Mystery of Giant South American Mounds Is Solved

May 13, 2016 | Erica Tennenhouse

A surale formed by earthworms on Orinoco Llanos
Photo credit: Professor José Iriarte

They are formed by giant earthworms!

If you were to visit Orinoco Llanos, a vast tropical grassland plain located in Columbia and Venezuela, you would immediately be confronted with a striking view of densely packed, regularly spaced mounds up to 16 feet (five meters) wide and 6 feet (two meters) high covering the landscape.

It was previously assumed that these mysterious mounds, called surales, were formed by erosion caused by flooding of the Orinoco River. But until now, neither the mounds nor their formation had ever been described scientifically.

A new study published in the journal PLOS ONE shows surales are largely made up of earthworm casts, which is a fancy word for poop.

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The researchers used aerial and satellite photography, along with data on the physical and chemical makeup of the soil to examine to formation of the surales. They found that the mounds were about 50 percent earthworm manure. Though nine species of earthworm were present at the surales, Andiorrhinus, a species of giant worm that can grow more than 3 feet (1 meter) long, was by far the biggest contributor to the mound biomass.

Surales form when the earthworms feed in the flooded soil and their excretions form towers above the water level. The researchers found that, as earthworms repeatedly return to the same locations to breathe and defecate, over time the tower becomes a mound.

"This exciting discovery allows us to map and understand how these massive landscapes were formed," said study co-author José Iriarte from the University of Exeter, in a press release. "The fact we know they were created by earthworms across the seasonally flooded savannahs of South America will certainly change how we think about human versus naturally built landscapes in the region."

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