In the hot dunes of Southern Africa’s Namib Desert, the black beetle Onymacris plana runs fast for its tiny size. Turns out, the speed not only helps the beetles find food but also, perhaps, cool down. The beetles’ temperature drops after a sprint, even in intense sunlight, researchers report July 9 in the Journal of Experimental Biology.
The cooling is “not just marginal,” says ecologist Carole Roberts, formerly of the Gobabeb Namib Research Institute in Walvis Bay, Namibia. “It takes them into a safety zone that guarantees their survival.” She and her colleagues conducted their experiments on the beetles nearly 40 years ago, but because no one had worked on this behavior since then, the team decided to go ahead and publish their findings.
O. plana forage during the day, when solar radiation peaks. To measure their temperature in their native habitat, Roberts’ team used a thermocouple — a device that converts temperature differences to electrical voltages. The device was inserted into the beetles’ thorax and attached to a fishing rod, allowing the researchers to follow from a distance without disturbance. After their sprints, the beetles’ temperature dropped by about 1.5 degrees Celsius, in contrast to dead beetles placed under the sun that heated up.
In the lab, Roberts and her colleagues set up an experiment to simulate the beetles running in the desert. The team used fans blowing winds at 1 meter per second toward the stationary insects to match their running speed. The team found that under moderate temperature, low ambient windspeed and high radiation — conditions similar to the desert — the beetles cooled by almost 13 degrees, the max cooling under ideal conditions.
Several features help the beetles beat the heat. They are efficient runners, generating little metabolic heat during sprints. Their bodies are wide and flat, providing “a larger surface area for [heat transferring to the air], so when running, it is able to cool off more rapidly,” says study coauthor Joh Henschel, an ecologist also at Gobabeb Namib Research Institute.
The flattened back also acts like an aerofoil. “Once they get to a certain speed, they kind of lift off, sort of flying [on the ground] with their legs,” says Roberts, who now works as a freelance editor.
That lift is useful. “It’s a cool windy breeze just above the surface,” says study coauthor Duncan Mitchell, a physiologist at University of the Witwatersrand in Johannesburg. “If animals that are on the surface can get themselves into that breeze, they can achieve a lot of cooling.”
Though O. plana may appear to fly while sprinting, it doesn’t — making this the first known case of a creature that runs to cool off.
“How many pedestrian animals do you need to find … to know that animals can cool by running? Just one,” Mitchell says. “Now it’s for the new generation [of scientists] to go and find other ones.”
