Soil translocation by pocket gophers in a Mima moundfield

We measured soil translocation due to the tunneling of valley pocket gophers (Thomomys bottae) in a Mima moundfield at Miramar Mounds National Landmark, San Diego, California, from December, 1984 through December, 1985. We placed 1-l soil plugs containing 20 11-g iron pellets into pocket gopher tunnels at locations between mound tops and points about one mound radius beyond mound edges. After about 4-10 d, sites to which the marker-containing soil had been translocated were located with a metal detector and the horizontal and vertical displacements measured. Between 1 October and 15 May (the cooler, wetter portion of the year), pocket gophers removed an average of 63% of the experimental plugs and moved an average of 38% of the markers that we recovered. From 15 May through 1 October (the hotter, drier portion of the year), only 32% of plugs were cleared and 12% of the recovered markers were moved. On average, markers that were moved were displaced 41 cm moundward and 4.9 cm upward in elevation. The intensity of moundward translocation increased with distance from the mound center. At a distance of 0.5-1.0 mound radius beyond the edge of the mound, the moundward translocation tendency averaged 71 cm. The intensity of moundward translocation was also inversely related to maximum mound height. These observations provide strong support for the fossorial rodent hypothesis of Mima mound origin, and constitute a first step in development of a mathematical model of mound formation.