A dung beetle (Kheper lamarcki) rolling a ball of dung at Thornwood Lodge, South Africa.
A dung beetle (Kheper lamarcki) rolling a ball of dung at Thornwood Lodge, South Africa.
Dung beetles may not be the first creatures that spring to mind when you think of insect acrobats, but when the beetles perform a handstand, before walking backward on their forelimbs while rolling a ball of dung before them with their hind limbs, they certainly are agile. And the most effective way to ensure that they get their ball of precious dung away to safety on a flat surface is to roll in a straight line. But the real world is rarely flat. Tufts of grass, rocks and uneven terrain get in the way, so Nienke Bijma, Alexander Filippov, Alexander Kovalev and Stanislav Gorb (all from Kiel University, Germany), with Poramate Manoonpong and Peter Billeschou (University of Southern Denmark) and Emily Baird (Stockholm University, Sweden) wondered how the tenacious creatures cope when they encounter a bump in the road. Do they simply roll over obstructions or must they recover after veering off course?
Heading to join Marie Dacke (Lund University, Sweden), at the game farm in South Africa that has been her field site for almost two decades, Bijma, Billeschou and Baird set the beetles some ball rolling challenges. First, Filippov, Kovalev and Gorb designed and printed three square 3D surfaces for dung beetles (Kheper lamarcki) to manoeuvre dung balls across: a gently rolling surface with 3.5 cm high peaks separated by valleys wider than the ball of dung, mimicking a surface strewn with large rocks; a second intermediate surface, with narrower peaks separated by valleys similar in width to the dung ball; and a third with hundreds of narrow sharp peaks, similar to gravel. Then Bijma, Baird and Dacke placed a dung beetle perched on a dung ball at the centre of each surface, before filming the insects as they selected in which direction to propel their trophy.
‘Although K. lamarcki conquered all surfaces, the shapes and distribution of the tracks were greatly influenced by the surface structure’, says Bijma, recalling that the insects followed largely straight courses heading in all directions as they scampered across flat terrain. However, the beetles were channelled into deviating from a straight line – following the wide valleys – in the gently rolling surface, with the occasional animal making a detour as it encountered a bump. Meanwhile, the beetles placed at the centre of the intermediate surface were even more tightly channelled into the narrower valleys, rarely maintaining the straight course they would have preferred. Negotiating the gravely surface, the beetles set off in random directions – as they had on the flat surface – but veered off course when encountering a sharp pinnacle head on.
In addition, the beetles often lost control of their dung balls on all three of the bumpy surfaces. They struggled most on the intermediate surface, losing their footing approximately twice per run, with the ball rolling ∼2.4 cm before they recovered their stride. And when the team tried tilting a flat surface (up to 10 deg) – to find out how well they coped rolling dung balls across, up and down a hillock – even though the beetles were reasonably successful rolling their balls directly up or down the slopes, their success plummeted when they tried to roll across the slope, often losing control of their dung ball.
When faced with uneven terrain dung beetles cannot roll their precious dung balls along the straight paths they set out on. Instead, they veer around obstructions as their balls are knocked off course. They may also benefit from moulding dung balls to suit the terrain, either shaping balls that are significantly larger than the gaps in gravelly surfaces or smaller than the channels between rocks, for the best getaway.
