Why Dung Beetles Like to Chill on Poop Balls
This dung beetle has little silicone mittens, added to test the researchers theories. Jochen Smolka
Though the smell might dissuade you, balls of feces make superbly effective foot coolers.
Dung beetles eat feces. Everyone knows this. But here's something you didn't know: newly published research reveals that dung beetles can use spheres of rollable poop-meals as portable AC units — and they're damn effective ones, at that.
The sands of the South African desert can exceed temperatures of 60 degrees Celsius, or 140 degrees Fahrenheit. That's ridiculously hot. In fact, for a dung beetle like Scarabaeus lamarcki — which transports its meal by rolling it into a ball and pushing it across the scorching desert landscape with its hind legs — it's too hot, as demonstrated in a study by functional zoologist Jochen Smolka in the latest issue of Current Biology. Using infrared thermography and behavioral experiments, Smolka and his colleagues have shown that dung beetles use their poo-ball as "a mobile thermal refuge" — a portable evaporative unit that cools the beetle slightly as it rolls, and dramatically when it clambers on top of it.
The researchers demonstrated this by observing beetle activity in two sandy, circular arenas of varying temperature. They found that at ground temperatures below 50 °C, beetles would roll their balls about the arena without pause. But as temperatures rose to over 50 °C, the time spent rolling the balls decreased, and the time spent perched atop the balls increased. "Overall," note the researchers, beetles "climbed onto their balls almost seven times as often in the hot arena as they did in the cool arena."
Thermal imaging reveals that beetles' front legs get up to ten degrees hotter during ball rolling, and that they quickly cool (about 7 °C in just 10 seconds) once they've climbed atop their poo-ball. Ball-climbing behavior was shown to be directly related to the temperature of the beetle's front legs; after applying insulating "boots" made from dental silicone to the beetles' front legs (pictured — in the form of what look like mint-green mittens — at the top of this post), "the average rolling time between consecutive ball climbs almost doubled, from 10.7 to 20.8 ."
So what's the secret to ball-cooling? The big one is evaporation. Dung balls are moist. As that moisture evaporates it keeps the ball very cool — around 32 °C, even when it's resting atop 60 °C soil. What's more, note the researchers, "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 around 1.5 °C.
All told, that means a beetle's ball of crap helps keep it cool in three ways. First: as a platform, elevated above the scorching desert sand. Second: as a heat sink, drawing heat from the beetle's forelimbs whenever they start to overheat. And third: as a mobile sand-cooling unit, paving a cooler path for the beetle as it pushes its prize ball of poo from one place to the next.
The researchers' findings are published in the latest issue of Current Biology. You can access the full text here, free of charge.