For decades nobody has been able to understand how the mysterious Surales mounds of South America were formed, but it seems their creators have been revealed

East of the Andes mountains, in the floodplains of great Orinoco River of South America, there lies a mysterious place.

The Surales is a landscape of green mounds alternating with deep pits. The mounds are organised in intricate patterns, which vary from simple rounds to veritable labyrinths.

So regular are the arrangements, you might think they had been built by human engineers. In fact, these mounds were built by humble earthworms.

But these are no ordinary earthworms. The builders of the Surales are earthworms up to 3ft (1m) long.

There are descriptions of the Surales dating back to the 1940s, but hardly any scientists have ever studied them.

So Anne Zangerlé of the Braunschweig University of Technology in Germany and Delphine Renard of McGill University in Montreal, Canada led an international team to explore them in detail, and find out how they were formed.

Their findings have been published in the journal PLOS ONE.

They found that the surface of each mound was covered by earthworm poops, called "casts", which were unexpectedly big.

"The casts had a mean diameter of over 5cm, which is unusually large for an earthworm cast," they say. "An average cast in Europe is rarely bigger than 1.5cm."

Such huge casts implied that the creature making them was also large. The largest earthworms found in the area belonged to the genus Andiorrhinus, and measured up to 3ft (1m) long.

This species was also the predominant one in the waterlogged pits between mounds. Zangerlé and Renard believe that they are primarily responsible for creating the mounds.

Over decades, the Andiorrhinus worms have stuck to rigid eating habits. They eat in flooded pits, and always poop in the exact same place.

Over time their poop accumulates into large mounds up to 16ft (5m) in diameter and 7ft (2m) tall.

"Given that earth is available in limited amounts, the whole landscape could not rise as a single mound," says Vincent Deblauwe of the University of Yaoundé I in Cameroon, who was not involved in the study. "Weathering processes erode high mounds, and eventually mounds of constant height emerge in the landscape."

The worms may build mounds for their own safety. The Orinoco basin floods every year, and the worms' mounds of poop jut out above the water level. That means the worms have a place where they can perch out of the water and breathe.

Over time, mounds formed next to each other coalesce and grow. The earthworms keep eating, making the pits between the mounds deeper.

If you fly over one of these labyrinths, you will realise how common and extended they are

The mounds are laid out in a regular pattern over space, again because of the eating habits of the earthworms.

"The earthworms collect earth only from a given radius distance from the mounds," says Deblauwe. "This distance imposes the scale of the pattern, [meaning] the size of inter-mounds and the size of the mounds themselves."

As the mounds grow, other creatures start using them to escape the flooded soil. Other earthworm species help build up the mounds, and plant roots consolidate and maintain them.

When an earthworm dies, another earthworm takes over its mound, ensuring the mounds keep growing.

Zangerlé and Renard found that the mounds and inter-mound regions are quite different habitats, even though they are right next to each other. Less than half of the region's plant species were found in both habitats.

"The authors… make a captivating and convincing case for the central role of the large Andiorrhinus earthworms in the development of the Surales landscape," says Visa Nuutinen of the Natural Resources Institute Finland, who was not involved with the study.

While some of their evidence is "circumstantial", he says it could be tested.

It is less clear why the Surales landscape takes different forms, varying between "well-defined round mounds" and "labyrinths". "If you fly over one of these labyrinths, you will realise how common and extended they are," say the authors.

Their aerial surveys suggest that labyrinths form in places where the topography of the grassland changes, they say. Smaller mounds also seem to be more likely to form labyrinths.

"How topography, and possibly other environmental factors, influence the shape of Surales is one of the fascinating questions we will have to explore next," say Zangerlé and Renard.

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