Mysterious mounds: not created by animals after all?

Published October 20, 2015

Bryan Moss and Tracey Byrne from the Seattle area stop along the walking path in the Mima Mounds Natural Area Preserve Dec. 29, 2013, near Littlerock, Wash. (AP Photo/The Olympian, Steve Bloom)

The mystery of the mounds lives on. A mere six months after researchers said computer modeling proved pocket gophers, over the course of several hundred years of scurrying and burrowing, formed the bizarre-patterned earthen "Mima mounds" in Washington state, a new team of researchers claims that plants are in fact the likely source.

These mounds—which are up to 6.5 feet tall and 55 feet wide—are found on every continent but Antarctica, and in his study, Michael Cramer of the University of Cape Town sets out to debunk the gopher theory.

He outlines a number of issues: Mima mounds appear in areas gophers don't inhabit; some of the mounds feature rocks bigger than the 2-inches-in-diameter stones pocket gophers are believed to be able to move.

And as for the previous claim that a series of gophers developed the mounds over hundreds of years, Cramer says there's no indication that abandoned mounds are repopulated over and over.

His theory: The mounds have formed due to what is called vegetation spatial patterning. The idea, reports LiveScience, is that plants and their roots alter how wind or water may carry soil to these patches of vegetation, thus the mounds grow bigger over time as the plants continue to trap sediment.

The vegetation could further stabilize the soil, thereby reducing erosion on the mounds while depleting the adjacent soil of water and nutrients, creating patterned dips. The researchers hope to test their theory on mounds in South Africa.

Whatever the source, the News Tribune reports that Washington state is pushing to protect its mysterious mounds: Its Department of Fish and Wildlife recently requested $3 million from the state to do so.

(Another mystery solved: the source of fairy circles?)