Morphology and anatomy of the beaver
Beavers are the largest living rodents at the northern hemisphere. Only the South-American capybara Hydrochoerus hydrochaeris grows
larger. A fully grown beaver can reach a body mass up to 30 kg and above, however, Hinze (1950) casts doubt on data of 40 kg
and above. The head-body length amounts to approx. 100 cm, the trowel has a length of 30 - 34 cm and a width of approx. 17 cm, whereas the
females are larger and heavier in average than the males (Freye 1978). The outer ears are small (2 -4 cm) and rounded and also
the eyes are comparatively small for an animal of this size.
Adult animals show an annual fluctuation in body mass of up to 3 kg, whereas the body mass reaches it's largest value by the end of summer and it's lowest by the end of winter (Djoshkin & Safonow 1972). This fluctuations in body mass can be attributed at least partly to the type of diet, which in winter contains almost exclusively the bark of woody plants.
Beaver scull in lateral view.
Notice the large incisors
and the diastema. Because of the semi-aquatic way of life, beavers show specific morphological and anatomical adaptations. As a protection against hypothermia they possess a dense fur with long guard hairs and fine, dense undercoat: dorsally there are growing approximately 12,000 and ventrally even 23,000 hairs per cm2 (Djoshkin & Safonov 1972). Additionally, during the winter a well developed subcutaneous fat tissue, which the animals obtain in autumn, provides protection against low temperatures.
Besides the long incisors with an orange coloured forefront, which have open roots and are growing during the whole life, the diastema, the toothless gap between the incisors and the molars, which is characteristic for rodents, hares and rabbits, is very conspicuous. The lips can be "dragged" inside of this gap and this way the oral cavity can be closed during gnawing. Thereby, the entering of wood shavings or water - for instance when cutting submersed plants - into the oral cavity can be prevented (Hinze 1950; Heidecke & Ibe, undated). The diastema is followed by a premolar and three molars in each half of the jaw. The dental formula, that is the arrangement of the all in all 20 teeth within the dentition of the beaver, sums up to:
1 0 1 3
1 0 1 3
When diving, the animals close their nose and ear orifices, so even during long lasting dives no water can enter the nose ore ear cavities. Beavers are able to dive up to 20 minutes, for instance during flights (Zahner et al. 2005), however, normal dives turn out to be significantly shorter. During diving the frequency of the heartbeat is reduced up to 20% (Zahner et al. 2005), whereby the animals can lower their oxygen consumption and extend their diving time.
Video of a swimming beaver. The animal is curiously observing the camera man and is therefore only swimming relatively slow. Therefore, a part of the pelvis of the animal is seen above the water line, which is normally not the case in swimming beavers (Video: free-hand recording of Christoph Elbert, Meppen).
The feet of the beaver are large, up to 20 cm long and webbed between the toes. The toes are ending in strong claws, and the second toe has a double claw, the so-called "brushing or grooming claw", which is used for the maintenance of the fur.
Schematic drawing of
a beaver hand, which holds a
decorticated stick. Notice that instead of the
thumb the little finger
will be opposed. The hands are relatively small. The thumb is short and can not be opposed. When holding a decorticated stick, the little finger is put under the stick and functions as a counter bearing. Similar to the toes also the fingers are equipped with strong claws, which are important for digging. The hands are not webbed and are kept pressed under the chin during swimming.
The propulsion in the water is only generated by the hind limbs: beavers are "leg swimmers". In being plantigrade, the animals touch the ground with the entire palm of the hand and sole of the feet respectively when moving on land.
Beavers feed exclusively on plants. Similar to all other rodents that mainly feed on a diet rich in cellulose, beavers also have a large caecum in which
cellulase-producing bacteria break down the cellulose in the cell walls of the ingested plant material and, in addition, produce different vitamins. Without
help of the microorganisms beavers (like all other mammals) would not been able to make use of the plant cellulose, because the mammalian organism in itself
is not able to produce the enzyme necessary for the breaking down of the cellulose. During the break down of the cellulose by the caecum symbionts mainly
short-chained, volatile fatty acids are produced, which were reabsorbed either directly in the caecum or in the following colon and be available for the
organism as a source of energy. Based on calculations, beavers can cover approx. 19% of their energy requirements with such volatile fatty acids
(Hoover & Clarke 1972).
Similar to several other rodents, beavers produce two types of faeces: the moulded waste faces and the paste-like caecal faeces - the caecotrophs. Whereas the waste faeces is directly discarded into the water, the caecal faeces are ingested again by the beavers. This occurs on land whereas the caecal matter is discarded on the trowel, taken up from there, and passes the digestives system again. Thereby further carbohydrates, resulting from the brake down of the cellulose, are digested as well as the bacteria ingested together with the caecal faeces and by that delivering additional proteins and amino acids. Furthermore, the beavers can also make use of the vitamins (mainly vitamins of the B-group as well as vitamin K) produced by those bacteria. The resorption of nutrients and cleavage products respectively takes place - like in all other mammals - in the small intestine, which can reach a length of more than 5 m in the herbivorous beaver.