Mammals - honyurui (English spelling)

Japanese: 哺乳類 - ほにゅうるい（英語表記）mammal

A general term for animals belonging to the class Mammalia in the phylum Vertebrate. Members of this class Mammalia are a taxonomic group that split off from reptiles in the Late Triassic Period of the Mesozoic era, acquired homeothermic and viviparous characteristics, and flourished by filling the vacant ecological niche (ecological position) created by the extinction of the dinosaurs. There are approximately 4,300 extant species, with terrestrial species naturally distributed on all continents and continental islands except Antarctica and New Zealand (except for two species of bats), and marine species distributed throughout all oceans.

[Yoshinori Imaizumi]

Features

They vary greatly in size and shape, from the little shrew, which is 4-5 centimeters long and weighs 1.5-1.8 grams, to the blue whale, which is 30 meters long and weighs over 110 tons, but females always have mammary glands that secrete milk to raise their young, and all, except the beluga, have hair on their skin. For this reason, they are also called "kemono" (furry things, or beasts). The lower jaw consists only of the dentary, and is directly articulated with the squamosal bone at the rear upper end. The articular bone and quadrate bone that formed the jaw joint in reptiles move to the middle ear and become the malleus (malleus) and incus (incus) of the auditory ossicles. This dramatically increased the ability of the eardrum to amplify vibrations and transmit them to the lymph in the inner ear, and this, combined with the development of the snail tube and organ of Corti and the appearance of the auricle, sharpened hearing. The audible range of barn owls is 200 to 10,000 Hz, and that of frogs is 200 to 1,500 Hz, while that of humans is 200 to 20,000 Hz, with the upper limit reaching tens of thousands of Hz in bats and over 200,000 Hz in whales. The body cavity is divided into the thoracic and abdominal cavities by a muscular diaphragm, allowing active breathing. The heart is divided into four complete chambers, with a tricuspid valve on the right and a bicuspid valve on the left between the atria and ventricles. There is only one aortic arch on the left side. The skull is immovable, with two occipital condyles and a secondary palate that separates the oral and nasal cavities. They have a penis that transmits semen, and their red blood cells are anucleate.

[Yoshinori Imaizumi]

Main Form and Function

Unlike reptiles, their limbs rotate backward at the elbows and forward at the knees, making it easier for them to move forward. Primitive species walk with the entire sole of the foot on the ground (plantigrade), but more advanced species become digitate or ungulate. Sometimes the front legs change into wings or fins, and the hind legs disappear. Their skin is soft and lacks scales, and they are equipped with sweat glands, sebaceous glands, scent glands, and hair, and their claws sometimes change into flat claws or hooves. Hair is newly formed near the scales, and differentiates into supercilium, underhair, and tentacles, and plays important roles in thermoregulation, skin protection, touch, and dissipation of odor. The ribs are limited to the thoracic vertebrae, forming the thorax. Except for sirenians and some sloths, there are always seven cervical vertebrae, the first cervical vertebra being the atlas and the second cervical vertebra being the axis. The coracoid bone becomes the coracoid process of the scapula, the humerus articulates with the lower end of the scapula, and the clavicle disappears in ungulates. Each finger has two phalanges, except in cetaceans, and the other fingers have three. Teeth occur only in the maxilla, premaxilla, and dentary (mandible), and their roots fit into the alveolar socket. They are differentiated into incisors, canines, premolars, and molars (heterodontia), and there are deciduous and permanent teeth except for the molars (monodontia). Molars have three or more processes (awls) arranged in a triangle or W shape, and the upper and lower awls interlock to cut food and crush it at the base, but this changes greatly depending on the diet. The oral cavity contains salivary glands such as the submandibular gland, sublingual gland, and parotid gland.

The stomach is often complex, and in ruminants, bacteria that break down cellulose live symbiotically. The kidneys are metanephros, which are highly functional, and even marine animals do not require salt glands. The lower part of the oviduct changes into the uterus and vagina, and in evolved animals, the testes leave the abdominal cavity and descend into the scrotum. The nasal cavity contains complex nasal turbinates, which warm the air sent to the lungs. The lungs are divided into numerous alveoli, and in large species the surface area reaches over 100 square meters. The pulmonary circulation and systemic circulation are completely separate, and there is a portal venous system connecting the intestines and liver. The cerebral hemispheres are large, and most of the cerebral cortex is made up of the neopallet. The general receptor for odor is the olfactory epithelium in the nasal cavity, but the receptor for sexual odor is the Jacobson's organ, located between the palate and nasal cavity. Color vision degenerates to various degrees in all animals except diurnal monkeys and squirrels, while nocturnal animals have a special membrane (tapetum) that reflects light behind the retina. The reason mammals prospered in place of reptiles is largely due to the development of constant body temperature, offspring rearing, and social life.

