哺乳动物(mammal)
哺乳纲的成员,温血脊椎动物,四肢(有些水生物种除外),与脊索动物门其他各纲的区别在於雌性分泌乳汁的腺体,在发育的一些阶段有毛发存在。其他特徵还包括:下巴直接铰接在头骨上,藉着中耳的骨头听声音,肌肉的横膈分开胸腔与腹腔,以及无核的成熟红血球细胞。哺乳动物小如鼩鼱,大至蓝鲸。单孔目(鸭嘴兽和针鼹)产卵,其他所有的哺乳动物都是直接产下幼雏。有袋目的新生幼子在子宫外面完成发育,有时候是在类似袋囊的构造之中。胎盘哺乳动物(参阅placenta)出生时是较为高等的发育阶段。最早的哺乳动物可追溯至三叠纪晚期(在2亿600万年前结束);最接近的祖先是兽孔目爬行动物。7000万年来,哺乳动物已经成为地球生态系的优势动物,主要有两个原因:哺乳动物幼子向长辈学习的能力(依靠母亲养育的结果)使其具有强大的行为适应性;温血的身体可适应各种气候与环境。亦请参阅carnivore、cetacean、herbivore、insectivore、omnivore、primate、rodent。
English version:
mammal
Any member of a class (Mammalia) of warm-blooded vertebrates having four limbs (except for some aquatic species) and distinguished from other chordate classes by the female's milk-secreting glands and the presence of hair at some stage of development. Other unique characteristics include a jaw hinged directly to the skull, hearing through bones in the middle ear, a muscular diaphragm separating the pectoral and abdominal cavities, and nonnucleated mature red blood cells. Mammals range in size from the tiny shrew to the enormous blue whale. Monotremes (platypus and echidna) lay eggs; all other mammals bear live young. Marsupial newborns complete their development outside the womb, sometimes in a pouchlike structure. Placental mammals (see placenta) are born at a relatively advanced stage of development. The earliest mammals date from the late Triassic period (ended 206 million years ago); their immediate ancestors were the reptilian therapsids. For 70 million years mammals have been the dominant animals in terrestrial ecosystems, a consequence of two principal factors: the great behavioral adaptability provided by the ability of mammalian young to learn from their elders (a consequence of their dependence on their mothers for nourishment), and the physical adaptability to a wide range of climates and conditions provided by their warm-bloodedness. See also carnivore, cetacean, herbivore, insectivore, omnivore, primate, rodent.
参考文章
- 哺乳动物是最高等的脊椎动物,靠母体的乳腺分泌乳汁哺育初生幼体。下面哪种是哺乳动物知识题库
- 澳大利亚哺乳动物化石地点群建筑与名胜
- 哺乳动物化石FossilMammal地球科学
- 哺乳动物细胞中糖原的分解机理是什么?生物知识
- 哺乳动物都有一套相对有效的调节温度的系统和手段,用来保持()的体温。党建知识
- 哺乳动物的眼睛一般都长在头的前部,这样便于观察前方的状况,利于避开敌害或捕获食物。所以当一种动物攻击其他动物时,常常会采用在()慢慢靠近的方式。党建知识
- 恒温动物的体表是散热的场所,当哺乳动物的体温()周围环境温度时,身体产出的能量就会变成热量,通过体表散发到周围环境中去。党建知识
- 生活在地下洞穴里的哺乳动物,由于环境微光或无光,眼睛常常长得很(),甚至完全看不见。不过,这些动物可以靠听觉、嗅觉、触觉来弥补。党建知识
- 哺乳动物的乳头对数,与其每胎产的崽数相关。乳头的对数越多,一胎产的崽数也越()。党建知识
- 哺乳动物是恒温动物,俗称温血动物,它们能保持恒定的体温,以满足高效率()的需要。党建知识