eNews
#04 2025
Running out of heat
By Joh Henschel, Research Associate, SAEON Arid Lands Node
They say that only mad dogs and Englishmen go out in the midday sun, but sprinting beetles do it too, though neither mad nor obstinate.
A dune where Namib Sprinting Beetles roam. (Picture: Joh Henschel)
In the Namib Desert, true to the saying, the midday sun beats down relentlessly, heating the dune surface to 60 ºC or more, with sufficient energy across two square metres to boil a two-kilowatt electric kettle. Even so, a tenebrionid beetle, kin to the toktokkie, continues its relentless search for food and mates by streaking through this furnace from one titbit or encounter to the next. It employs a cool trick that entails sprinting, literally beetling the heat.
As temperatures rise globally, attention is increasingly focused on learning how to cope with the heat. What better than to study the pitch-black Namib beetle, Onymacris plana, which is active during the heat of the day in a desert with insufficient water to afford evaporative cooling. Even though it can survive body temperatures of up to a searing 51 ºC, it heats up beyond this temperature within minutes when standing in the scorching sun on a windless day near high noon. To prolong its foraging, it must somehow dissipate the heat it builds up. Surely, taking a fast sprint cannot be the answer? Or is it?
This question brought our team of six together to study Onymacris plana, also known as the Namib Sprinting Beetle, an endemic species to the Namib Sand Sea. We clocked it as one of the fastest-running terrestrial insects at speeds of 1 m/s and more − equivalent to over 50 body lengths per second − while sprinting over distances of tens of metres.
Field surveys in summer revealed that its activities increase during the morning until the blistering heat of early afternoon, at which time it swims through the sand to cooler depths to take a siesta, emerging later to resume its activities until dusk, when nocturnal predators emerge. Winter days are shorter and present less time to forage, and solar radiation is not as intense, so the beetle can skip siesta. Over a third of the beetle’s summer activities take place when the wind is still, before the onset of afternoon onshore breezes, and when the sun’s power ranges between 600 and 1000 Watts/m².
Apart from its sprinting ability, this flightless Namib beetle is notable for its discoid shape, particularly in the male, which is formed by its broad, fused elytra (the sealed back covering). This shape is purported to act as an aerofoil that provides lift during running, while the beetle’s long legs enhance lift so its body moves 15 mm off the baking dune surface into air that is 10−15 ºC cooler.
A male Onymacris plana, showing its broad back that may act as an aerofoil and spiked feet that grip the sand. (Picture: Joh Henschel)
A Namib Sprinting Beetle in action. (Picture: Joh Henschel)
The beetles’ body temperatures, measured with fine thermocouples inserted into the thorax, were 1.5 ºC cooler at the end of sprints in full sun than those of beetles standing at 15 mm elevation in no wind. This degree of cooling shown in running beetles was similar to that achieved by beetles moving into the shade of a plant for 30 seconds. However, shade in the Namib’s nearly vegetationless dunefield is sparse, and there is little point keeping to the shade when most of the beetle’s widely dispersed food source, detritus, is scattered far afield. On a windless summer day at high noon, a beetle sprinting in the sun gets the same thermal benefits as it would standing in the shade, but the sprinter finds more food.
Cooling by running seems counterintuitive. How can it work? Surely sprinting requires much energy, raising metabolism and generating heat, in addition to the heat produced by the absorption of solar radiation? Indeed, that is true for most animals. Earlier research, however, revealed that Onymacris plana is an exception. It hardly increases its energy consumption when running faster than 0.13 m/s. To boot, its energy requirements at a body temperature of 40 ºC are no higher than at 20 ºC. Remarkably, it costs the beetle no extra energy and produces no extra heat to run in hot conditions.
But how does the running beetle rid itself of the heat absorbed from the sun?
We moved to controlled laboratory conditions to examine this, where solar radiation could be simulated at various intensities using a heat lamp, and headwind (which would be created if the beetle were running) and various ambient wind speeds could be simulated with frontal and lateral fans, respectively.
Astonishingly, we found that, under conditions of high radiation equivalent to high noon on a dune and a headwind of 1 m/s equivalent to running, a beetle’s body temperature drops by as much as 13 ºC compared with that of a beetle with no headwind (i.e. not running). When the lateral wind reached 1 m/s or higher, the headwind effect was negligible. There was no significant difference between treatments in the gain and loss of heat between males and females, despite their differences in morphology, nor any difference between live and dead beetles. The laboratory test confirmed that sufficient airflow over the beetle’s body, whether by running in still conditions or by standing in the wind, cooled the beetle by convective heat exchange.
Although this is the first time it has been demonstrated that a pedestrian animal can cool by running, we suspect that there may be other animals that use this method to enable them to continue going about their business in intense heat. For instance, hot-rod ants (Ocymyrmex) of the Namib, Karoo and Kalahari perform high-speed dodging when foraging for other insects that die of heatstroke. Like the Namib Sprinting Beetle, they skirt a fine thermal boundary between tolerable and lethal while foraging.
Our study should serve to prompt further research, even biomimetic engineering research concerning convective air cooling.
While there is no question that our Earth is not about to run out of heat, rather the opposite, there is comfort in knowing that at least one animal can cope by running out of heat.