[Yoshinori Imaizumi]

Thermostatic

Body temperature is endothermic, and camels and temperate bats are poikilothermic to some extent, but others are homeothermic, independent of air temperature within a certain range. Rectal body temperature is about 30°C for monotremes, about 32°C for armadillos and sloths, about 36°C for kangaroos, hedgehogs, and elephants, about 37°C for chimpanzees and orangutans, and reaches 38°C or higher for pikas, rabbits, cats, wolves, wild boars, deer, bison, etc. A drop in body temperature is prevented by standing their hair upright to increase the amount of air contained in their fur, and reindeer prevent the loss of body heat by lowering the temperature of their extremities. When their body temperature becomes too high, humans and horses sweat from sweat glands all over their body to lower it, but dogs and cats only have sweat glands in the pads of their feet, so they cool their blood by evaporating water with their tongues and lungs. Whales, manatees, and golden moles do not have sweat glands. Homeothermy allows mammalian mothers to safely maintain pregnancy and produce milk continuously, but it also allows the offspring to grow much faster than reptiles. Maintaining endothermy requires a lot of energy, but mammals' digestive, respiratory and circulatory systems are more functional than those of reptiles to meet this demand.

[Yoshinori Imaizumi]

Childcare

The composition of milk varies depending on the species and the stage of development, but always meets the needs of the young. Milk also contains antibodies against infectious diseases acquired by the mother. Reptile young grow slowly and are vulnerable to predators for a long period of time, but mammal young grow safely and quickly under the protection of their parents, and become independent after learning the knowledge and skills necessary for life (such as hunting techniques). This care forms a close bond between parent and child. Teeth do not grow in until weaning is approaching and the jaws have grown to a certain size. At that time, the rate of jaw growth has slowed, so the upper and lower teeth can be adjusted so that they fit together properly and do not interfere with chewing.

[Yoshinori Imaizumi]

Social life

The bonds formed between parents and offspring and siblings during parenting are maintained for a long time, and animals often form tight-knit groups. Individuals in the group are identified primarily by their sense of smell, with secretions from sebaceous, sweat, and scent glands playing an important role. Scent glands may be concentrated in specific areas on the surface of the skin, but it is not uncommon for them to be distributed on the inside of a pouch, expanding the surface area, as in the subocular glands of Japanese serows, metatarsal glands of deer, anal glands of dogs, cats, and weasels, perineal glands of civets, and preputial glands of male musk deer. Secretions from scent glands are left on trees and stones within a territory as sign posts. The scent continues to communicate the individual's social status, state of estrus, and several other pieces of information for several days after the sender has departed.

[Yoshinori Imaizumi]

Evolution and phylogeny

Mammals are the first reptilian therapsids to split off from those with a double jaw joint (the dentary-interstele joint and the dentary-intersquamosal joint) in the Late Triassic, leaving only the dentary-intersquamosal joint. The oldest mammal with a complete skeleton is Eozostrodon (Tricomydont) from South Africa, which was about 10 cm long and semi-arboreal. Its molars have three teeth arranged vertically, whereas the contemporary Queneotherium (Odonta) has three teeth arranged in a triangle (triangular). The latter gave rise to the Theria, and the former to the other members of the Prototheria. Haramiya (Microkleptes), which appeared around the same time, has molars with three large teeth on one side and five small teeth on the other. From this, it is thought that the herbivorous Multicomydonts, which flourished until the beginning of the Tertiary Period of the Cenozoic Era, evolved. Because the relationship of the Multicomydonts to other species is unclear, they are sometimes distinguished as Allotheria (some sources also consider Tricomydonts to be included). From the osseodonts, holotherians with a trapezoid tooth heel behind the triangular molars appeared in the Jurassic, which gave rise to the Metatherian (marsupialia) and Eutherian in the Cretaceous. The Eutherian includes all living mammals except for monotremes and marsupials, and flourished rapidly in the Cenozoic Era, giving rise to the Age of Mammals.

