Monday, January 28, 2013

Dung Beetles and Ball-Rolling: Star Light, Star Bright


Lately, it seems that poo is a popular topic in science news sections, and the dung beetle seems to be up front and center. I suppose that, if you are a dung beetle, you've solved all sorts of poo-related problems. If you recall the dung beetles and ball-cooling post from November, you will remember that these insects use their dung balls to help cool off their feet on the blazing hot African sands. But what if you are a beetle that works at night? You can chuck out the hot feet problem and worry about a whole new one: navigation. A new paper published in Current Biology suggests how dung beetles may solve this navigation dilemma.

For African ball-rolling dung beetles (Scarabaeus satyrus), the best ball rolling strategy is the straight line. The straighter the path the beetle uses to roll the ball away from the dung pile the less likely it is their ball will be stolen by rival beetles. They spent all that time to pinch and roll the poo together, it is a waste of time and energy if it is stolen. Competition is fierce near the dung heap, so a quick and straight exit strategy is best. Getting that ball to roll straight isn't simply a matter of putting one tarsus in front of another, it usually involves exploiting such celestial features as the sun and the moon to orientate. However, it has been observed that many beetles will still manage to orientate along straight paths on clear moonless nights. So what are they using to help them navigate?

To answer this question, a group of researchers set up some beetles in arenas. I know, it already sounds good. On a starlit night, they placed dung beetles with their dung balls in a flattened, leveled, and enclosed circular arena.They first wanted to know how accurately the dung beetles could orientate along straight paths when they were prevented from seeing any celestial cues at all. So they made little hats for them. No kidding. They made little caps from small pieces of cardboard and attached them to the beetles' heads so that their dorsal field of view was obscured but their ventral eyes were unimpeded. Then they let them roll, filming them from above so that the rolling paths could be reconstructed and measured. The sight impeded beetles had path lengths almost 4 times longer than beetles that could see the moonless night sky. Okay, so maybe the beetles are using landmarks, like trees, to help them. To test this, the researchers made another arena that removed all visual cues (including the observer), enclosing it with a circular black cloth wall. Because they removed all observer cues, like the camera, they had to design the arena such that it could tell them when the beetles were at the edge without the researchers filming or looking. So they made the arena wall with a slightly larger diameter than the floor so that there was a gap large enough to allow the beetles reaching the edge to fall from the floor into a trough below, resulting in an audible thump sound. Since ball rolling speed is relative to path straightness (the straighter your path the faster you get to the edge), they just had to time the thumps. Clever. Under a full moon, starry night the beetles took 21.4 seconds to exit the arena and on a moonless, starry night they took a reduced, but not significantly so, 40.1 seconds. The story changes when you put the little beetle hats back on. With the caps they take a significantly longer 124.5 seconds (note: this is not significantly different than an overcast night at 117.4 seconds).

Figure 2 from Dacke et al. (2013) showing the effect of stars on dung beetle orientation

Now we know that stars are important in getting a dung beetle to roll its ball straight. Good. But we also know that most stars are too dim for tiny beetle eyes to discriminate. It is probably unlikely that the beetles are picking out constellations for their navigating needs. So what orientation information are they extracting from a starry sky? To answer this question, the researchers grabbed their beetle arena and took it to the Johannesburg planetarium, where they could manipulate the sky the beetles were seeing. Again, clever. They performed the experiments under five different conditions: (1) complete starry sky, with more than 4,000 stars and the Milky Way, (2) Milky Way only, (3) dim stars, with the brightest 18 stars excluded, (4) 18 brightest stars only, or (5) total darkness. They found that the beetles took the same amount of time to exit the arena, irrespective of whether they could see the full projection of the starry sky or only the Milky Way. This means that the dung beetles are using the bright band of light produced by the Milky Way. The Milky Way is a bright band because it is made up of stars, and when the Milky Way part of the projection (a diffuse streak of light) is removed you still see a sky where a higher density of stars defines the galaxy's axis. In this case, the beetles were still able to use the star density but it took them somewhat longer to reach the edge of the arena. It is high density of light forming into the streak across the sky that is visible, and therefore usable, to the beetles.

There is all sorts of navigating going on in the animal kingdom. Now, it appears that we've found another one. One that may be more widespread than we yet know. We just need to go looking.

ResearchBlogging.orgDacke, M., Baird, E., Byrne, M., Scholtz, C., & Warrant, E. (2013). Dung Beetles Use the Milky Way for Orientation Current Biology DOI: 10.1016/j.cub.2012.12.034

Some press stories on this paper:
National Geographic: "Dung Beetles Navigate Via the Milky Way, First Known in Animal Kingdom"
ScienceNOW: "Dung Beetles Navigate by the Milky Way"
The Naked Scientists: "Dung Beetles Navigate by the Light of the Milky Way"
The New Yorker: "Dung Beetles, Dancing to the Milky Way"
Wired: "Lowly Dung Beetles Are Insect Astronomers"

(dung beetle hat photo credit to Eric Warrant via the NatGeo link above)
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