草稿:哺乳动物

哺乳动物
如何讀生物分類框

化石时期:侏罗纪晚期英语Late Triassic至今;2.25亿或1.67亿年前至今 正文中的相关讨论
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科学分类 e
界: 动物界 Animalia
门: 脊索动物门 Chordata
总纲: 四足总纲 Tetrapoda
演化支 爬行形类 Reptiliomorpha
演化支 羊膜动物 Amniota
演化支 合弓类 Synapsida
演化支 哺乳形类 Mammaliaformes
纲: 哺乳纲 Mammalia
Linnaeus, 1758
现生子类

哺乳动物哺乳类是对脊椎动物哺乳纲学名Mammalia,出自拉丁语mamma,即“胸部”)动物的统称。与同属羊膜动物、在石炭紀(約30億年前)分化的爬蟲類鳥類不同的是,哺乳类具有新皮质毛皮、三個听小骨和乳腺,雌性还會用乳腺分泌乳汁餵養幼崽(棕梠果蝠的雄性也可以餵乳[1][2]。目前,人们共发表了约 6400 个哺乳類物种[3],其中物种数量最多的目有囓齒目翼手目真盲缺目英语Eulipotyphla刺猬鼹鼠鼩鼱等),多樣性略低於以上三目的有灵长目(包含人類在內的等)、偶蹄目鯨豚類和其他偶蹄目)和食肉目海豹等)。

支序學的角度來看,哺乳類是合弓綱中的唯一成員,而合弓綱和蜥形綱組合成了一個更大的演化支羊膜動物。合弓綱哺乳動物的先祖為楔齒龍類中的盤龍目,包含非哺乳類的異齒龍屬。在 30 億年前,石炭紀末期,合弓綱從蜥形綱中分化出來後,其中一個分支演化成了哺乳類,而蜥形綱則成為了現生的鳥類和爬蟲類。在二疊紀早期獸孔目出現之前,楔齒龍類的基群演化成了好幾支多樣性高的非哺乳類合弓綱,這些生物偶爾會被誤認為類似哺乳類的爬蟲類。現生的哺乳類起源於新生代的古近紀和新近紀,大約6千6百萬年前,也就是非鳥恐龍滅絕之後,且自出現以來便是陸生動物中最具優勢的類群。

哺乳动物的基本体型为四足步行型,大多数哺乳动物会用其四肢在陆地上移动英语terrestrial locomotion。不过,一些生活在别的栖息环境的哺乳动物,如海里空中英语aerial locomotion树上英语arboreal locomotion地下英语fossorial,以及靠双足移动的动物英语bipedalism等,它们的四肢末端演化出了不同的形态,来适应其不同的生活环境。哺乳动物小至30~40 mm(1.2~1.6英寸)的凹臉蝠,大到30米(98英尺)的蓝鲸(可能是地球上生活过的最大的动物)。其寿命在2年(鼩鼱)到211年(弓頭鯨)之间不等。除了单孔目下的五个产蛋物种外,其他所有的现生哺乳动物在分娩时都会直接诞下幼崽。哺乳动物中最大的一群动物是一群被称为胎盤動物分类阶元,它们的胎盤可以在孕期为胎儿供养。

绝大多数哺乳动物具有认知能力,有的还有大容量的脑和自我意识,会使用工具英语tool use in animals。哺乳动物可用多种不同的方式发声、交流,如发出超聲波用气味标记领地发出警告信号英语alarm signal鸣唱回声定位英语Animal echolocation等。哺乳动物会组织形成裂变融合社会英语fission-fusion society一雄多雌集團英语harem (zoology)[有没有更好的翻译?]等级制度——但也有的更独来独往,有着自己的领地。大部分哺乳动物实行“一夫多妻制英语polygyny in animals”,但也有的是“一夫一妻制英语monogamy in animals”或者“一妻多夫制英语Polyandry in nature”.

