WHEN A CAPE buffalo goes down in South Africa, lions, hyenas, and painted dogs will soon arrive to fight over the carcass. But what most people don’t realise is that the same thing happens every time the Cape buffalo takes a poo—only it’s beetles battling over the spoils.
Using scent detectors on their antennae, dung beetles home in on a pile of feces and descend en masse. Each beetle then races to tear off a chunk, roll it away, bury it, and devour the droppings before the other dung beetles can steal it for their own. But how they are able to navigate away from the fray, and do so in efficient, straight lines, has been the topic of intense study.
In 2003, a scientist by the name of Marie Dacke discovered that nocturnal dung beetle species like Scarabaeus zambesianus can navigate by the polarised light of the moon. A decade later, Dacke found that another species, Scarabaeus satyrus, uses the light of the Milky Way when the moon isn’t available. A year after that she revealed that day-rolling species such as Scarabaeus lamarcki look to the sun as their guiding star.
And now Dacke has discovered yet another tool in the same dung beetle’s sensory array.
According to a study published today in the Proceedings of the National Academy of Sciences, when the sun is at its highest point in the sky and thus useless for navigation, dung beetles abandon the ball of fire and decide to follow the wind instead.
The findings suggest that the animals can interpret two different kinds of signals and opt to use one or the other depending on the conditions around them.
“These systems appear to be extremely flexible,” Dacke says. “Which is fascinating when you imagine that their brain is about the size of a sesame seed.”
BEETLES BATTLE FOR A PRIZED BALL OF DUNG
Watch two dung beetles fight over a ball of dung—a valuable resource they use both as food and as a nursery for beetle babies. Read more about the fight here.
Against the wind
To figure out how dung beetles navigate without their sun compass, Dacke and her colleagues had to first control for variables.
They started by constructing two-foot-wide domes that mimicked the beetles’ natural habitat. Next they performed a series of experiments where an adjustable LED stood in for the sun, and fans created different speeds and directions of wind flow. This allowed the scientists to measure the dung beetles’ performances under a variety of conditions and show that when the artificial sun was at high noon, the beetles chose to roll their dung balls into the wind—even when it shifted.
“They will follow the wind, because that is now the cue that they are using,” says Dacke, who is an expert in neuroethology, or the study of how the nervous system controls behaviour. “If we change the direction of the wind by 180 degrees, the beetle will change its direction by 180 degrees as well.”
But things really got interesting when the team prevented the beetles from accessing some of their sensory systems.
In one experiment, the scientists removed the club at the end of the insect’s antennae, which is thought to help them sense odours. Like the others, these beetles successfully followed the wind when the sun was high. This suggested that the beetles were feeling the wind rather than smelling something in it.
Beetles that had their entire antennae removed, however, rolled their dung balls every which way, as if lost. The animals could not sense the wind and were rudderless as a result.
Inside the mind of a beetle
Paul Graham, who studies ant navigation at the University of Sussex in the United Kingdom, says the study’s findings are perfectly clear.
“One of the beauties of this dung beetle rolling system is that the behaviour is easy to interpret, so a consistent rolling direction shows us clearly that the beetles are using the wind as a directional signal,” Graham says in an email.
What’s more, it’s important that dung beetles seem to be merging two different kinds of information—one mechanical, the other visual—into a single brain region. Graham says this suggests “even small-brained animals are undertaking sensory processing that is reminiscent of what some people would call cognition.”
Next up, the dung beetle researchers plan to investigate how a dung beetle evaluates the information provided by either the wind or sun and how it determines which one to use.
“We’re going to do recordings from the neurons themselves, inside the compass of the beetle,” Dacke says.
After more than 15 years of studying these seemingly simple insects and the way they perceive the world, plenty of mystery still remains.