Visually, male and female beaver are indistinguishable, and, because their sexual organs are located internally, determining a beaver’s sex in the field can be challenging.
There are a number of different ways to ascertain their sex. The easiest field method is to locate the four dorsal mammary glands on females; however, this method is of limited value because the glands are visible only during a brief lactation period in the summer and non-breeding females do not develop conspicuous teats.
Two other methods are reliable ways to determine sex but require training and a sedated or constrained animal. These methods are (1) checking the color and consistency of anal gland secretions, and (2) palpating the baculum or penile bone.