Enormous mounds found in the tropical wetlands of South America have long puzzled onlookers, but now scientists finally know what the gigantic mounds are mostly made of: worm poo.
A new study on the mounds, which are called surales, marks the first ever research to describe their formation.
"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 verses naturally-built landscapes in the region," José Iriarte from the University of Exeter's Archaeology Department said in a press release.
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For the study, published in the journal PLOS ONE, Iriarte and his team used remote sensing techniques, satellite images and aerial photographs taken by a drone to study the surales, which cover large areas of the Orinoco Llanos tropical grassland plain region in Columbia and Venezuela. The Orinoco is one of South America's longest rivers.
(Trivia: The title of the 80's song "Orinoco Flow" by Enya referred both to this river as well as to a London recording studio named Orinoco.)
The researchers also collected data on the physical and chemical makeup of the region's soil to learn more about how the mounds form and develop.
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They found that up to a half of each mound consists of worm poop, which is technically known as vermicast. This material, also called "worm manure," is the end product of the breakdown of organic matter by an earthworm. It contains reduced levels of contaminants and a higher saturation of nutrients than do organic materials before the breakdown process, so it is coveted by gardeners.
The scientists further determined that surales form when large earthworms feed in shallowly flooded soils. As the worms release vermicast, towers of the material form above water level. As each earthworm returns repeatedly to the same spot to feed and poo, the towers become mounds that can expand up to 16 feet in diameter.
What's more, when mounds are situated close together, the basin between them can fill, causing two big mounds to form one gigantic one. The surales are so large and plentiful that they actually alter much of the South American tropical wetlands landscape.
Iriate said, "This exciting discovery allows us to map and understand how these massive landscapes were formed."