From the dungheap to the stars

Dung beetle

A scarab beetle – photo by Udo Schmidt

What a great way to end the week – with the discovery that scarabs (dung beetles) navigate by the stars to make sure their precious spheres of poo don’t get stolen. Finding this paper (by Marie Dacke of the University of Lund and her colleagues) on Twitter today made me unreasonably happy – because it perfectly encapsulates everything that’s great about science.

Who couldn’t be impressed by the contrast between a beetle rolling a ball of dung, guided by the enormous arm of a spiral galaxy? It’s lovely that these two vastly different things are connected – a link revealed by a brilliant experiment that involved beetles wearing hats and wandering around in a planetarium.

This isn’t just charming –  it’s also pretty big news. As the study authors say, ‘only  birds, seals and humans are known to use stars for orientation… a feat that has, to our knowledge, never been demonstrated in an insect.’ It’s also  ‘the first documented use of the Milky Way for orientation in the animal kingdom’ (apart from humans, of course). That’s right – the list of things that only humans can do is looking less and less impressive (no bad thing, in my opinion).

So how did the experiment work? The researchers thought it was likely that beetles used the stars for navigation, because they seemed rather good at rolling their dung balls in straight lines, even when there was no sun or moon visible to guide them. To find out, they observed the beetles in their natural environment (or as close as they could make it while still being able to measure the results accurately). They compared two groups of beetles – ones wearing cardboard caps to obscure their view of the sky, and beetles that could see all around them. They found that the beetles wearing hats were less good at rolling their balls in a straight line.

Clearly the hats were obscuring something important – the next question was exactly which celestial objects the beetles were using to navigate. As the authors rather poetically explain, ‘the night sky is sprinkled with stars, seen from Earth as point sources of light varying in size and intensity, but the vast majority of these stars should be too dim for the tiny compound eyes of a beetle to discriminate.’ So what were the insects seeing?

In order to have a bit more control over the experimental conditions, the researchers moved to the Johannesburg planetarium. They tested 5 different artificial skies – complete starry sky with the Milky Way, Milky Way only, dim stars with the brightest 18 stars excluded, the 18 brightest stars only, or total darkness. The beetles walked in straighter lines when they could see the full sky or the Milky Way only – and other experiments outdoors showed that when the Milky Way was naturally not visible (when it was close to the horizon), beetles were less able to roll in straight lines.

As Sherlock Holmes would say, ‘when you have eliminated the impossible, whatever remains, however improbable, must be the truth’. So it looks like dung beetles really are using the Milky Way to navigate. And, as the authors suggest, other animals (such as frogs, moths and locusts) might also turn out to have this rather useful ability.

What a beautiful discovery – a simple yet ingenious experiment, with an unexpectedly impressive result. This is everything that’s great about science – stretching from the sublime to the ridiculous (from the dungheap to the stars) and making you appreciate all the wonders of the world a little bit more every day.


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