新石器革命期间,人类驯化了多种哺乳动物,导致农业活动取代狩獵採集成为人类的主要食物来源,社会结构亦由游牧转向定居,各部落之间开始互相合作,发展出了人类的首个文明。自那时起,驯化的家养哺乳动物一直劳作在人类社会的交通与农耕上,它们向人类社会输出了食物(如肉类奶制品)、毛皮皮革。人们还会狩獵哺乳动物、将其用于体育竞技中,或是将其作为模式生物进行研究。自旧石器时代伊始,哺乳动物即广泛在各种艺术创作中亮相,在文学作品、电影、神话、宗教中频频现身。然而,人类对哺乳动物栖息地的破坏(以森林砍伐为主)和偷獵活动导致了哺乳动物数量锐减乃至灭绝

分类编辑

 
超过七成的哺乳动物物种[來源請求]属于啮齿目(蓝色)、翼手目(即蝙蝠,红色)和鼩形亚目(黄色)。

自从卡尔·林奈首次将哺乳类分为一纲以来,哺乳动物的定义曾多次变动。目前,人们对哺乳动物的分类方式还没有明确的共识;最新有关方面的研究由麦肯纳与贝尔(1997年)和威尔森与里德尔(2005年)作出。[4]喬治·蓋洛德·辛普森的“哺乳动物分类与分类原则”(AMNH Bulletin v. 85, 1945)从系統分類學上阐述了哺乳动物的起源、关系,他的理论直到20世纪末都还出现在各个教科书中。自辛普森发明了分类系统以来,化石记录已几经修正。分类系统理论本身也经多方讨论、发展壮大,在支序分類學出现后更是愈演愈烈。虽然实地考察中新发现的生物证据让辛普森的分类系统逐渐落伍,但他的分类方式依然是哺乳动物分类体系中最为权威的。[5]

大多数哺乳动物,包括物种最多的六个,属于胎盘动物。其中,物种最多的三个目是啮齿目豪豬河狸水豚等啃咬哺乳动物)、翼手目(蝙蝠)和鼩形目鼩鼱鼹鼠溝齒鼩等)。The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes, monkeys and lemur英语lemurs; the Cetartiodactyla including whales and even-toed ungulates; and the Carnivora which includes cats, dogs, weasels, bears, seals and allies.[6] According to Mammal Species of the World英语Mammal Species of the World, 5,416 species were identified in 2006. These were grouped into 1,229 genera, 153 families and 29 orders.[6] In 2008, the International Union for Conservation of Nature (IUCN) completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species.[7] According to research published in the Journal of Mammalogy英语Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 including 96 recently extinct.[8]

定义 编辑

The word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes (echidnas and platypuses) and therian mammals (marsupials and placentals) and all descendants of that ancestor.[9] Since this ancestor lived in the Jurassic period, Rowe's definition excludes all animals from the earlier Triassic, despite the fact that Triassic fossils in the Haramiyida have been referred to the Mammalia since the mid-19th century.[10] If Mammalia is considered as the crown group, its origin can be roughly dated as the first known appearance of animals more closely related to some extant mammals than to others. Ambondro英语Ambondro mahabo is more closely related to monotremes than to therian mammals while Amphilestes英语Amphilestes and Amphitherium英语Amphitherium are more closely related to the therians; as fossils of all three genera are dated about 167 million years ago in the Middle Jurassic英语Middle Jurassic, this is a reasonable estimate for the appearance of the crown group.[11]

T. S. Kemp has provided a more traditional definition: "synapsids that possess a dentarysquamosal英语squamosal bone jaw articulation and occlusion between upper and lower molars with a transverse component to the movement" or, equivalently in Kemp's view, the clade originating with the last common ancestor of Sinoconodon and living mammals.[12] The earliest known synapsid satisfying Kemp's definitions is Tikitherium英语Tikitherium, dated 225 Ma, so the appearance of mammals in this broader sense can be given this Late Triassic英语Late Triassic date.[13][14]

麦肯纳/贝尔分类编辑

In 1997, the mammals were comprehensively revised by Malcolm C. McKenna英语Malcolm McKenna and Susan K. Bell, which has resulted in the McKenna/Bell classification. Their 1997 book, Classification of Mammals above the Species Level,[15] is a comprehensive work on the systematics, relationships and occurrences of all mammal taxa, living and extinct, down through the rank of genus, though molecular genetic data challenge several of the higher level groupings. The authors worked together as paleontologists at the American Museum of Natural History, New York. McKenna inherited the project from Simpson and, with Bell, constructed a completely updated hierarchical system, covering living and extinct taxa that reflects the historical genealogy of Mammalia.[5]