[Yoshinori Imaizumi]

"Encyclopedia of Animals, Vols. 1-6" by D.W. MacDonald (1986, Heibonsha) "Modern Biology Series 4: Vertebrates B" edited by Yasusaburo Sugi, Yoshinori Imaizumi, and Masaharu Nishiwaki (1976, Nakayama Shoten)

Mammalian morphology
© Tadaaki Ohkata ">

Mammalian morphology

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

脊椎(せきつい)動物門哺乳綱に属する動物の総称。この綱Mammaliaの仲間は、中生代三畳紀後期に爬虫(はちゅう)類から分かれて恒温性、胎生性を獲得し、恐竜類の絶滅で生じた空白のニッチ（生態的地位）を埋めて栄えた分類群である。現生種約4300種、陸生種は南極大陸とニュージーランド（2種のコウモリを除く）以外の大陸、大陸島に自然分布し、海生種は全海洋に分布する。

［今泉吉典］

特徴

体長4～5センチメートル、体重1.5～1.8グラムのチビトガリネズミから、全長30メートル、体重110トン以上のシロナガスクジラまで、大きさ、体形とも変化に富むが、雌はかならず乳腺(にゅうせん)を備え、乳汁を分泌して子を育て、シロイルカ以外は皮膚に毛を生ずる。このため「けもの」（毛物、転じて獣）ともいう。下顎(かがく)は歯骨だけからなり、後上端で鱗骨(りんこつ)と直接関節する。爬虫類で顎関節を形成していた関節骨と方形骨は中耳に移動し、耳小骨のツチ骨（槌骨）、キヌタ骨（砧骨）となる。このため鼓膜の振動を拡大して内耳のリンパに伝える能力が飛躍的に高まり、かたつむり管とコルチ器の発達、耳介の出現と相まって聴覚が鋭くなった。可聴範囲はメンフクロウで200～1万ヘルツ、カエルで200～1500ヘルツなのに対し、ヒトで20～2万ヘルツ、その上限はコウモリで十数万ヘルツ、クジラでは20万ヘルツ以上に達する。体腔(たいこう)は筋肉質の横隔膜で胸腔、腹腔に分かれ、活発な呼吸が可能となる。心臓は完全な4室に分かれ、心房・心室間には右に三尖弁(せんべん)、左に二尖弁がある。大動脈弓は左側の1個しかない。頭骨は不可動性で、2個の後頭顆(か)、口腔と鼻腔を区画する二次口蓋(こうがい)を備える。精液を通す陰茎をもち、赤血球は無核である。

［今泉吉典］

主要形態と機能

四肢は爬虫類と異なり肘(ひじ)が後方、膝(ひざ)が前方へ回転し、前進が容易になる。原始的なものでは足底全部を地につけて歩く蹠行(しょこう)性であるが、進んだものでは指行性、蹄行(ていこう)性となる。ときに前肢が翼やひれに変化し、後肢が消失する。皮膚は柔らかく鱗(うろこ)を欠き、汗腺、皮脂腺、臭腺、毛を備え、つめはときに平づめやひづめに変化する。毛は鱗のそばに新規に生じたもので、上毛、下毛、触毛などに分化し、体温調節、皮膚の保護、触覚、においの発散などに重要な働きをする。肋骨(ろっこつ)は胸椎に限られ胸郭を形成する。頸椎(けいつい)は海牛類とナマケモノ類の一部以外はつねに7個、第1頸椎は環椎、第2頸椎は軸椎となる。烏口(うこう)骨は肩甲骨の烏口突起になり、上腕骨は肩甲骨の下端部と関節し、有蹄類などでは鎖骨が消失する。各指の指骨はクジラ類などを除き、第1指が2個、ほかの指は3個である。歯は上顎骨、前顎骨、歯骨（下顎骨）にのみ生じ、歯根部は歯槽に収まる。また門歯、犬歯、前臼歯(ぜんきゅうし)、臼歯に分化し（異歯性）、臼歯を除き乳歯と永久歯がある（一換性）。臼歯には3個またはそれ以上の突起（錐(すい)）があって三角形かW字形に並び、上下の錐がかみ合って食物を切断し、基部で押しつぶす砕錐歯が原形であるが、食性に応じた変化が甚だしい。口腔内には顎下腺、舌下腺、耳下腺などの唾液(だえき)腺がある。

　胃はしばしば複雑化し、反芻(はんすう)類などではセルロースを分解するバクテリアが共生する。腎臓(じんぞう)は後腎で機能が優れ、海生のものでも塩腺を必要としない。輸卵管の下部は子宮と腟(ちつ)に変化し、精巣は進化したものでは腹腔から出て陰嚢(いんのう)内に降下する。鼻腔内には複雑な鼻甲介があり、肺へ送る空気を温める。肺は多数の肺胞に分かれ、大形種では表面積が100平方メートル以上に達する。肺循環と体循環が完全に分かれ、腸と肝臓を結ぶ門脈系がある。大脳半球が大きく、大脳皮質は大部分が新外套(がいとう)からなる。一般的なにおいの受容器は鼻腔内の嗅(きゅう)上皮であるが、性的なにおいのそれは口蓋と鼻腔の間にあるヤコブソン器官である。色覚は昼行性のサル類、リス類など以外では種々の程度に退化し、夜行性のものでは網膜の後ろに光を反射する特殊な膜（タペータム）がある。哺乳類が爬虫類にかわって繁栄したのは恒温性、子の保育、社会生活の発達によるところが大きい。

