Rodents

A general term for animals in the order Rodentia. Members of this order Rodentia are small, with a pair of long incisors (front teeth) on both the top and bottom, which are rootless and continue to grow throughout life. The hard enamel is only on the front of the incisors, making them chisel-like, and suitable for gnawing at hard-shelled nuts and seeds such as walnuts and chestnuts. They lack canines and premolars, and there is a toothless area behind the incisors (a gap) where the lips fit in to prevent the chewing debris from going further in. The left and right mandibles are usually not fused and are mobile, and the tips of the incisors can be freely opened and closed, so they can insert the incisors into cracks in the shells of acorns and other foods, then open them and skillfully crack the shell. The jaw joints and the powerful masseter muscles, some of which reach the snout, are suitable for moving the lower jaw forward and using the lower incisors like a chisel. Their molars are adapted to grinding fibrous foods, and their crowns are long and often grow throughout their lives. Their elbow joints are freely movable, and most are plantigrade. They have three to five fingers and claws. Their tails, which are usually covered in scales, are useful for self-defense as they can easily be cut or skinned when held down by an enemy. They have a single stomach and usually have a cecum. Their testes are often located in the groin, and their penis has a baculum. Most are herbivorous, and generally have high reproductive abilities and grow quickly. Excluding the domestic rats that have spread their distribution to humans, they are found all over the world except Antarctica and New Zealand, and live in deserts, grasslands, forests, alpine mountains, and tundra from the tropics to the polar regions. Many live in holes and feed above ground, but there has been a remarkable adaptive radiation, including the mole rat, which lives underground, the arboreal squirrel, the aquatic water rat, the beaver, the leaping jerboa, and the gliding scaly squirrel and flying squirrel. Some damage crops and transmit infectious diseases and parasites, but some are important as furbearers (chinchillas, beavers, squirrels, etc.) and laboratory animals (guinea pigs, mice, rats, etc.). Their ancestors were Paramys (family Paramysidae), which lived in North America during the Paleocene. They are the most numerous of all living mammals, accounting for about 40% of all mammals, with at least 1,600 species, and are usually classified into the following three suborders and 28 families, but the classification is confusing as many of them are of unknown lineage.

(1) The suborder Sciuridae consists of seven families: the Albiidae (close to the Paramysidae), Sciuriidae, Gopheridae, Pocket Mice, Beavers, Scyllidae, and Leporidae.

(2) Muridae (suborder Muridae) There are five families: Muridae (sometimes called separate families for species such as the crucian mouse, the mole rat, the bamboo rat, and the spiny dormouse), the Dormouse, the Desert Dormouse, the Long-eared Mice, and the Jerbomorphidae.

(3) Suborder Hystridae There are 11 families in the New World: Hystridae, Caviaridae, Capybaraidae, Pacaranaidae, Pacasidae, Chinchillaidae, Capromyidae, Octodontidae, Ctenomyidae, Chinchillaridae, and Echymidae, and 5 families in the Old World: Hystridae, Deutziaidae, Bucconidae, Mole Rats, and Gundiidae. However, some people consider the 11 New World families to be the suborder Hystridae, and the Gundiidae, one of the 5 Old World families, to be included in the suborder Sciuridae, with the other 4 families considered to be of unknown affiliation.

[Yoshinori Imaizumi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

哺乳(ほにゅう)綱齧歯目に属する動物の総称。この目Rodentiaの仲間は小形で、切歯（門歯）が上下とも1対あり長大で根がなく終生伸び続け、堅いエナメル質がその前面にしかないため、のみ状を呈し、クルミ、クリのような堅い殻の実や種子をかじるのに適する。犬歯と前位の前臼歯(ぜんきゅうし)がなく、切歯の後ろに歯のない部分（歯隙(しげき)）があり、ここに口唇が入り込んでいて、かじりかすが奥に入るのを防ぐ。左右の下顎(かがく)骨は普通癒着せず可動性で、切歯の先端を自由に開閉できるため、切歯をそろえてどんぐりなどの殻の割れ目に差し込み、それを開いて巧みに殻を割る。あごの関節と、一部が吻(ふん)側に達する強大な咬筋(こうきん)は、下顎を前方に動かし、下の切歯をのみのように使うのに適する。臼歯は繊維の多い食物をすりつぶすのに適し、歯冠部が長くしばしば終生成長する。肘(ひじ)の関節は自由に動き、多くは蹠行(しょこう)性である。指は3～5本で鉤(かぎ)づめを備える。通常は鱗(うろこ)がある尾は、敵に押さえられると簡単に切れたり皮がむけることが多く、護身に役だつ。胃は単一、普通は盲腸がある。精巣は多くは鼠径(そけい)部に位置し、陰茎には陰茎骨がある。多くは植物食で、一般に繁殖力が強く、成長が早い。人間について分布を広げた住家性のネズミを除いても、南極大陸とニュージーランド以外の世界中に分布し、熱帯から寒帯までの砂漠、草原、森林、高山、ツンドラなどにすむ。穴にすみ地上で食物をとるものが多いが、もっぱら地下にすむメクラネズミ、樹上生のリス、水生のミズネズミ、ビーバー、跳躍するトビネズミ、滑空するウロコオリス、ムササビなど適応放散が著しい。農作物を食害し、伝染病、寄生虫を媒介するものもあるが、毛皮獣（チンチラ、ビーバー、リスなど）や実験動物（テンジクネズミ、マウス、ラットなど）として重要なものがある。祖先は暁新世に北アメリカにいたパラミス（パラミス科）である。現生哺乳類中もっとも種類が多くその約4割を占め、最低1600種あり、普通は次の3亜目28科に分類されるが、系統の不明なものが多く、分類は混乱している。

(1)リス亜目　ヤマビーバー科（パラミス科に近い）、リス科、ホリネズミ科、ポケットネズミ科、ビーバー科、ウロコオリス科、トビウサギ科の7科がある。

(2)ネズミ亜目　ネズミ科（キヌゲネズミ、メクラネズミ、タケネズミ、トゲヤマネなどを別の科とすることがある）、ヤマネ科、サバクヤマネ科、オナガネズミ科、トビネズミ科の5科がある。

(3)ヤマアラシ亜目　新世界にキノボリヤマアラシ科、テンジクネズミ科、カピバラ科、パカラナ科、パカ科、チンチラ科、カプロミス科、オクトドン科、クテノミス科、チンチラネズミ科、エキミス科の11科、旧世界にヤマアラシ科、アフリカアシネズミ科、イワネズミ科、デバネズミ科、グンディ科の5科がある。しかし新世界の11科をテンジクネズミ亜目とし、旧世界の5科中グンディ科をリス亜目に含め、他の4科を所属不明とみる人もある。

［今泉吉典］

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