning on and off certain celestial features, stars and what have you, it was clear that the dung beetles were happy when the Milky Way was on display, but became confused when only the brightest stars in the sky were illuminated.

So clearly the Milky Way plays a pivotal part in dung beetle orientation, but what specific part  are these beetles using? James Foster and his team at Lund University in Sweden had a theory. Beetles, bless them, don’t really have complex enough brains or eyes to spot and recognise individual stars, or constellations. So instead was the changing brightness gradient from the centre of the galaxy moving outwards. They can see where they are in relation to this white line above them, and maintain course keeping the line in the same place. And when dung beetles were duped into looking at artificial galaxies, they couldn’t recognise stars within the Milky Way, but were very happy to use a simulacrum of the entire galactic mass.

So there you have it. If you ever look out at the night’s sky and wonder if we’re the only creature that does so, rest assured, there’s a dung beetle heading out for the night shift doing the very same.