Friday, November 9, 2012

Dung Beetles and Ball-Cooling: The Secret of the Poo

You’re a dung beetle. That isn’t an insult, it’s a visualization aid. You are a dung beetle, you live in South Africa, you roll up feces into balls, you push those balls to a storage location, and you use the balls as food or for brooding. Now, as a human visualizing yourself as a dung beetle, consider the environment you are rolling your dung ball across: the sands of the South African desert. Are your feet hot? How do you cool them down?

The authors of a new paper in Current Biology asked just these questions. The hot desert sands of South Africa can exceed temperatures of 60°C (140°F). Even for the resident dung beetle (Scarabaeus lamarcki) that’s hot. It is known that many species will seek refuges to cool down in these hot climes. For example, desert ants will spend up to 75 percent of their foraging time cooling down on elevated thermal refuges (like stalks of grass). It would make sense that dung beetles, which work under similar hot conditions, would seek refuges as they roll their poo-balls across the sand. Returning to your imagined-dung-beetle-state, what do you do to cool off?

The researchers used infrared thermography and behavioral experiments to see how dung beetles use their dung ball as a mobile thermal refuge onto which they climb to cool down. Jochen Smolka and his colleagues set up two sandy, circular, 3 meter diameter arenas in a natural South African habitat. One of the arenas was shaded in the morning to keep the ground temperature cooler. The other arena was exposed to full sunlight. They found that at the cooler ground temperatures, below 50°C, the beetles roll their dung balls straight across the arena without stopping. On the hotter ground, the beetles were observed to occasionally stop, climb up onto their ball and preen their front legs with their mouth-parts. It is likely that this preening covers the legs in regurgitated liquid, cooling them down by evaporative cooling. After the preening, the beetles perform an orientation dance and continue to roll their balls across the arena.

Fig 1. The dung ball as a mobile thermal refuge. (A) With rising soil temperature, beetles climb onto their dung balls more frequently while rolling (B) Temperature of the right front leg (red) and thorax (blue) of a beetle during its first three ball climbs (periods of rolling are grey) (C) Front leg temperature profile averaged over 84 ball climbs from 7 beetles (D) With silicone ‘boots’ on their legs, beetles perform fewer ball climbs. Similarly, beetles climb onto cool balls less often than hot balls

Ground temperature also significantly affected the frequency of this ball climbing behavior. At progressively high temperatures, the beetles climbed up on their balls more often, spending almost 70 percent of their time on top of their balls when the ground temperature went above 60°C.

So why climb balls? Answer: Ball-cooling. Infrared thermography shows that when the beetles roll their dung balls, the surface temperature of the beetles’ front legs increases by as much as 10°C, but when they climb up on their balls that temperature decreases again. That’s quite a bit, but is it really their hot feet that causes the beetles to ball climb? To this, they applied dental silicone to the beetles’ front legs. Pause: Beetle-booties, fun to say and I’m sure fun to see, and reminds me of the awesomeness that is ants on stilts. They found that these beetle-boots doubled the beetles’ ball rolling time, decreasing their ball climbing by 35 percent. This suggests that the ball climbing behavior is related to ground temperature and the heating up of beetle feet. As it turns out, the poo-balls are acting as thermoregulators in three ways:

1. They are portable, elevated platforms that can be used to escape the hot sand.

2. They are heat sinks. The moist dung ball undergoes evaporative cooling, keeping it the much cooler temperature of 31.8°C. This is substantially cooler than the beetle and the sand.

3. They are sand-coolers. Essentially, they are performing another heat sinking duty, sort of a heat vacuum, if you will. The dung ball draws the heat from the sand so it is cooler for the beetles to walk on.

If the poo-balls are actually acting as heat sinks, both during rolling and while the beetle is on it, then warmer balls should be less efficient heat sinks and the beetles should climb on them more often. The researchers tested this by giving beetles cold balls and hot balls. They found that the beetles climbed the hot balls 73 percent more often than the cold balls, supporting the heat sink hypothesis. “Because beetles roll their ball rather than drag it, the ball, preceding the beetle, cools down the sand the beetle is about to step on” by 1.5°C.

Put together, these mechanisms allow dung beetles to operate during a time of day when most arthropods, and other animals for that matter, seek a cool shelter. I guess there are a lot more good things about poo than I ever realized. And it appears that dung beetles have uncovered the secret of the poo.

ResearchBlogging.orgSmolka, J., Baird, E., Byrne, M., el Jundi, B., Warrant, E., & Dacke, M. (2012). Dung beetles use their dung ball as a mobile thermal refuge Current Biology, 22 (20) DOI: 10.1016/j.cub.2012.08.057

Here are a few news outlets that have picked up the story:
From Wired UK "Study: Dung beetles cool their heels atop balls of poo"
Discovery News' story "Why Dung Beetles Like to Chill on Poop Balls"
LiveScience's "That's Hot! Beetles Dance on Poop Balls to Keep Cool"
The Naked Scientists' report "Beetles use dung balls to keep cool"

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