Likely Builder Of Mima Mounds? Hint: It’s Furry, Tiny And Tireless

Dec 9, 2013

Mima mounds near Olympia, Wash.
Mima mounds near Olympia, Wash.
Credit Flickr Photo/Washington State Department of Natural Resources

For centuries, Mima mounds have mystified.

How were they created? How do they stay so impeccably maintained?

Mima mounds are hillocks, piles of dirt upchucked from the ground. They are sometimes covered in grass, giving them the illusion of a knoll. There are millions in California’s Central Valley; near Olympia, Wash. is the Mima Prairie, where the mounds got their name.

Some Mima mounds are petite, and others rise to six feet. From above, Mima-fied terrain looks like a pock-marked desert or like a photograph of giant slugs racing through a sand dune. Impressively, they are the largest structures built by non-human mammals.

Now scientists say they are fairly certain about what has been creating – or at least maintaining – these Mima mounds: half-pound, foot-long rodents better known as pocket gophers.

The Mima mounds' supposed architect and custodian: the pocket gopher.
The Mima mounds' supposed architect and custodian: the pocket gopher.
Credit From Wikipedia.

According to an article in Geomorphology by Emmanuel J. Gabet and J. Taylor Perron, pocket gophers have been tirelessly burrowing and pushing up dirt into piles. The scientists say that “gophers construct Mima mounds as a response to seasonally saturated soils; in the winter, the mounds sit above perched water tables and afford the gophers relatively dry conditions.”

Gophers were originally floated as the likely builders, the researchers say, because the surface area of an average mound – just more than 130 feet – is the home range of the solitary, and very territorial, gopher.

Although the scientists say they cannot confirm whether the gophers originally built Mima mounds, they’re confident the gophers are their custodians. (Actually, the scientists say the gophers may be “living there opportunistically.”)

However, it seems the mounds actually take on lives of their own: “Because the time scale of mound construction (100 years) is much greater than a gopher's lifespan (10 years), the existence of a mound is essentially independent of the actions of any individual gopher; in a sense, then, it is the mounds that compete with each other for available soil.”

Field observations indicate that mounds compete for soil, which controls their growth and that sometimes larger mounds “capture and subsume smaller ones.”

Produced for the Web by Isolde Raftery.

From ground level, Mima mounds look like grassy knolls.
From ground level, Mima mounds look like grassy knolls.
Credit Flickr Photo/Washington State Department of Natural Resources