Some African dung beetles roll their feasts of dung away to avoid the hordes of other hungry bugs at the pile. But now researchers who report their findings in the October 23 issue of Current Biology, a Cell Press publication, have discovered that the beetles also use the balls in another, rather clever way: The moist balls keep the bugs cool even as they push a weight up to 50 times heavier than their own bodies across the scorching sand.
"Like an air conditioning unit, the moist ball is cooled by evaporative cooling," said Jochen Smolka of Lund University. "The beetles climb their cool balls whenever their front legs and their head overheat from pushing this huge dung ball across the hot South African sand."
The discovery marks the first example of an insect using a mobile thermal refuge in this way. It is also a demonstration of the remarkably sophisticated strategies that insects and other cold-blooded creatures employ to maintain their body temperatures.
Smolka's team stumbled upon this unusual behavior completely by accident. In fact, dung beetles also climb atop their dung balls to perform an "orientation dance" that the insects use to work out where they are going. As they watched for the dancing, the researchers began to notice that the beetles climbed their balls of dung much more often in the heat of the midday sun.
Further experiments showed that this midday phenomenon only held true when the beetles were crossing hot ground. In fact, the researchers report, beetles on hot soil climb their balls seven times as often as those on cooler ground.
Once on top of a ball at midday, the beetles were often seen "wiping their faces," a preening behavior that the researchers suspect spreads regurgitated liquid onto their legs and head to cool them down further. That's something the insects never do at other times of day.
To show that it was the beetles' hot legs that made them climb the ball, the researchers applied some cool (as in temperature) silicone boots to their front legs as alternative protection from the heat. "To our great surprise, this actually worked, and beetles with boots on climbed their balls less often," Smolka said.
The findings are yet another reminder of the many creative solutions found in nature, Smolka added. "Evolution has an astonishing ability to make use of existing structures for new purposes—in this case using a food resource for thermoregulation."
Smolka et al.: "Dung beetles use their dung ball as a mobile thermal refuge."
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