Dung Beetles and The Milky Way

One of the benefits of having kids, other than having someone to go and get things from the other side of the room for you, is the interesting conversations that a young and inquisitive mind can generate. I genuinely enjoy trying to explain things to young ones and will normally indulge the incessant parade of why? why? why? which means that conversations all end one of two ways; either, “because of the Big Bang” or “because of evolution by natural selection” depending on whether the initial question had to do with the physical or biological world.

So it was that I found myself a part of an in depth conversation about dung beetles. Dung beetles are super cool. They’re vital for helping to fertilise the land as they can take a big pile of crap, break it up, spread it out and bury it all in under ten minutes. We were speculating on the most likely source of the dung (camels or My Little Ponies) when in the back of my mind I saw a brief glimpse of a dusty factoid long forgotten. There was something about dung beetles and the Milky Way buried in my brain hidden behind some Elvis lyrics and all six Lord of the Rings appendices.

A quick Google reminded me dung beetles are the only animal other than us humans that can use the Milky Way to navigate by. That’s pretty awesome in my book and so, whilst this news is a few years old now, I thought I’d tell you all about it.

In 2013 researchers from Sweden and South Africa published a paper in Current Biology (open access) making the bold claim above. They had already shown that dung beetles used polarised sunlight during the daytime to navigate and they wanted to see if there was some kind of equivalent system in place for nightfall. I should perhaps say why dung beetles should need to navigate at all. Well, once the My Little Pony (more likely than a camel apparently) has made its deposit all the beetles in the area smell it and come flying in. Once there they need to start sculpting the leavings into a ball. They do a pretty damn good job of this and the result is verging on a perfect sphere to the untrained eye which makes it all the easier, of course, for rolling.

However, some beetles won’t bother with the considerable effort of making their own dung ball, they’ll just wait for some other mug to make the investment and then steal off with it when an opportunity presents. To defend against this chicanery the best strategy is for the beetle to speed off in a straight line from the dung pile but this is easier said than done. The dung ball tends to be many times larger than the beetle itself and the way they push it doesn’t exactly lend itself to accurate navigation. They back up to the ball and do a sort of handstand before pushing it along with their hind legs so their head is down and they’re facing the wrong way.

On sunny days and clear nights they seem to have no problem in making a beeline away from their competitors. On overcast days and nights they tend to roll around in circles and not get very far leaving themselves open to theft of their precious dung ball. The researchers speculated that at night they used the moon to manoeuvre but observation revealed that the beetles are still perfectly capable of forging a straight line even during a new moon.

The team was initially puzzled as the only other light source up in the sky was the stars but they knew that the visual system of the beetle wasn’t capable of resolving that kind of detail. We know that there are some species of birds and seals that can do this but it just wasn’t an option for the plucky poo roller. They then speculated that it could be the strong band of light the Milky Way forms that allows them to orient themselves for the get away. How, though, to test this hypothesis?

The obvious first step was to make little cardboard baseball caps for the beetles so that they couldn’t see the sky and the second was to take them on a trip to the local planetarium. No, really, that’s what they did. The figure below shows a beetle donning its trendy headwear in section C. Sections A and B are traces of the paths of the beetles when they can either see the stars or not. As is evident, they do a lot better at getting away from the dung pile when they can see a starry sky.

Dacke_Figure1
Image from Current Biology

To test whether or not it really was the Milky Way part of the sky that they were using they took some beetles to the planetarium in Johannesburg where they could control exactly what parts of the night sky are visible. They ran a series of tests from no stars at all or just the 4000 dimmest, to just the 18 brightest stars or just the Milky Way. The beetles were able to navigate away from their start point about 3 times quicker when the Milky Way was present.

As a final test they ran their experiments in the field again in October when the Milky Way is low on the horizon and barely visible; the beetles struggled to navigate under such conditions.

So there you have it. The only other species shown to use the Milky Way as a way to find your way around whilst doing a handstand and pushing a ball of crap around 5 times the size of your own body. Go science!

Headline image courtesy of Rafael Brix
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