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As the largest mammalian order, rodents are nearly cosmopolitan in distribution, can exploit a broad spectrum of foods, and can often reach high population densities. One sole representative of the most primitive family of rodents, Aplodontidae, does not share some of these common rodent characteristics. The aplodontoid rodents in the family Aplodontidae and Mylagsulidae radiated during the Miocene from the Allomyinae family (Carraway and Verts 1993). The extinct Mylagaulidae represents the earlier radiation of these rodents who exhibited great specialization (Carraway and Verts 1993). Unlike the other members of the rodent order, mountain beavers are not prolific breeders; nor are they broad-spectrum habitat invaders, retaining in their morphology the primitive condition of the masseter muscle originating entirely on the zygomatic arch. The mostly extinct Aplodontidae family is now made up of the monotypic genus Aplodontia which has been able to survive since the early Oligocene and in some areas is even considered a pest.
Mountain beavers (Aplodontia rufa) are known from a variety of common names including boomer, whistler, and the Native American names sewellel and showt’l. Although the common name suggests relationship to true beaver (Castoridae), this semi-fossorial rodent shares only the behavior of tree clipping with the stream beaver and is usually more abundant at lower elevations than in mountainous areas. In Oregon, mountain beavers have retained the designation of boomer even though their vocalizations do not include booms or whistles. Very few people actually know about, much less have seen, this compact fossorial rodent. In this chapter I will present information on the ecology and human-wildlife conflicts of this little-known rodent species.