Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Species: Pachysoma endroeydi , P. hippocrates and P. glentoni
Habitat: The coastal deserts of Namaqualand in South Africa and Namibia
Everyone walks differently, so much so that we can often identify our closest friends purely from their walks. Out in the animal kingdom, the diversity is far greater. There's a vast difference between the graceful trot of a gazelle and the splayed-leg waddling of a newt, while some animals have gaits straight out of Monty Python's ministry of silly walks.
But within the major groups of animals, there's often a certain consistency. Take insects. Almost all of the known species walk in pretty much the same way, in what is known as the "tripod gait", where the front and back legs on one side of the body move in synchrony with the middle leg on the other side.
Three species of dung beetle buck that trend. They are the only insects that can gallop. But bizarrely, their ability to gallop means they are slower than other insects.
Deserts and dung
The three species belong to a genus called Pachysoma. There are four other species in the genus, and these walk with the typical tripod gait. Female Pachysoma beetles dig a curved burrow into moist sand and fill it with a combination of dung and leaf litter – unlike other dung beetles, they don't roll the material into a ball
This lifestyle demands skilful navigation, and dung beetles are among the animal kingdom's best route-finders. One species uses polarised light from the moon to find its way, while, famously,
Marcus Byrne of the University of the Witwatersrand in Johannesburg, South Africa, took two colleagues on a field trip to Namaqualand to study the species P. striatum. The team was interested in how the beetle navigates from food sources back to its nest, but the experiment wasn't going too well.
"We were poking around in the sand looking for other species of Pachysoma which might be more cooperative," says Byrne. His colleague Clarke Scholtz of the University of Pretoria, South Africa, spotted a P. endroeydi "bobbing along, happily galloping across the sand". Surprised, Jochen Smolka of Lund University in Sweden highlighted the beetle's unusual gait. "He pointed out to us that something really weird was going on, right before our eyes," says Byrne.
Further exploration revealed that two other Pachysoma species, hippocrates and glentoni, walked in the same unusual way.
Run for your life, slowly
The dung beetles moved two pairs of legs alternately: front, then middle, then front. They used their hind legs for dragging dung, but if they weren't carrying anything they simply dragged the hind legs behind them.
This is completely different from the tripod gait, in which a given leg never moves at the same time as its pair.
The beetles didn't spend any time in the air – unlike galloping mammals such as horses
But there is one difference between the motion of the dung beetles and that of their mammalian counterparts: the beetles gallop slowly, and cannot keep up with their tripod-walking relatives. In experimental tests, the galloping P. endroeydi moved at 7.6 centimetres per second, while P. striatum got up to 9.1 cm/s with its tripod gait.
So the galloping gait probably didn't evolve to help the dung beetles move faster. That is puzzling, because on the face of it the inept galloping seems to put them at a disadvantage. It seems likely that there must be some subtle advantage to it.
Smolka suggests that it helps them to navigate back to their burrows. "It is possible that the galloping gait could help them to count their steps more easily, even when slipping with each step on shifting sands, or help them to stabilise their head and therefore their compass system," he says.
For now that's an untested idea, so we don't know for sure why the dung beetles move in this strange way. They also can't fly, which Byrne says is "weird for a dung beetle". Maybe one day their descendants will learn to jump
Journal reference: Current Biology, DOI: 10.1016/j.cub.2013.09.031
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