Photograph by José Iriarte
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Enormous mounds (as seen from the air) in the tropical wetlands of South America have long puzzled scientists.

Photograph by José Iriarte

Mysterious Mounds in South America Are Likely Worm Poop

The human-size towers in Colombia and Venezuela wetlands are actually piles of excrement, a new study says.

As an earthworm biologist and soil scientist, Anne Zangerlé has spent most of her research career looking down. 

Trekking through the steamy tropical wetlands of Colombia and Venezuela in search of strange, six-foot-tall (two-meter-tall) towers called surales, however, forced her to look up. 

Lucky she did. 

Zangerlé and colleagues have discovered that the mysterious mounds are probably piles of earthworm poop. (See "Mystery Picket Fence in Amazon Explained.") 

“Nobody knew what they were or where to find them," says Zangerlé, who first heard about the mounds several years ago from another scientist.

She and her team studied obscure descriptions in old books about South America and did some digital sleuthing on Google Earth.  

Armed with this information, in 2012 the team explored the seasonally flooded wetlands via plane and Jeep, eventually identifying thousands of surales dotting an area roughly the size of Ireland.

After performing chemical analyses of the structures and studying the animals that called them home, the team determined that Andiorrhinus earthworms are the likely architects. 

Although other animals can modify the landscape, such as beavers in North America and termites in Africa, earthworm surales are unique to South America, the team adds.

Human-Size Mounds

María Jesús Briones, an earthworm biologist at the University of Vigo in Spain, first heard about these odd mounds several years ago, when she began work on the Global Soil Biodiversity Atlas.

“As far as I knew, they were originally built by farmers,” Briones says. "After abandoning them, I thought soil animals like termites, ants, and earthworms had colonized them and maintained them.” 

Since South America's indigenous peoples frequently built large earthworks, farming and landscaping on a massive scale, one of Zangerlé’s first tasks was to rule out that these towers—which range from small bumps on the ground to the massive spires—were simply the remnants of a long-ago civilization. (Also see "Exclusive: Lost City Discovered in the Honduran Rain Forest.")

"It's incredible. You can feel so small next to them," says Zangerlé, of the University of Montpelier in France and the Technical University of Munich in Germany. 

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From the ground, surales look like large, vegetated mounds separated by swampy ditches. 

Analysis of small particles of fossilized plants in the soil by study co-author and archaeologist José Iriarte of the U.K.'s University of Exeter revealed that no one had farmed the soil for a thousand years. 

Further analysis showed that earthworm castings make up a very high proportion of the mounds' soil. The invertebrates tend to stick in the same spot, which means they always deposit their castings—or nutrient-rich excrement—in the same place. 

Over time, although the researchers don't yet know how long, these castings can rise higher than a tall human before becoming unstable and toppling over, Zangerlé says. And if two mounds form very close to one another, they merge into a single tower. (Also see "Pablo Escobar's Escaped Hippos Are Thriving in Colombia.") 

The researchers identified other species in the surales, but Andiorrhinus earthworms were the most abundant and thus the probable creators of these structures, Zangerlé and colleagues concluded in their paper, published May 11 in the journal PLOS ONE.

Molding the Landscape

Other experts agree that earthworms are making these structures. 

“It's a fantastic paper—really exciting and compelling findings,” Corina Tarnita, a biologist at Princeton University, says in an email. 

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A person holds a young Andiorrhinus earthworm, the likely architect of the surales. 

To Tarnita, the variety of physical patterns and sizes of the surales are reminders of the ecological complexity that can be created by relatively small animals.

Earthworm expert Patrick Lavelle, of France's Pierre and Marie Curie University, adds “it takes a lot of invisible cooperation among soil organisms to make forests grow." (See "The Dirt on Dirt: 5 Things You Should Know About Soil.") 

Towers of earthworm poo aren’t just architectural wonders: They also have an important role in ecology by altering soil moisture, which can affect the types of plants that can grow, adds Lavelle, who has studied earthworms for four decades but wasn't involved in the new study.

“It is a very important work that has great meaning with regards to the view we have on soils and their management,” he says. 

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