Seasonal foraging responses of beavers to sodium-enhanced foods: an experimental assessment with field feeding trials

Salt drive is a seasonal phenomenon common to several classes of wild herbivores. Coincident with shifts of nutrient quality when plants resume growth in the spring, sodium is secondarily lost as surplus potassium is excreted. The beaver (Castor canadensis) is an herbivore whose dietary niche closely follows that of other herbivores that are subject to salt drive, but no published studies to date have assessed the likelihood of its occurrence. To quantify if beavers experience seasonal salt drive, we designed a field experiment to measure the foraging responses of beavers to sodium-enhanced foods. We used sodium-treated (salted) and control (no salt) food items (aspen [Populus tremuloides] and pine [Pinus spp.] sticks) during monthly feeding trials at beaver-occupied wetlands. If conventional ontogeny of salt drive was operant, we expected to observe greater utility of sodium-treated food items by beavers in May and June. Further, if water lilies (Nymphaea spp. and Nuphar spp.) supply beavers with sodium to meet dietary requirements as is widely speculated, we expected foraging responses to sodium-treated food items at wetlands where water lilies were absent to be greater than at wetlands where water lily was present. Aspen was selected by beavers in significantly greater amounts than pine. There was no difference between the mean percent consumed of salted and control aspen sticks by beavers at lily and non-lily wetlands, and no differences in temporal consumption associated with salted or control pine sticks at either wetland type. Salted pine was consumed in greater amounts than unsalted pine. We propose that the gastrointestinal or renal physiology of beavers may preclude solute loss, thereby preventing salt drive.