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Determination of age of the Eurasian beaver (Castor fiber)

The article presents data on body size and weight of beavers of different ages from Ukraine (Dnipro basin), as well as features of determining the age of males by the size of the baculum and taking into account the bonding lines in the periosteal zone of the mandible. In addition, this paper summarizes the methods for determining the age of the Eurasian beaver, which are used in different countries. Among them are methods that determine the age of beaver by size and body weight, by traces, by craniological features, by the length of the sagittal ridge, by the developmental features and functional state of teeth, by the width of the incisor and the interorbital width of the skull, by the stratification of dentin or cement, and by size and weight of the baculum. The body weight of beavers from Ukraine was 7.0 ± 0.31 (5.5–7.9) kg in individuals of 6–7 months of age, 9.1 ± 0.26 (8.6–9.5) kg in one year old individuals, 12.7 ± 0.30 (11.0–13.9) kg in two years old beavers, 15.6 ± 0.45 (14.4–17.0) kg in individuals of 2.4–3.0 years of age, and 21.8 ± 0.81 (16.2–30.0) kg in beavers over 3.0 years of age. The size of the skull in subsequent age groups was characterized by a significant transgression by the interorbital and zygomatic width of the skull and they clearly differed from one another only by the total length of the skull. Curiously, the mass of the baculum in the Elbe beaver (С. fiber albicus Matschie, 1907) significantly exceeds that of beavers from Ukraine, which may be a results of geographic variation of the animals. In the 1970s, a new method of post-mortem determination of the age of beavers was proposed by studying the polished section of the first molar measuring the height of dentin-enamel loops as well as the thickness of the layer of cement and secondary dentin. It was found that in beavers the cement layer appears in the second year in the lower part of the tooth and, gradually increasing with age, almost completely displaces the dentin-enamel loops by the end of the animal's life. At 3–4 years, secondary dentin appears, which occupies an intermediate position between the cementum and the lower part of the tooth, and, when the latter is chipped, it fills certain areas of its occlusal surface. In addition, there is a method of classification of beavers by age groups (up to 1 year, 1–3 years, over 3 years) based on measuring the radii of the external and internal curvature of the incisors. Depending on the age of the animals, all methods allow to obtain fairly accurate results.