Belowground Herbivory: The Adaptive Geometry of Geomyid Burrows

An analysis of the relationship between the energy costs and benefits of below-ground herbivory is undertaken for the rodent family Geomyidae. Given that geomyids attempt to maximize the net energy gain per segment of burrow excavated, I conclude that (for habitats featuring a uniform horizontal and unimodal vertical component of the dispersion pattern of belowground plant biomass) a unique combination of burrow radius, depth, and segment length is optimal. If, on the other hand, geomyids are attempting to maximize the net gain per unit volume of soil excavated, both burrow radius and segment length should be as close to zero as possible. An examination of the available information on burrow geometry indicates that burrow radius is as small as possible, but that segment length may not be. This paradox can be resolved if it is assumed that burrowing costs rise dramatically for burrows with diameters larger than just necessary for free passage. Environments promoting one or the other of the energy maximization strategies will then lead to differences in body sizes, a widely observed phenomenon among geomyid species and populations.