The mystery of the "fairy circles" in grasslands across Namibia in Africa -- circles of barren ground surrounded by a ring of healthy vegetation -- continues, as researchers rule out a suggestion of termites.
The circles, as large as 65 feet across and lingering for years, have long puzzled scientists, who've suggested causes ranging from grass-killing upwellings of underground hydrocarbon gases to carnivorous insects.
One biologist suggested a species of termite found living near the circles might be feeding on the roots of the grasses, creating the rings.
However, aerial images show the distribution of the circles is unexpectedly regular, which would seem to rule out termites as a cause.
"The occurrence of such patterning in nature is rather unusual," says Stephan Getzin from the Helmholtz Center for Environmental Research in Leipzig, Germany. "There must be particularly strong regulating forces at work."
Because termites are normally found in irregularly-spaced colonies, Getzin says, it's unlikely they could create the regular spacing of circles apparent in the aerial images.
"There is, up to now, not one single piece of evidence demonstrating that social insects are capable of creating homogenously distributed structures on such a large scale," he says.
Getzin and his fellow researchers say they have their own theory about the circles, suggesting the grasses, as they compete for limited water sources across the grasslands of southwest Africa, propagate in self-regulating configurations, thinning themselves out to make the most of the available resources and space.
That's supported by the fact the mystery rings are usually found in zones where grassland is transitioning to desert, they said.
Using computers to model competition underground for scarce water resources, they researchers said they could create distribution patterns that look like what has been observed in Namibia and other African regions.
Still, Getzin acknowledges, that's just another theory for the circle's formation.
Since no one has actually observed a circle in the act of forming or growing, there's no firm answer even after years of study by a number of researchers.
"Although scientists have been trying to answer this question for decades, their mystery remains as yet unresolved," Getzin says.
Getzin and his collaborators have published their latest investigations of the mysterious circles in the journal Ecography.
