Mima Mound Mystery Solved

Busy critter. It takes scores of generations of gophers (inset) to build Mima mounds, meter-high hillocks of soil in which gophers burrow.

Busy critter. It takes scores of generations of gophers (inset) to build Mima mounds, meter-high hillocks of soil in which gophers burrow.

Dominic Sherony/ Creative Commons; (inset) Washington State DNR

Over the decades, 50 or more explanations have been offered for the fields of broad, meter-high mounds of soil found across the western United States and on every continent except Antarctica. The ideas have ranged from earthquakes to glaciers to UFOs. But now it seems that generation upon generation of gophers built the millions of mounds seen today. And it took a computer model programmed to act like the burrowing rodents to unearth the truth.

“I love these cute problems of geomorphology,” says Robert Anderson, referring to the study of how Earth’s surface got its shape. “Everybody and their dog has taken a shot at this,” says Anderson, a geomorphologist at the University of Colorado, Boulder. The problem has been that no one has ever seen a Mima mound—named after examples on the Mima Prairie in Washington state—actually grow. Either the mounds aren’t growing today or they grow very, very slowly.

Emmanuel Gabet of San Jose State University in California—also a geomorphologist—and his colleagues had no grad students willing to keep an eye on a Mima mound for decades on end, so they set out to build Mima mounds in a computer model. Pocket gophers had been a leading candidate for Mima mound builders. A lone gopher (they are highly territorial) spends its short life burrowing through a mound and eating plant roots. In a 1987 study, researchers salted mounds with small iron pellets, waited while the gophers did their burrowing thing, and relocated the pellets using a magnetic detector. It turns out that pocket gophers tend to push their burrow tailings uphill rather than downhill despite the extra effort required.

So Gabet and colleagues created a computer model of a flat field populated by randomly distributed gophers behaving like the gophers in the iron-pellet study. In a paper in press in Geomorphology and in a presentation last week at a meeting of the American Geophysical Union, they reported that their virtual gophers built mounds that bear a striking resemblance to the real thing: The model’s mounds were a close match in height and diameter. The distances between model mounds and the pattern the mounds formed across a field were also very similar to those of real Mima mounds.

Thanks to an inadvertent experiment in central California, the researchers could even show that their model mounds grew at the same rate as natural ones. In the 1980s, farmers stopped tilling fields that would become part of Carrizo Plain National Monument. During the subsequent 30 years, gophers built mounds—unobserved—24 to 25 centimeters high, the same growth rate as in the model. “There’s no way to prove” gophers build mounds, Gabet says, but “I think this is as close as we’re going to get.”

Anderson agrees. “I would say they’ve got a strong case,” he says. “I think they’re right. It’s an example of how nature can organize itself into patterns that are striking.”

Now on to the next mystery: Why do gophers bother? It would take 500 to 700 years for the animals to build a full-size mound, Gabet estimates. That’s a couple of hundred generations of gophers. Gabet, Anderson, and others speculate that the creatures are creating a new habitat better suited to their needs than the unaltered terrain. Mima mounds typically overlie a subsurface soil layer so impervious that it impedes the drainage of winter rains, leading to seasonally waterlogged, uninhabitable soils. A mound, on the other hand, stands high and dry in winter above surrounding pools. Just how pocket gophers acquired their inclination to build for the benefit of distant future generations, Gabet can’t say. Geomorphologists know their limits.

For a story about another unusual formation, check out our article on fairy circles.

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