Persistent paramesonephric ducts (masculine uterus) in the male North American beaver (Castor canadensis)

Remnants of the paramesonephric (Mullerian) ducts can be found in the male asa uterus-like structure. Historically, these have been known as a masculi ne uterus. We examined the reproductive tracts of 24 male North American be aver, Castor canadensis Kuhl, obtained from trappers between 1980 and 1997 from four areas extending across Ontario. Persistent paramesonephric ducts were found in 20 of these tracts. Prevalence was not related to geographic location. Grossly, the remnant appeared as one or two thin, uterus-like tub es medial to the ducti deferentes, with or without a medial corpus lying be tween the ampullae. Histologically, the paramesonephric remnants resembled a normal female uterus, but the endometrium consisted primarily of amorphou s extracellular matrix. Previous descriptions of the prevalence of the para mesonephric duct remnants (male uterus) in C. canadensis were found to be c ontradictory.