Study: Earthworms responsible for elaborate mounds in South America

EXETER, England, May 11 (UPI) -- Researchers say earthworms create the unusual and elaborate mounds found in the tropical wetlands of South America.

Dotting the Los Llanos, a vast grassland plain in Venezuela and Colombia, are large mounds of mud, densely packed and marked by a bubble-like texture. The mounds are called surales, and until now their origin was unknown.

An excavation of several surales by researchers from the University of Exeter turned up dozens of earthworm casts. Their study suggests the mounds are composed of mud expelled by the guts of earthworms.

Each earthworm returns to the same spot to empty its entrails, shed its casts and respire. Over time, excreted dirt and casts pile above the water level in areas of shallow, soggy soil. Mounds that begin forming close together can coalesce into one.

Satellite imagery and drone surveys revealed the variety of sizes of surales found in Los Llanos. The mounds range in width from 0.5 to 5 meters and rise between 0.3 and 2 meters in height.

Researchers shared their findings in the journal PLOS ONE.

"This exciting discovery allows us to map and understand how these massive landscapes were formed," José Iriarte, a professor of archaeology and palaeoethnobotany at Exeter, said in a news release. "The fact we know they were created by earthworms across the seasonally flooded savannas of South America will certainly change how we think about human verses naturally-built landscapes in the region."