［今泉吉典］

恒温性

体温は内温性で、ラクダや温帯生のコウモリではある程度変温性であるが、ほかは気温とある範囲内で無関係な恒温性である。直腸内体温は単孔類約30℃、アルマジロ、ナマケモノ約32℃、カンガルー、ハリネズミ、ゾウ約36℃、チンパンジー、オランウータン約37℃、ナキウサギ、ウサギ、ネコ、オオカミ、イノシシ、シカ、バイソンなどでは38℃以上に達する。体温の低下は、毛を直立させ毛衣に含まれる空気を増やして防ぎ、トナカイは四肢の温度を下げて体熱のロスを防ぐ。体温が上がりすぎたとき、ヒトやウマは全身の汗腺から汗を出して下げるが、イヌやネコは汗腺が足底の肉球にしかないため、舌や肺で水分を気化して血液を冷却する。汗腺はクジラ、マナティー、キンモグラにはない。恒温性のため哺乳類の母親は妊娠を安全に継続し乳汁を継続的に製造できるが、このため生まれた子も爬虫類より成長がずっと速い。内温性を保つには多量のエネルギーを必要とするが、哺乳類の消化器、呼吸器、循環器は爬虫類より機能が優れていて、この要求を満たしている。

［今泉吉典］

保育

乳汁の組成は種によっても、成長の段階によっても変化し、つねに子の要求に合致している。また乳汁には母親が獲得した感染病の抗体が含まれている。爬虫類の子は成長が遅く天敵に殺されやすい期間が長いが、哺乳類の子は親の保護下で安全かつ速やかに成長し、生活に必要な知識や技術（狩りの技術など）を学んだのち独立する。この保育により親子の緊密なきずなが形成される。歯は、離乳が近づき、あごがある程度大きくなるまで生えない。そのころにはあごの成長速度が落ちているので、歯は上下の錐が正しくかみ合い、そしゃくに支障ないよう調整できる。

［今泉吉典］

社会生活

保育で形成された親子、兄弟のきずなは長い間保たれ、しばしば緊密な群れが形成される。群れの個体は主として嗅覚(きゅうかく)で識別され、皮脂腺、汗腺、臭腺の分泌物が重要な働きをする。臭腺は皮膚表面の特定部位に密集していることもあるが、カモシカの眼下腺、シカの中足腺、イヌ、ネコ、イタチなどの肛門(こうもん)腺、ジャコウネコの会陰(えいん)腺、ジャコウジカの雄の包皮腺などのように袋の内面に分布し、表面積を広げていることもまれでない。臭腺の分泌液は縄張り内の木や石につけてサインポストとする。そのにおいは送り主が立ち去ったあとも、数日間その個体の社会的地位、発情の状態、そのほかいくつかの情報を伝え続ける。

［今泉吉典］

進化と系統

爬虫類獣弓目のなかの二重の顎関節（関節骨・方骨間関節と歯骨・鱗骨間関節）をもったものから三畳紀後期に分かれ、顎関節が歯骨・鱗骨間関節だけになったのが哺乳類である。全身骨格が知られている最古の哺乳類は南アフリカのエオゾストロドン（三丘歯目）で、体長約10センチメートル、半樹上性であった。臼歯には縦一列に並んだ3錐があるが、同時代のクエネオテリウム（相称歯目）では3錐が三角形に並ぶ（錐三角）。後者から獣亜綱が、前者から原獣亜綱のほかの仲間が生じた。同じころ現れたハラミヤ（ミクロクレプテス）の臼歯には片側に大きな3錐、反対側に小さな5錐がある。これから、新生代第三紀の初めまで栄えた、草食性の多丘歯目が進化したと思われる。多丘歯目はほかとの関係が不明なため、異獣亜綱（三丘歯目を含める考え方もある）として区別されることもある。相称歯目からは、臼歯の錐三角の後ろに台状の歯踵(ししょう)を備えた全獣目がジュラ紀に現れ、これから白亜紀に後獣下綱（有袋目）と真獣下綱が生じた。真獣下綱には単孔目、有袋目以外のすべての現生哺乳類が含まれ、新生代に急速に繁栄して哺乳類時代を出現した。

［今泉吉典］

『D・W・マクドナルド著『動物大百科1～6巻』（1986・平凡社）』▽『杉靖三郎・今泉吉典・西脇昌治監修『現代生物学大系4　脊椎動物B』（1976・中山書店）』

哺乳類の形態図

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

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