Even-toed ungulates

An order of ungulates in the mammalian class, Artiodactyla is so named because they usually have an even number of toes on their front and back feet, usually two or four. Members of the even-toed ungulate order Artiodactyla are ungulate, walking with only their hooves on the ground, and many of them are adapted for running, with the forearm and shank bones longer than the humerus and femur. The axis of the foot passes between the third and fourth toes, which are larger than the other toes. The third and fourth metacarpal bones of the forefoot and metatarsal bones of the hindfoot are also larger, and in more evolved species they are united to form a single long tubular bone. They have no clavicles, and their forelimbs can only move forward and backward. The talus at the heel of the hindfoot has trochleas at both the top and bottom, and is mobile at the joints with the tibia and the tarsus bones. This allows the tibia and hindfoot to bend more strongly than in other mammals, allowing them to take larger strides. The femur does not have the third trochanter (where the gluteal muscle attaches) seen in odd-toed ungulates, and the gluteal muscle attaches to the tibia. The upper incisors have disappeared, and the lower canines tend to become incisor-like, and many have stomachs divided into several chambers for ruminating food. They have a cecum and often a gallbladder. More advanced species have scent glands in the undereye area, between the digits, midfoot, and groin, and males often have bony horns on their heads.

Phylogenetically, like the Perissodactyla, they branched off from the Ankylodes order, and the Palaeodontidae of the Suidae suborder appeared in North America in the early Eocene of the Cenozoic era. They soon declined, but the Ruminantidae suborder, which appeared in the middle Eocene, gradually replaced the Perissodactyla and flourished, coming to make up the majority of the Ungulates. In addition to domestic pigs, camels, llamas, alpacas, reindeer, cattle, water buffalo, yaks, bantengs, gayales, sheep, and goats, there are more than 184 extant wild species known, distributed almost throughout the world except for Antarctica and the Australian region, and there are the following two suborders and 10 families.

(1) Suidae: The upper incisors are well developed and the canines are large and tusk-like. There is no esophageal groove in the stomach and they do not ruminate. The limbs are relatively short and have no tubular bones, making them unsuited for fast running. They have no horns. There are the omnivorous Suidae (8 or more species, Eurasia, Africa) and Peccariidae (3 species, North and South America), and the herbivorous Hippopotamusidae (2 species, Africa).

(2) Ruminantidae: All are herbivorous, have degenerated upper incisors, and have an esophageal groove at the entrance to the third stomach, through which they chew their food. They have long limbs and tubular bones, which allow them to run at high speeds. Many of them have horns. There are four suborders: Camelidae (4 species, Asia and South America), Mouse Deer (4 species, South Asia and Africa), Musk Deer (3 species, Asia), Cervidae (34 or more species, South and North America, Eurasia, the Sunda Islands, New Guinea, and North Africa), Giraffidae (2 species, Africa), Pronghorn (1 species, North America), and Bovidae (123 or more species, Africa, Eurasia, and North America).

[Yoshinori Imaizumi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

哺乳(ほにゅう)綱の有蹄類の1目で、前後足の指の数が普通2本または4本の偶数のためにこの名がある。偶蹄目Artiodactylaの仲間は、ひづめだけを地につけて歩く蹄行性で、多くは走るのに適し、前腕骨と下腿(かたい)骨は、上腕骨と大腿骨より長い。足の軸は、他指より大きい第3、第4指の間を通る。前足の中手骨と後ろ足の中足骨も第3と第4が大きく、進化した類では合一して1本の長い管骨となる。鎖骨がなく、前肢はほとんど前後にしか動かない。後ろ足のかかとにある距骨は、上下の両端が滑車になり、脛骨(けいこつ)との関節のほか、足根骨との関節も可動性である。このため脛骨と後ろ足は他の哺乳類より強く屈曲でき、歩幅を大きくできる。大腿骨には奇蹄類にみられる第3転子（臀筋(でんきん)の付着部）がなく、臀筋は脛骨に付着する。上の切歯が消失し、下の犬歯が切歯状に変わる傾向がみられ、胃が数室に分かれて食物を反芻(はんすう)するものが多い。盲腸と、多くは胆嚢(たんのう)がある。進化した類では、眼下部、指間部、中足部、鼠径(そけい)部などに臭腺(しゅうせん)があり、雄は頭に骨質の角(つの)をもつことが多い。

　系統的には、奇蹄類と同じく踝節(かせつ)目から分かれ出たもので、新生代の始新世前期にイノシシ亜目のパラエオドン類が北アメリカに現れた。これはまもなく衰えたが、始新世中期に現れた反芻亜目はしだいに奇蹄類を駆逐して栄え、有蹄類の大部分を占めるに至った。家畜のブタ、ラクダ、ラマ、アルパカ、トナカイ、ウシ、スイギュウ、ヤク、バンテン、ガヤル、ヒツジ、ヤギなどのほか、現生の野生種は184種以上知られ、南極大陸とオーストラリア区以外のほとんど世界中に分布し、次の2亜目10科がある。

(1)イノシシ亜目　上の切歯はよく発達し、犬歯は大きく牙(きば)状。胃には食道溝がなく、反芻しない。四肢は比較的短く、管骨がなく、早く走るのには適さない。角はない。雑食性のイノシシ科（8種以上、ユーラシア、アフリカ）とペッカリー科（3種、南・北アメリカ）、草食性のカバ科（2種、アフリカ）がある。

(2)反芻亜目　すべて草食性で、上の切歯は退化し、胃には第3胃の入口に食道溝があり、食物を反芻する。四肢が長く、管骨があり、高速で走るのに適する。多くは角をもつ。ラクダ科（4種、アジア、南アメリカ）、マメジカ科（4種、南アジア、アフリカ）、ジャコウジカ科（3種、アジア）、シカ科（34種以上、南・北アメリカ、ユーラシア、スンダ列島、ニューギニア島、北アフリカ）、キリン科（2種、アフリカ）、プロングホーン科（1種、北アメリカ）、およびウシ科（123種以上、アフリカ、ユーラシア、北アメリカ）がある。

［今泉吉典］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例