Sloth - sloth (English spelling)

A general term for animals belonging to the order Rodents of the class Mammalia, family Slothidae. This family Bradypodidae has two genera and five species, and they mainly live in the trees of the dense forests of South America, feeding on leaves and flowers. The genus Bradypus includes the white-throated three-toed sloth B. tridactylus found in Brazil, the maned sloth B. torquatuc in southern Brazil, and the brown-throated three-toed sloth B. variegatus in Brazil, Argentina, and Bolivia, while the genus Choloepus found in the Amazon basin, Venezuela, and Guyana includes the two-toed sloth C. didactylus and the Hoffman's sloth C. hoffmanni found from Nicaragua to Ecuador.

Body length is 50-64cm, tail is absent or short if present. Weight is 4-9kg. All species have three digits on their hind legs, but the front legs have two digits in the two-toed sloth genus and three digits in the three-toed sloth genus, with large curved claws on all digits. Since sloths live in trees and hang upside down by their claws, the long hair that covers their body grows from the belly to the back, the opposite of that of normal animals. The hair has two types of green algae attached to it, so it looks green in the rainy season and turns brown in the dry season. There are five pairs of teeth on the upper jaw and four pairs on the lower jaw, which lack enamel and continue to grow throughout their lives. One of the characteristics of mammals is that they have seven cervical vertebrae, but sloths have six to nine, with three-toed sloths having nine, two-toed sloths having seven, and Hoffmann's sloths having six. Their stomachs are compound and complex, and they do not have a cecum.

Their movements in the trees are slow, moving at an average speed of 900 meters per hour, and they sleep hanging down for 18 hours a day. They hang upside down in the trees, but when on the ground or in the water they lie on their bellies and are good swimmers. Their body temperature is known to be the lowest of any mammal, and has been observed to vary between 24 and 35 degrees Celsius. It is believed that the long gestation period of two-toed sloths, at 263 days, is related to this low body temperature. They normally give birth to one baby, and there is a record of a lifespan of 32 years for Hoffman's sloths in captivity.

[Masaru Saito]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

哺乳(ほにゅう)綱貧歯目ナマケモノ科に属する動物の総称。この科Bradypodidaeには2属5種があり、おもに南アメリカ大陸の密林地帯の樹上で生活し、木の葉、花などを餌(えさ)としている。ミユビナマケモノ属Bradypusには、ブラジルに分布するノドジロミユビナマケモノB. tridactylus、ブラジル南部のタテガミナマケモノB. torquatuc、ブラジル、アルゼンチン、ボリビアのノドチャミユビナマケモノB. variegatusが、アマゾン流域、ベネズエラ、ガイアナのフタユビナマケモノ属Choloepusには、フタユビナマケモノC. didactylus、ニカラグアからエクアドルにかけてのホフマンナマケモノC. hoffmanniが含まれる。

　体長50～64センチメートル、尾はみられないか、あっても短い。体重4～9キログラム。後肢はいずれの属も3指であるが、前肢はフタユビナマケモノ属では2指、ミユビナマケモノ属の場合3指で、前後肢とも全指に曲がった大きな鉤(かぎ)づめがある。樹上で生活するナマケモノは、この鉤づめで逆さにぶら下がった状態で生活するため、体を覆う長い体毛は普通の動物とは逆に腹から背のほうに向いて生える。毛には2種類の緑藻類が付着しているため雨期には緑色にみえ、乾期には褐色になる。歯は上顎(じょうがく)に5対、下顎に4対あり、エナメル質を欠き、一生伸び続ける。哺乳動物の特徴の一つに頸椎(けいつい)が7個であることがあげられるが、ナマケモノの頸椎は6～9個で、ミユビナマケモノで9個、フタユビナマケモノ7個、ホフマンナマケモノの場合は6個である。胃は複胃であり複雑で、盲腸はない。

　樹上での動作は鈍く、平均時速900メートルで移動し、1日18時間はぶら下がって眠る。樹上では逆さにぶら下がるが、地面や水中では腹ばいになり、泳ぎはうまい。体温は哺乳動物のなかでももっとも低いものとして知られ、24～35℃の間で変化することが観察されている。フタユビナマケモノの妊娠期間が263日と長いのも、この体温の低さと関係があると考えられている。1産1子が普通で、寿命はホフマンナマケモノの飼育例では32年という記録がある。

［齋藤　勝］

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