Chill out: climb a dung ball

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It is hard to make generalisations about insects. with unparalleled diversity – blanket statements are very rarely correct. Take for example dung beetles. Most people would recognise that dung beetles eat dung. This is mostly true, but some dung beetles prefer other diets: carrion, rotting fruit, mushrooms, and decaying vegetation are popular choices. One species of dung beetle has evolved carnivorous behaviour: feeding exclusively on millipedes.

A fascinating study from South Africa has illuminated a wonderful peculiarity about dung beetles. A common reproduction strategy for dung beetles in tropical ecosystems is telecopry. This is the process of forming dung into a ball, then rolling it away to safety. The main function of this behaviour is rolling dung to a safe place, where breeding can take place before the ball is buried safe within the soil. This also reduces the risk of a competitor stealing the valuable resource, the mate, or even another species parasitising the young.

In the case of the dung beetle Scarabaeus lamarcki, telecopry has taken on another role through being paired with a novel behavioural adaption. In a paper aptly named “Dung beetles use their ball as a mobile thermal refuge” the researchers demonstrate the vast utility of a dung ball. Scarabaeus lamarcki is known to climb the dung ball ball between short bursts of rolling, quickly preening, then continuing on its merry rolling way.

The first experiment compared beetles on hot soil, and soil cooled by shading. The experiment observed beetles as they rolled a dung ball from the centre of a 3 m diameter  circle. Beetles on cool soil exited the circle along an almost perfectly straight 1.5-m path. Beetles on hot soil stopped periodically to climb the ball and preen, taking three times longer on average to roll the dung ball out of the circle.

By looking at infrared images, the team found temperate of the beetles front legs climbed by roughly 10°C when running on hot ground. The beetles legs would cool quickly once they had climbed on top of the dung ball. However, that didn’t explain whether heat avoidance was the reason the beetles were climbing the dung ball. The researchers worked out a clever way of testing whether the beetles were climbing the dung to avoid heat stress: giving the dung beetles heat resistant booties.

Using dental silicon the researchers painted ‘boots’ on the dung beetles which prevented the transfer of heat from the ground to the beetles feet. Beetles with boots climbed dung balls 35% less often than their bootless counterparts. Demonstrating that the beetles were indeed using the dung ball as a mobile, thermal refuge.

These experiments are a powerful example of the wonderful ways beetles have achieved such widespread global diversity, and success. It also demonstrates that seemingly straight forward behaviours can mask latent, and rather sophisticated evolutionary adaptation. It would appear that there is truly more than one way to use a dung ball.

Reference:
Smolka, J., E. Baird, M. J. Byrne, B. el Jundi, E. J. Warrant, and M. Dacke. 2012. Dung beetles use their dung ball as a mobile thermal refuge. Current Biology : CB 22:R863–4.

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