Implications of Subfossil Coleoptera for the Evolution of the Mima Mounds of Southwestern Puget Lowland, Washington

The origin of Washington State’s Mima Mounds has been debated for over a century, with numerous mechanisms being postulated. Subfossil Coleopteran (beetle) remains recovered from the base of a mound at Mima Prairie consist of species that would be expected in rodent burrows and nests; all but one species are obligate burrow inhabitants. These results suggest the past presence of fossorial rodents (probably pocket gophers, Thomomys mazama) in the mounds, although none live there at present.  Whether or not the gopher created the mounds, they may well have been instrumental in maintaining mound geometry until very recently.