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Dung-hoarding beetle caught galloping

Off and racing A species of South African dung beetle has given up the ability to fly to instead gallop across the sand grasping coveted bits of dried faeces.

"This species of Pachysoma grabs bits of poo and gallops forward with it," says Marcus Byrne of the University of Witwatersrand, Johannesburg.

"That is really odd. Most insects walk with a tripod gait. They plant three legs in a triangle, while swinging the other three legs forward. It's an incredibly stable way of walking because you've always got three legs on the ground."

"For an insect to abandon the tripod gait and use its legs together in pairs like a galloping horse is really radical. The big question is: why are they doing it?" asks Byrne, who along with his colleagues, studied the unusual insect.

Most dung beetles gather "fresh" poop and pack it into one wet ball that they roll along the ground before their big escape. Pachysoma, on the other hand, collects bits of dry dung and hoards it in a nest. The beetle goes on repeated foraging trips instead of one major journey.

Byrne and his colleagues from Lund University in Sweden think the species might have changed its mode of navigation because it needs to be able to find its way back and forth from its nest.

"For most dung beetles, it's always a one way trip — grab the poo, run away and never go back," he explains. "The very marked pacing of Pachysoma's gallop might be giving it a better signal in terms of estimating the return distance from the food to its nest. When it gallops, it slips less in the soft sand."

Finding its way home

Prior research has found that ants count their steps as a way to navigate back and forth from home, and bees use the optical flow of scenery across their retinas to measure how far they've travelled to forage from the hive. Pachysoma dung beetles seem to be using a version of both techniques.

"Bees use optic flow as a measure of how fast and how far they've flown," Byrne says. "Dung beetles have two eyes on each side of their head, one on top and one on the bottom, looking at the sand and we think Pachysoma might be registering optic flow with its bottom eye over the sand."

But Pachysoma has not only changed the way it moves across land, it has also lost its ability to fly.

"There are 800 species of dung beetle in South Africa and most of them fly," says Byrne. "To fly makes sense because poo is a very ephemeral resource. It's only useful for a few days and it's very patchy — you don't know where you're going to find the next dropping. That's why Pachysoma is so weird. Why would anyone give up flying?"

It could be that Pachysoma sealed up its wings in order to conserve moisture in its arid environment.

In any case, its unique ways have worked. Like a businessman settling into a niche market, this flightless dung beetle has cornered the market on dried poo in its ecosystem.

The findings are reported in the journal Current Biology.