Wednesday, April 10, 2013

Not Quite Like a Rolling Stone

Dung beetles are competitive little critters. And who can blame them? When a fresh pile of poo is at stake, wouldn’t we all be a bit competitive? …Okay, maybe not. But animal dung is actually chock-full of nutrients, which makes it a precious resource to the animals that can make use of them. The approximately 6,000 species of dung beetles and their babies are among the animals that make excellent use of those resources.

Mmmm... A poo-pile worth fighting for! Image by Duwwel at Wikimedia.
But even animal dung is a limited resource. When it is plopped out, dung beetles gather from far and wide (okay, maybe not that far) to compete over the miraculous manna from heaven. Many dung beetle species, such as the South African dung beetle, forms round balls of poo to prepare them for transport, and then rolls them away from the poo-pile. This process of forming the poo-ball and getting it away from the poo-pile must be quick. Otherwise, dung beetles that are either less fortunate or less-inclined to work for their poo-balls will try to steal pre-made poo-balls from those that worked to form them.

But this isn’t a story about competition. This is a story about navigation.

The fastest way to get a poo-ball away from the poo-pile is in a straight line, and dung beetles are experts at pushing their poo-balls in straight lines. Researchers have played countless tricks (like adding obstacles and spinning floors) on these guys to try to confuse them into going the wrong way, but to no avail. These guys always seem to know where they want to go. But how?

There are many methods animals use to navigate. Some use celestial cues, like the placement of the sun or the stars, to know what direction they are facing. Some remember the placement of landmarks in places they visit often, such as their home. But many combine strategies to get a more accurate idea of where they are going.

Marie Dacke and Marcus Byrne from the University of the Witwatersrand in South Africa, and Jochen Smolka, Eric Warrant, and Emily Baird from Lund University in Sweden set out to test the relative importance of landmarks and celestial cues in South African dung beetles.

A dung beetle rolls his poo-ball backwards. Photo by Dewet at Wikimedia.
South African dung beetles face backwards when they roll their poo-ball away from the poo-pile. In this position, they should always be able to see the poo-pile and perhaps use the pile itself as a landmark. To test this, the researchers placed beetles on dung piles and waited for them to make a poo-ball and roll it away. Once they got 75 cm away, they moved the dung pile 45° to the left or to the right with respect to the beetle. If the beetles use the dung pile as a landmark, the new location of the pile should make the beetles change course by 45° as well. But they didn’t.

Next, the researchers tested whether dung beetles rely on more distant landmarks to know where they’re going. They created two adjacent, but different, testing arenas: One had an unobstructed view of the surrounding landscape (called the “landmark arena”), and the other was surrounded by a beige featureless wall (called the “no-landmark arena”). Both arenas had a full view of the sky and its celestial cues. They placed beetles and their poo-balls at the center of the landmark arena and allowed them to roll away at least 80 cm. Then they picked them up and placed them in the no-landmark arena and waited to see if the removal of landmarks caused them to roll in the same direction or change bearings. They repeated this in the opposite direction to see if the addition of landmarks could improve accuracy. To check if simply moving the beetles affected their rolling directions, they also picked beetles up and returned them to their original arenas (these were the control beetles). In the end, the beetles that changed arenas did not navigate any differently than the beetles that were picked up and returned to their original arenas, indicating that as long as the beetles can see the sky, they don’t seem to rely on landmarks to navigate.

To test if the sky provides important navigational information to the dung beetles, the researchers put little cardboard hats on some of them to block their view of the sky and had them roll their poo-balls from the center of a wall-free arena with full view of landmarks. Other beetles were allowed to roll their poo-balls without the obstructive hats. But just to be sure the hats themselves weren’t causing the beetles trouble (other than by blocking their view of the sky), the researchers also tested a group with transparent plastic hats. They found that the beetles with no hats or with clear hats rolled their poo-balls just fine. However, the beetles with dark cardboard hats spun in circles, whirled and twirled, apparently having the darnedest time going in a straight line. But a dung beetle doesn’t have to wear a cardboard hat to lose track of the sky. The researchers tested beetles on a clear day with hats, on a clear day without hats, and on an overcast day without hats. The overcast sky screwed up the little buggers almost as much as the cardboard hats did!

Combined, these studies show that the South African dung beetles rely almost entirely on celestial cues and don’t seem to rely on landmarks at all. Even the dung pile, which is always a large, central easy-to-see landmark, seems to be completely ignored by the poo-rolling dung beetles. This heavy reliance on a single type of navigational cue is unusual in the animal world, and you can imagine the havoc it would wreck if all animals got so lost every time there was a cloudy day. But for the South African dung beetle, the consequences aren’t as high as they are for other species. They aren’t taking their poo-balls to a specific location, just away quickly, so they’re never truly lost as long as they’re with their poo. And if the worst happens and someone takes their poo-ball, they can go make another.

We can learn a lot from these guys. In life we face obstacles, we go unknown directions, and we get lost. But no worries… The sun will come out again soon.

Want to know more? Check these out:

Dacke M, Byrne M, Smolka J, Warrant E, & Baird E (2013). Dung beetles ignore landmarks for straight-line orientation. Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology, 199 (1), 17-23 PMID: 23076443


  1. Hello

    Can I point out that a paper by Eric Matthews from 1963 paper showed that the position of the sun is what dung beetles use to orient during ball rolling.

    1. Absolutely! All great research is on the shoulders of previous great research. Thanks!