Optimal foraging of beaver (Castor canadensis) in northern lower Michigan.

Abstract

An energy-maximizing central place model of optimal foraging predicts that animals will maximize their food energy intake per unit foraging time. We studied the beaver (Castor canadensis) in northern lower Michigan to test predictions of this model. Specifically, we tested whether beaver prefer to feed on certain tree species, whether they prefer to feed near or far from shore, whether there is a relationship between diameter and distance from shore of trees cut by beaver, and whether beaver are more selective by size at increasing distances from shore. We set up transects perpendicular to the bodies of water near two active beaver lodges, one on a lake and one on a river. We determined genus, diameter, distance from shore line and status of each tree in the transects. At the river site aspen (Populus spp.) was the preferred tree genus, while at the lake site, alder (Alnus spp.) was preferred; these genera are known to be nutritionally superior to some of the other genera available to beaver. At both sites, beaver utilized a significantly greater number and proportion of trees near to shore than far from shore, presumably minimizing energy expenditure per unit of food energy gained. Although we found no significant relationship between diameter of trees cut by beaver and distance to shoreline for any genera, beaver were more selective in terms of size of trees cut at greater distances from shore, which is consistent with a model of size-distance relationships in optimal foraging. Future investigations of beaver foraging in relation to optimal foraging theory should consider factors such as tree chemical defense and how beaver treat very large trees when attempting to quantify relationships between size and distance from the shoreline of beaver forage.