RYBCZYNSKI, N.; Duke Univ., Durham, North Carolina: Wood-Cutting Performance in Two Castorids (Rodentia, Mammalia)

Fossilized cut wood found associated with the skeletal remains of the Pliocene beaver, Dipoides, provides the first evidence of wood-cutting behavior in an extinct beaver. Unlike the modern beaver, Castor, the lower incisor tips of Dipoides are not squared-off in anterior view, but are rounded. In addition, Dipoides is approximately 1/3 smaller than Castor, and is likely to have a lower maximum bite force. The aim of this study was to compare the wood-cutting performance of these two incisor shapes. Models of the lower incisors were made in steel and honed to reflect incisor sharpness measurements. The models were of similar widths to one another, and were intermediate in size between the lower incisors of adult Dipoides and Castor. I investigated cutting performance of the models using a motorized test apparatus, fitted with a dynamometer for measuring cutting forces. Fixed parameters, defining the relative orientation of the wood sample and model, were derived from behavioral observations. Parameters that varied were wood-type (sweetgum, white oak, redcedar or pine), and width of cut. Analysis of 257 cutting events found that for cuts of similar size average peak cutting-forces were significantly higher for the Castor-tooth model than for the Dipoides tooth-model. However, unlike the Castor-tooth model, the Dipoides-tooth model was often unable to complete wide cuts in most wood samples (redcedar excepted). Therefore, although cutting with the Dipoides model is generally more efficient than the Castor-tooth model, in terms of force requirements, its cuts are usually smaller. The possibility that rounded incisors might promote specialization on certain trees is also discussed. I thank North Carolina Univ., and NSF (DIG-0073119).