Insectivores - Insectivores

A general term for animals belonging to the order Insectivora in the class Mammalia. The members of this order Insectivora are small insectivorous animals that are considered to be the most primitive of all mammals other than monotremes and marsupials (Eutheria). They are generally small in body size, including the smallest mammals with a head and body length of about 4 cm, such as the pygmy shrew and the Tokyo shrew. Their limbs are short and plantigrade, and they walk with their bellies touching or almost touching the ground. They usually have five fingers with claws, and the front part of their snout is long and protrudes further forward than their mouth, making them mobile. Their eyes are small and their eyesight is poor, their ears are usually small, and they have scales on the tops of their feet and tails. They usually have 44 teeth (basic type of eutheria), and their molars have 3 to 5 sharp projections, which allow them to cut food but are not suitable for grinding it. Their cerebrum is small and smooth, and they are not intelligent. They lack a scrotum, often have a cloaca, and the placenta is disc-shaped.

They are found almost all over the world, except for Australia, Antarctica, and central and southern South America, and live in all environments from the tropics to the polar regions, and from lowlands to high mountains. There are no arboreal species, but they live on land or underground, and occasionally in freshwater. They usually live alone, are active both day and night, and feed mainly on invertebrates such as insects and earthworms. Species such as Solenodon and the Bralina shrew bite their prey and inject venom secreted from their submandibular glands to weaken it and capture it. Shrews also emit ultrasonic waves to find their way, and hedgehogs and other species hibernate.

It was once thought that all other eutherians branched off from this class, but in recent years it has been determined that the only orders that are certain to have descended from this class are Dermoptera, Chiroptera, and Primates, with only indirectly related orders such as Ungulates and Carnivora, and that Edentata, Squamata, Rodentia, and Lagomorpha are not descendants of this class. There are about 360 species in seven families: Solenodontidae (2 species of Solenodon), Tenrecidae (33 species including Tenrec and Potamogare) in Madagascar and Africa, Gnathidae (17 species of Golden Moles) in Africa, Erinidae (17 species of Gymnura and Hedgehog) in Eurasia and Africa, Soricidae (about 246 species including Shrew, Shrew, Musk Shrew, Water Mouse, etc.) in Africa, Eurasia, North America and northern South America, Tallidae (about 29 species including Mole, Shrew, Desman, etc.), and Elephantidae (15 species of Elephant Shrew) in Africa. The last of these, Elephantidae, has a cecum, well-developed eyes, is semi-digital, jumps on its hind legs like a kangaroo, and has different heel joints from other insectivores, so in recent years it has been considered an independent order similar to Lagomorpha.

[Yoshinori Imaizumi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

哺乳(ほにゅう)綱食虫目に属する動物の総称。この目Insectivoraの仲間は、単孔類、有袋類以外の哺乳類（真獣類）のうちでもっとも原始的とみられる食虫性の小獣類である。一般に体は小さく、なかにはコビトジネズミやトウキョウトガリネズミなど頭胴長4センチメートルほどの最小の哺乳類を含む。四肢は短小かつ蹠行(しょこう)性で、腹を地面につけるか、ほとんどつけるようにして歩く。指は普通5本で鉤(かぎ)づめを備え、吻(ふん)の前部は長く、口よりも前に突出していて可動性。目は小さく視力が弱く、耳殻が普通小さく、足の甲や尾には鱗(うろこ)がある。歯は普通44本（真獣類の基本型）、臼歯(きゅうし)には3～5個の鋭い突起があって、食物を切り砕くことはできるが、すりつぶすには適していない。大脳は小さく表面が平滑で、知能が低い。陰嚢(いんのう)がなく、しばしば総排出腔(こう)があり、胎盤は円盤状をしている。

　オーストラリア区、南極大陸、南アメリカの中部以南を除き、ほとんど世界中に分布し、熱帯から寒帯、低地から高山までのあらゆる環境にすむ。樹上生のものはなく、地上または地下生、まれに淡水生。普通単独で生活し、昼夜とも活動して、昆虫、ミミズなどの無脊椎(むせきつい)動物を主食とする。ソレノドン、ブラリナトガリネズミなどは、獲物にかみついて顎下腺(がっかせん)から分泌する毒液を注入し、弱らせてとらえる。またトガリネズミは超音波を発して進路を探り、ハリネズミなどは冬眠する。

　かつては、この類から他のすべての真獣類が分かれ出たと考えられていたが、近年では、この類から出たことが確かなのは皮翼目、翼手目、霊長目ぐらいしかなく、間接に関係があるのは有蹄(ゆうてい)目、食肉目などで、貧歯目、有鱗(ゆうりん)目、齧歯(げっし)目、ウサギ目などはこの類の子孫ではないと考えられるようになった。現生のものは、西インド諸島のソレノドン科（ソレノドン2種）、マダガスカル島とアフリカのテンレク科（テンレク、ポタモガーレなど33種）、アフリカのキンモグラ科（キンモグラ17種）、ユーラシアとアフリカのハリネズミ科（ジムヌラ、ハリネズミ17種）、アフリカ、ユーラシア、北アメリカ、南アメリカ北部のトガリネズミ科（トガリネズミ、ジネズミ、ジャコウネズミ、カワネズミなど約246種）、ユーラシアと北アメリカのモグラ科（モグラ、ヒミズ、デスマンなど約29種）、およびアフリカのハネジネズミ科（ハネジネズミ15種）の7科約360種がある。最後のハネジネズミ科は盲腸をもち、目がよく発達し、なかば指行性でカンガルーのように後肢で跳躍するほか、かかとの関節なども他の食虫類と違うので、近年ではウサギ目に近い独立の目とされることがある。

［今泉吉典］

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