The effect of hillslope angle on pocket gopher (Thomomys bottae) burrow geometry

One way for animals to decrease energy expenditures is to minimize the cost of movement. For animals dwelling on slopes, gravity can impart a large en ergetic cost to movement. For this reason, animals traveling aboveground al ter their movement patterns in response to the steepness of terrain (specif ically hillslope angle) so as to minimize their energetic costs. Subterrane an animals should also benefit from choosing optimum movement paths in rela tion to hillslopes but concurrently must factor the cost of excavation into their movement decisions. In cases where the excavation costs are much hig her than the costs of working against gravity, excavation costs may overrid e the consideration of gravitational costs and movement of subterranean ani mals may be independent of hillslope angle. To determine the response of a subterranean animal to hillslope angle, we excavated tunnels in the burrow systems of 19 pocket gophers in southern California that occupied hillslope s ranging from 2 to 30 degrees. At each excavation we measured several char acteristics of burrow geometry and used these data in a model of pocket gop her energetics to calculate the cost of tunnel construction at the various hillslope angles. We found that the cost of tunnel construction was indepen dent of hillslope angle, and that the costs of shearing soil and pushing so il horizontally through the tunnels were 3 orders of magnitude greater than the costs of lifting the soil against the force of gravity. Accordingly, p ocket gopher foraging tunnels were oriented independently of the hillslope. The decoupling of the movement patterns of subterranean animals from the e ffects of gravity is a distinctive feature of the subterranean habit compar ed to the movement of above-ground animals. Because of the important effect s of tunnel construction on soil processes, this unique biological feature of subterranean animals has implications for basic physical processes, such as soil erosion. We found that the rate of soil flux generated by pocket g opher activity was invariant to hillslope. This relationship is in contrast to the most common model of soil movement generated by purely physical pro cesses.