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| 11 chapter 2 Form, Weight, and Special Adaptations . . . their powerful incisor teeth not only serving them to strip off and divide the bark of trees, but also enabling them, when urged by their instinct of construction, to gnaw through trunks of considerable thickness, and thus obtain the timber of which they stand in need for the building of their habita­ tions. These important organs contribute, therefore, in an especial manner, to supply them both with food and shelter. E. T. Bennett, 1835 Size and Body Weight The weight of an adult North American beaver ranges between 40 and 50 lb (~18–23 kg). Recorded beaver weights are 96 lb (43.6 kg) in 1960 in Missouri1 and about 110 lb (50 kg) in 1921 at the Iron River in Wisconsin.2 The body including the tail reaches about 48 inches (1.2 m) in length. The tail itself is about 16–17 inches (~40 cm) long, about 6–7 inches (16 cm) wide, and ¾ inch (1.9 cm) thick. In pairs of Eurasian beavers, the female is almost always larger than the male.3 Tail Most distinctive for the beaver, the tail is flat and scaly (See Fig. 3.2). It is a multipurpose tool. Beavers use their tail as prop to balance themselves when cutting trees. It is also important as a rudder during diving and underwater maneuvering around all three body axes. The tail signals alarm when it is slapped on the water surface (see chapter 6). In addition, fat reserves are stored in the tail. It also serves in heat exchange through a countercurrent arrangement of blood vessels4 (see chapter 3). By this mechanism the beaver can reduce the 25% heat loss via the tail in the summer to 2% in the winter.5 Nostrils, Ears, and Eyes When the beaver is under water, valves close the nostrils and ears. Fur-lined lips can be closed behind the teeth. This permits gnawing under water. Part of the tongue and the epiglottis prevent water from entering the larynx and trachea. “Diving goggles,” a nictitating membrane, protects the eyes under water. 12  |  The Organism Brain The brain of a 17-kg North American beaver weighs about 45 g, and that of an 11.7-kg animal, 41 g.6 The size of the brain relative to body weight is considered to indicate how well a species processes information and solves complex problems posed by the environment, often shortened to the term intelligence. To determine how “intelligent” the beaver is in relation to other related rodents of similar size, the brains are compared using an encephalization quotient (EQ). This quotient relates the measured brain size (weight) of a species to the expected brain size. The expected brain size is an average for all rodents (or all mammals), computed as brain weight in relation to body weight. The EQ of the aquatic beaver (0.9) is intermediate between that of terrestrial rodents of similar size and arboreal rodents such as squirrels. Muskrat and nutria have lower EQs (0.679 and 0.779, respectively). The EQ (0.8) of the Eurasian beaver is slightly lower than that of the North American beaver.6 The size and anatomical appearance of the various brain parts do not betray any specific adaptations to the beaver’s semiaquatic lifestyle. For instance, the olfactory bulb of beavers is neither smaller nor larger than that of other mammals of similar size that are not adapted to water. The ratio of the length of the olfactory bulb to the length of the cerebrum is 0.35, the same as in the Alpine marmot (Marmota marmota) and very similar to the ratio in the 13-lined ground squirrel (Citellus tridecemlineatus; 0.33) and the common European red squirrel (Sciurus vulgaris; 0.34).7 Certain brain measurements have been used to infer intelligence. If the evolutionarily older hypothalamus is relatively small in comparison to the cerebrum, which contains the “neocortical” higher centers, the brain is considered more advanced . Among a number of related squirrel-like rodents, the beaver scores highest in this regard. The ratio of hypothalamus length to cerebrum length ranged from 0.20 to 0.24 in 12 North American beavers from Mississippi.7 The cerebellum, involved in the coordination of locomotion in three-dimensional space, is well developed, although arboreal rodents such as squirrels have equally large, if not larger cerebellum hemispheres relative to body weight.7 Large areas of the brain (neocortex) are dedicated to processing somatic...


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