在以下列表中,已滅絕的演化支由劍標(†)標註。

哺乳綱 Mammalia

对胎盘类的分子生物学分类编辑

As of the early 21st century, molecular studies based on DNA analysis have suggested new relationships among mammal families. Most of these findings have been independently validated by retrotransposon presence/absence data英语retrotransposon marker.[17] Classification systems based on molecular studies reveal three major groups or lineages of placental mammals—Afrotheria, Xenarthra and Boreoeutheria英语Boreoeutheria—which diverged in the Cretaceous. The relationships between these three lineages is contentious, and all three possible hypotheses have been proposed with respect to which group is basal. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria英语Epitheria (basal Xenarthra) and Exafroplacentalia英语Exafroplacentalia (basal Afrotheria).[18] Boreoeutheria in turn contains two major lineages—Euarchontoglires and Laurasiatheria.

Estimates for the divergence times between these three placental groups range from 105 to 120 million years ago, depending on the type of DNA used (such as nuclear英语nuclear DNA or mitochondrial)[19] and varying interpretations of paleogeographic data.[18]

哺乳綱 Mammalia

单孔目 Monotremata  

獸亞綱 Theria

有袋類 Marsupialia  

胎盤動物 Placentalia
大西洋獸類 Atlantogenata

非洲獸總目 Afrotheria    

异关节总目 Xenarthra    

北方真獸高目英语Boreoeutheria Boreoeutheria
靈長總目 Euarchontoglires

灵长动物 Euarchonta    

啮齿动物 Glires    

勞亞獸總目 Laurasiatheria

真盲缺目英语Eulipotyphla Eulipotyphla  

有陰囊大目英语Scrotifera Scrotifera

蝙蝠 Chiroptera  

有蹄類 Euungulata

偶蹄目 Cetartiodactyla    

奇蹄目 Perissodactyla    

猛獸類 Ferae

穿山甲 Pholidota  

食肉目 Carnivora    

分支图係根據於 Tarver等人在2016年的研究[20]

第1類群:非洲獸總目 Afrotheria[21]

第2類群:异关节总目 Xenarthra[21]

  • 披毛目 Pilosa:樹懶、食蟻獸(新熱帶界)
  • 有甲目 Cingulata:犰狳(美洲)

第3類群:北方真獸高目英语Boreoeutheria Boreoeutheria[21]

演化编辑

起源编辑

包含了現存的哺乳綱以及其他已滅絕近親的合弓綱最早起源於約323~300百萬年前的賓夕法尼亞世,與其他爬蟲類及鳥類的演化支分歧。哺乳綱的冠群演化自侏儸紀早期的哺乳形类物種。底下為以哺乳形類做為冠群的分支圖[22]

哺乳形类 Mammaliaformes

摩爾根獸科 Morganucodontidae  

柱齒獸類英语Docodonta Docodonta

霍爾丹齒獸屬英语Haldanodon Haldanodon

哺乳綱 Mammalia

原獸亞綱英语Australosphenida Australosphenida(包括单孔目 Monotremata)  

弗魯塔獸屬英语Fruitafossor Fruitafossor

祖賊獸屬英语Haramiyavia Haramiyavia

多瘤齒獸目 Eutriconodonta  

裂齒獸屬英语Tinodon Tinodon

真三尖齿兽目 Eutriconodonta(包括戈壁尖齿兽類英语Gobiconodonta Gobiconodonta) 

完獸超類英语Trechnotheria Trechnotheria(包括獸亞綱 Theria)  

从羊膜动物中分化编辑

 
The original synapsid skull structure contains one temporal opening behind the orbitals英语eye socket, in a fairly low position on the skull (lower right in this image). This opening might have assisted in containing the jaw muscles of these organisms which could have increased their biting strength.

