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Steifetten and Rosell • Territorial response toward conspecific and heterospecific 2Differential territorial response toward conspecific and heterospecific scent marks by the Eurasian beaver (Castor fiber) Abstract: Territorial behavior directed toward conspecific scent marks is common in both the Eurasian (Castor fiber) and the North American beaver (C. canadensis), but how the two species react to scent marks of the other species is unknown. We tested the hypothesis that C. fiber allopatric to C. canadensis would discriminate between scent marks of the two species (both males and females) and predicted that it would show a more aggressive response toward conspecific than to heterospecific scent marks. We presented 46 different C. fiber colonies with two pairs of experimental scent mounds (ESMs) placed within the territory boundaries during 1999 (N=24) and 2000 (N=22). One pair was marked with anal gland secretion (AGS) and the other with castoreum from the castor sacs. Results showed that C. fiber (1) did not spend significantly longer time sniffing conspecific over heterospecific ESMs, (2) spent significantly longer time responding aggressively to conspecific over heterospecific ESMs, and (3) responded significantly more aggressive to conspecific over heterospecific ESM