Anyone who's been to the beach on a hot day knows the feeling of scorching sand underfoot. But do beetles that cross the sunny savanna or dwell in the desert feel it, too? Biologists have found that not only do dung beetles—which famously feed on feces rolled across the sand until it becomes a smooth sphere— feel the burn, their snack lets them beat the heat.
Dung beetles such as Scarabaeus (kheper) lamarcki typically select a pile of poo, often from a local herbivore, then roll it in a straight line to escape competitor beetles as quickly as possible. Along the way, the beetle occasionally clambers up, stretches out its head, rotates 360 degrees, and repeats—a maneuver referred to as the orientation dance (it's hard to know you’re moving in a straight line when your head’s down and you’re pushing a ball). Biologists at Lund University in Sweden and the University of Witwatersrand in South Africa were studying this behavior in an effort to understand the insect’s visual navigational skills only to discover that the treasured dung ball serves as a sort of portable AC unit.
The scientists were observing S. lamarcki's natural behavior when they noticed that around noon each day, beetles would climb up and begin preening—as you can see in the video below.
To test whether the heat might be affecting the beetle's behavior, the scientists set up an experiment in which beetles rolled their dung balls at midday in either a well-shaded or sunny arena. They found that dung beetles in the hot arena climbed up on their ball more often, swiping at their faces and body.
Next, researchers decided to test whether the beetles climbed to get their feet off of the hot sands. In a bold scientific and sartorial experiment, they affixed dental silicone "boots" on the beetles' front legs. When comparing boot-wearing to barefoot insects, the scientists found those in boots climbed up 35 percent less often and could push their dung ball for about twice as long without taking a break.
But what exactly does the dung ball do? For one, dung is moist, meaning as it rolls along it loses heat through evaporation. In other words, the same principle—evaporative cooling—that works in your AC, is at work here—albeit in a more portable format. Thermal video footage confirmed that the typical dung ball was distinctly cooler than either the beetle itself or the ground. In addition, the scientists found that beetles climbed up the dung ball significantly more often if the ball was heated (left in the sun for half an hour) as opposed to chilled (set in the freezer), which suggests the ball may also cool the sands as the beetle rolls it, giving the insect’s feet a little extra relief. The result, as the researchers call it, is a "mobile thermal refuge."
The findings are published today in the journal Current Biology. Whether more beetle boots are in the research group’s future remains unclear. Jochen Smolka, a biologist at Lund University and an author of the study, notes that putting on the boots was easier said than done: "It's actually hard to put them on their legs and hold out their forearms—they have very strong forearms." For future studies, other beetle apparel is a distinct possibility—the scientists have also used dental silicone (which can be easily applied and peeled off without harming the insect) to fashion "blinders" for vision tests. "This group has a history of creating clothing for beetles—we’ve already created caps—it will be interesting to see what comes next," Smolka says.