The Namib desert of south-western Africa can be extremely hot - the surface temperature can be over 50°C. But a surprising number of around 200 beetle species live on its bare, inhospitable-looking sand dunes.
Scientists studying them were perplexed by the astonishing behaviour of one of the beetle species - a darkling beetle, Onymacris plana.
Like most desert darkling beetles, it is black - a colour that absorbs heat. And it has a flattened body, a big surface area exposed to heat. Scientists didn't expect to find it active on the sand surface in the dangerous heat of the day. But it sprints in the sun, sometimes pausing in the shade of a desert shrub.
In fact, it's the fastest of all the invertebrates of the Namib desert sands. This tiny beetle can run as fast as a human can walk.
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When humans and other animals run, the fuel burning in our muscles produces heat. The faster we run, the more oxygen we use and the hotter we get.
But not so these beetles.
In an astonishing discovery, we established that the beetle in fact gets cooler when it exercises. This is the first land animal to have been found with this capability (and the first research of its type on a pedestrian animal).
Their cooling system enables them to move around to find their wind-blown food before it's covered by sand. And they can be active when other animals (predators and competitors) are not. Finally, males can spend more time looking for mates. So we believe they are adapted to move in the sun because it's good for survival.
The hunt
In the early 1980s, entomologist Sue Nicolson and her co-workers from various universities and research institutes went out on the dunes in the hot sun to measure the temperature of the beetles. They used a thermometer in a fine hypodermic needle to measure each beetle's temperature without harming it. The needle went into the beetle's thorax, from underneath. They looked for beetles that had just finished a sprint and others that had rested for the same time in the shade of a shrub. The beetles that had finished a sprint were no hotter than those that stayed in the shade.
In the 1980s, comparative physiologist George Bartholomew and his co-workers from various universities measured how much oxygen the beetles used while running on a treadmill. Running fast took hardly any more oxygen than running slowly. So, running faster would not make the beetles much hotter.
So, we knew how hot the beetles were after a sprint (not very hot), and how much oxygen they used while running (not much). But what no-one had done was to measure the temperature of the beetles while they were running.
We're a team of scientists who work on how animals' bodies cope with heat. Much of our desert research is done in the Namib Desert. We wanted to know how the beetles achieved something that looked impossible physiologically: run in the Namib sun.
We attached a fine thermocouple thermometer fed through the end of a fishing pole.
One of our team followed the beetle while it was running in the sun, keeping the weight and drag of the thermometer off it. But the beetles did not get hotter when they ran - they got cooler.
Run like the wind
We calculated what should have happened to the temperature of the beetle. Because it was black, we could estimate how much of the sun's radiation it would have absorbed. The Namib's sun is so intense that the radiation falling on a tabletop would boil a kettle.
We measured how far the beetles had run and in what time, so we knew their speed. We could calculate how much heat they were generating in their muscles. Adding the sun's heating to the heat coming from the muscles, we calculated that, in the hottest Namib sun, the beetles' temperature should have risen by 5°C per minute. That should have killed them.
The Namib desert's sand can be burningly hot but its air, blowing in off the Atlantic Ocean, is cool. Running generates a wind over the body. We concluded that the heat from the sun and from the muscles must be carried away by that cool wind.
The males have an especially flattened body shaped like the wing of an aircraft so that they almost float, clear of the hot sand.
We needed to confirm that what we had observed on the sand dune did not conflict with what engineers know about heat transfer (moving thermal heat from one object to another). So, we took beetles into the laboratory. We put them under a lamp which heated them as much as the sun would have done.
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Then we blew cool air over them from the front at the speed at which they would have run. So, we mimicked the cool wind they would have felt when they were running on the dune. Switching on the fan dropped the temperature of the beetles by as much as 13°C.
Our laboratory experiments confirmed that the wind generated by running could carry away all the heat that the beetles absorbed from the desert sun. But to survive on the dunes, they had to run. Standing still in the sun in windless conditions would have meant death by overheating.
So evolution has delivered an animal that is cooled by running. This is unique for a pedestrian animal so far, though we think that some desert ants may also be able to do it. Many aquatic animals do cool by swimming and some insects cool by flying.
Carole S. Roberts, Mary Seely, Liz McClain and Victoria Goodall of the Gobabeb Namib Research Institute, Walvis Bay, Namibia, contributed to this research and article.
Duncan Mitchell, Honorary Professorial Research Fellow, University of the Witwatersrand