最早的陸生脊椎动物羊膜動物,和其他四足類一樣,牠們也具有肺臟及四肢。然而羊膜動物的卵內層具有膜可將水分鎖於其中,因此得以產卵於乾燥的陸地上,而胚胎仍能正常發育;相較之下,兩棲類僅能將卵產於水中。

最早的羊膜動物出現於石炭纪的賓夕法尼亞世,由早期的爬行形类演化而來[23],而當時在陸地上已有許多昆蟲及其他無脊椎動物棲息,陸棲的植物則包括链束植物藓类植物门等。樹百萬年後,羊膜動物分歧為兩個演化支:合弓綱,包含了所有哺乳動物的共同祖先;以及蜥形纲,包含了蜥蜴恐龙鳥類[24]。合弓綱在顱骨的兩側後方僅具有單一開孔,稱為顳顬孔。合弓綱下的盤龍目包括了二叠纪早期最大型與最兇悍的物種[25]。非哺乳類的合弓綱物種過去曾被統稱為「似哺乳爬行動物[26][27]

合弓綱中的獸孔目,由盤龍目演化而來,出現於二疊紀中期(約265百萬年前),為當時最為興盛的陸生脊椎動物[26]。牠們在顱骨及下顎上有些許特徵與真盤龍亞目基群物種相異,包括:較大的顱骨以及相同大小的門齒(真盤龍亞目具有相異大小的門齒)[26]。最早的獸孔目物種外型與盤龍目十分類似,而晚期的獸孔目物種如犬齒獸亞目新頜獸小目英语probainognathia中,有些外形已經與哺乳動物無異。從獸孔目到哺乳動物的演化依據下列特徵分為多個階段[28]

  • 第二骨質硬颚的漸進式發展
  • 四肢逐漸由身體兩側移轉至下側,使動物於移動時能避免卡里爾約束英语Carrier's constraint進而提高運動的持久度。但這項特徵的演化過程十分緩慢且非常不穩定,比方來說,所有草食性的非哺乳類形獸孔目物種四肢仍為兩側延展(部分較晚期物種則為半直立)、二疊紀時期的肉食性獸孔目物種期前肢為兩側延展、較晚期的肉食性物種則具有半直立的後肢。而現存的單孔目則仍然保留半直立式的四肢。
  • 下颌骨逐漸成為下顎主要的骨骼組成,在三疊紀時,哺乳類的下顎僅包含有下颌骨,而其他原本組成下顎的骨頭則後移形成中耳。

哺乳动物祖先编辑

252百萬年前的二叠纪-三叠纪灭绝事件,肇因於多種滅絕事件的發生與連鎖效應,導致了肉食性獸孔目物種的滅絕[29]。三疊紀早期時,中型與大型的陸生肉食生態位逐漸被主龍類取代[30],這包括了鱷形超目[31]翼龍目以及恐龍[32]。少數大型犬齒獸亞目物種如犬齒獸屬英语Trucidocynodon橫齒獸科的物種仍分別保有大型肉食與草食性動物的生態位,但到了侏儸紀時,所有大型陸生草食動物的生態位均被恐龍所取代[33]

最早的哺乳動物(克坎普的定義下)出現於約莫225百萬年前的三疊紀晚期,在最早的獸孔目出現四千萬年之後。這些哺乳動物於侏儸紀中期時保有夜行性食虫动物的生態位[34],但也逐漸有多樣化的發展。例如,生存於侏儸紀時期的獺形狸尾獸,具有與現存水獺類似的外型,擅長游泳、挖掘及捕捉魚類[35]。然而,當時幾乎所有的哺乳動物均為夜行性,稱為夜行瓶頸英语nocturnal bottleneck,許多夜行性特有的特徵於現存的哺乳動物(縱使為晝行性)上仍然可觀察到[36]。生存於中生代的哺乳動物主要為多瘤齒獸目真三尖齒獸目鼴獸類英语spalacotheriid[37]。最早已知的後獸下綱中國袋獸,發現於中國東北辽宁省地層年代125百萬年前白堊紀早期的页岩中。該化石十分完整,甚至保留了毛簇以及軟組織的印痕[38]

 
目前已知最古老(距今160百萬年前)的真獸下綱物種中華侏羅獸的復原圖[39]

