Beetles use dung balls to keep cool
South African dung beetles scuttling across scorching hot desert sands use their dung balls not just as food but also as a way to help keep themselves cool, a new study reveals.
The research team led by Jochen Smolka from the University of California Berkley watched dung beetles - also known as scarab beetles - rolling balls of dung across two circular arenas drawn into the sizzling sand at midday in the South African desert.
Publishing their results in the journal Current Biology, the team found that when they shaded the arena and kept the temperature below 50 degrees, the beetles crawled straight along without stopping. But at higher temperatures the beetles periodically stopped and climbed on top of their dung balls, where they preened their front legs - perhaps licking fluids onto themselves to cool down by evaporation.
At progressively higher temperatures, the beetles climbed up on their balls more frequently until at 60°C they spent 70% of their time on top of their balls.
Using thermal imaging cameras the team found that while standing on sand, the beetles' legs can get 10 degrees hotter and then they cool down again quickly when they climb onto the ball.
To test whether getting hot feet is what causes the beetles to climb on their balls, the researchers gave the beetles little silicon booties to kept their feet insulated against the hot sand.
Putting on these boots reduced the number of times the beetles climbed up on their balls by 35%.
There are a few ways that the balls work to help keep the beetles cool. Firstly, they provide a platform so they can take a break from standing on the hot sand - desert ants do something similar by climbing up blades of grass.
The balls also provide the beetles with a heat sink because being moist they cool down by evaporation to around 31°C. To test whether cooler balls are better heat sinks, the team gave the beetles refrigerated and heated balls to push - the beetles climbed onto the hot balls (40°C) 73% more often than the cool balls (15°C). And since the beetles push rather than pull, the normal balls cool down the sand that the beetles then tread on by a couple of degrees.
So it seems that carrying around its own personal cooling device gives a dung beetle an advantage over other animals since it doesn't have to scurry off in search of shelter when things get too hot - all it has to do is take a break from the pushing and hop on top of its dung ball.