Beaver - Bi-baa (English spelling) beaver

A general term for animals in the genus Beaver, order Rodentia, family Beaveridae. Members of this genus Castor are adapted to aquatic life and are also called sea raccoons. There are two species: the European beaver C. fiber , found in Germany, France, Norway, and Lake Baikal in the Russian Federation, and the Canadian beaver C. canadensis , found in Alaska, Canada, and the northern United States. The two species are very similar in morphology and ecology, and some scholars consider them to be the same species. The Canadian beaver was introduced to Finland and has become habituated. It has a body length of 74 cm and a tail length of 21 to 30 cm. It is large for a rodent, with a thick and sturdy body. Its eyes and ears are small, and its tail is wide, horizontally flat, and covered with scales. It has well-developed webbed feet between the toes of its hind feet. The body color is diverse, including chestnut, reddish brown, and blackish brown, with individuals from southern regions tending to be lighter in color. The fur is dense and soft.

They live in rivers and lakes in lowland forests, and spend most of their time in the water. They are slow runners on land, but skilled swimmers and divers. In large rivers, they dig burrows on the riverbank, and in smaller rivers, they build sturdy islands in the river with piles of soil, stones, and wood, and in both cases, they build burrows lined with bark and other materials. The entrances (usually two or more) of both are below the waterline to prevent predators from entering. The ceiling and walls are highly waterproof, and the ceiling has ventilation holes. In addition, to keep the water level constant, beavers gnaw down trees near the river, stand them on the riverbed, and fix them with stones, mud, and algae, and build dams by piling up branches and other materials. The dams are usually 20 to 30 meters long, but sometimes reach 700 meters. In the United States, where there were once a huge number of beavers, these dams helped to prevent many floods. Beavers' incisors are suitable for gnawing hard wood, and they can gnaw down trees with a diameter of 8 to 20 centimeters in a short time. A single burrow can contain a pair and sexually immature young, or a family group consisting of three generations. They mate in the water from January to March, and give birth to one to eight young after a gestation period of about 105 days. At birth, the young have body hair and incisors. On land, the female is said to carry her young on both her forelimbs and walk using only her hind legs. Their diet consists of bark, buds, etc. In winter, they become less active and spend their time eating the bark of branches stored in dams.

Beavers were once widespread across Eurasia and northern North America, but overhunting for their fine fur has led to them becoming extinct in many areas, including the UK. They are now protected in many places, and their populations are increasing.

[Yoshiharu Imaizumi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

哺乳(ほにゅう)綱齧歯(げっし)目ビーバー科ビーバー属に含まれる動物の総称。この属Castorの仲間は水中生活に適応し、ウミダヌキ、カイリ（海狸）ともよばれる。ドイツ、フランス、ノルウェー、ロシア連邦のバイカル湖などに分布するヨーロッパビーバーC. fiberと、アラスカ、カナダ、アメリカ合衆国北部に分布するカナダビーバーC. canadensisの2種がある。両種は形態、生態がきわめて似ており、同一種とみなす学者もいる。フィンランドにはカナダビーバーが移入され、馴化(じゅんか)している。体長74センチメートル、尾長21～30センチメートル。齧歯類としては大形で、太く頑丈な体をもつ。目と耳は小さく、尾は幅広く水平方向に扁平(へんぺい)で、鱗(うろこ)に覆われる。後足の指の間には水かきが発達する。体色は栗(くり)色、赤褐色、黒褐色など多様で、南方産の個体ほど明色の傾向がある。毛は密生し、柔らかい。

　低地の森林内の川や湖にすみ、おもに水中で生活する。地上を走るのは遅いが、泳ぎと潜水は巧みである。大きな川では川岸に巣穴を掘り、比較的小さい川では川の中に土、石、木を積んで頑丈な島を築き、ともに内側に木の皮などを敷いた巣室をつくる。どちらも出入口（普通二つ以上ある）は水面下にあけられ、外敵の侵入を防ぐ。天井と壁は防水性に優れ、天井には通気孔がある。さらにビーバーは、水位を一定に保つために、川近くの木をかじり倒して川床に立て、石、泥、藻などで固定し、枝などを積み重ねてダムを築く。ダムの長さは普通20～30メートル、ときには700メートルに及ぶ。かつて莫大(ばくだい)な数のビーバーが生息したアメリカ合衆国では、そのダムのおかげで多くの洪水を免れた。ビーバーの門歯は堅い木をかじるのに適しており、直径8～20センチメートルの木を短時間でかじり倒す。一つの巣穴にはつがいと性的に未成熟な子、または3世代からなる家族群ですむ。1～3月に水中で交尾し、約105日の妊娠期間ののち1～8子を産む。出生時の子は体毛を有し、門歯もみられる。雌親は地上では子を両前肢にのせて、後肢だけで歩いて運搬するといわれる。食物は樹皮、芽などである。冬には活動量が減少し、ダム内に蓄えた枝の樹皮を食べて過ごす。

　ビーバーはかつてはユーラシアと北アメリカの北部に広く分布したが、良質の毛皮を求めての乱獲によって、イギリスなど多くの地域では絶滅した。現在は各地で保護され、個体数は増加しつつある。

［今泉吉晴］

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