Examining preferences of Castor canadensis for tree diameter, distance, and species: Optimal foraging behavior?

Abstract

Optimal foraging is a valuable tool in the study of adaptation. Many generations of selective pressure force organisms to maximize foraging efficiency. The beaver, Castor canadensis, provides an excellent model for the study of foraging behavior. They harvest trees for food, dams, and lodges, with visually obvious results. Since the profitability of foraging behavior for beavers is a function of tree size, distance of the tree from shore, and tree species, these variables were examined. Following optimal foraging theory, we believe the beaver should prefer small trees to large, trees closer to the shoreline than far, and demonstrate species preference. Our study was conducted at Grapevine Point in Cheboygan County, northern lower Michigan, USA. Each tree in four plots approximately 20 m wide and 20m in depth was examined, and we recorded the tree diameter, perpendicular distance of each tree from the shore, tree species, and treatment of each tree by beaver. Trees were categorized as being either only foraged, foraged and felled, foraged and removed, or unforaged. Chi square analysis showed a significant preference by beaver for the small and large trees (X2 calc = 55.47, df = 2, P < 0.05), and avoidance of the middle size trees. However, no difference in preference was found for trees based on distance (X2 calc = 3.81, df = 2, P < 0.05). Species preference was found for the bigtooth aspen and American beech. In addition, removed trees were found to be statistically significantly smaller in diameter than unremoved trees (U=276.5, P < 0.001). Results differing from the expected hypotheses may be due to the downhill terrain towards the shore or a lack of predators, reducing the preference for nearby trees. Preference for aspen trees agrees with its characteristics of high nutritive value and easy digestibility. In addition, beaver have been shown to prefer aspen when it is especially abundant. However, studies of optimal foraging in beavers have been conducted in a variety of ecosystems, and the results differ widely; the species and abundance of trees present in an area seems to play a large role. Nonetheless, beaver in our study did exhibit behaviors that would be favored by natural selection.