目前已知最早的真獸下綱為外型類似鼩鼱的中華侏羅獸,生存於160百萬年前的侏儸紀晚期[39]。更為晚期出現的始祖獸,出現於距今125百萬年前的白堊紀早期,具有許多有袋類共有而有胎盤類缺乏的特徵;這代表著有部分有袋類的特徵曾經存在於有袋類與有胎盤類的共同祖先上,而這些特徵在之後有胎盤類的演化支中逐漸退化或喪失[40]。例如,由骨盆向前延伸的上恥骨英语epipubic bone並不存在於現存的有胎盤類物種上,但可於有袋類、單孔目、非獸亞綱哺乳動物、多瘤齒獸目以及烏克哈獸英语Ukhaatherium(生存於白堊紀早期的真獸亞綱亞洲掘蝟目英语Asioryctitheria物種)上發現[41]。上恥骨主要用於在運動時能支持腹部,但這也限制了妊娠期胎兒發育的空間。狹窄的發育空間代表胎兒出生時的體型必須非常小,也因此不得有太長的妊娠期,為現存有袋類的共有特徵。而胎盤的演化可能發生於上恥骨的退化消失之後[42]

目前已知最早的單孔目物種為泰諾脊齒獸英语Teinolophos,生存於距今120百萬年前的澳洲[43]。單孔目物種仍然保留有許多早期羊膜動物的特徵,包括用於排泄、排遺、生產的單一開孔(泄殖腔[44],以及革質未鈣化的[45],這些特徵在現存的爬蟲類或鳥類身上可以發現。

哺乳動物特徵的出現编辑

生存於距今195百萬年前侏儸紀早期的吴氏巨颅兽,為最早發現下顎僅由鱗骨與齒骨組成的物種,早期合弓綱常見的關節骨已經退化[46]

最早明確的毛髮化石證據來自於距今164百萬年前侏儸紀中期的獺形狸尾獸哺乳形巨齒獸英语Megaconus。1950年代,位於犬齒獸亞目物種上颌骨前上頜骨上的小孔被認為是其感覺毛血管及神經的通道,是為間接毛髮存在的證據[47][48];然而,這些小孔其實並無法證明感覺毛的存在,例如現存的雙領蜥其上颌骨上也具有和三尖叉齒獸類似的小孔,但是並不具有感覺毛[27][49]。不過現今大多的復原圖仍然會將三尖叉齒獸加上感覺毛[50]。針對二疊紀粪化石研究顯示,當時的非哺乳類合弓綱已經具有毛髮,這代表毛髮的存在可以向前推至二齒獸下目[51]

目前仍無法得知哺乳動物是在何時演化為內溫動物,但多半認為獸孔目是哺乳動物演化支上最早出現的內溫動物[51][52]。現存的單孔目物種比起有袋類與有胎盤類具有較低的體溫與變化率較大的代謝速率[53],然而有證據顯示有些較古老的單孔目物種,包含獸亞綱的祖先,可能具有與現存獸亞綱相近的體溫[54]。此外,獸亞綱中非洲獸總目與異關節總目部分現存的物種具有為了適應環境重新演化出的低體溫[55]

哺乳動物直立式四肢的演化並不完全,現存與已滅絕的單孔目四肢均為兩側延展。向下延展的四肢特徵最早約出現於侏儸紀晚期至侏儸紀早期,生存於距今125百萬年前的始祖獸與中國袋獸均具有向下延展的四肢[56]上恥骨英语epipubic bone最早發現於三瘤齒獸科,為牠們與哺乳形類共有衍徵,於現存的非有胎盤類哺乳動物身上仍可發現。然而於大帶齒獸英语Megazostrodon紅層獸英语Erythrotherium中上恥骨已經退化消失[57]

一般認為乳汁的產生最早是為了讓產下的卵保持潮濕,之後才發展出哺乳的行為。這樣的論據主要來自於對現存單孔目(卵生哺乳動物)的觀察[58][59]

哺乳动物的崛起?编辑

獸亞綱哺乳動物於距今6千6百萬年前白垩纪﹣古近纪灭绝事件發生後的新生代開始填補原先由非鳥恐龍、爬蟲類及其他哺乳動物所遺留下來的中大型生物生態位空缺[60],並且體型上有十分顯著的成長(巨型動物群[61]。在滅絕事件發生後,尚未滅絕的鳥類與哺乳類展現了十分快速的輻射演化[60]。例如,目前已知最古老的蝙蝠,爪蝠伊神蝠,發現於約5千萬年前,在滅絕事件發生後僅經過了1千6百萬年,然而外型已接近現存的蝙蝠並具備了完整的飛行能力[62]

分子類緣關係之研究原先認為大多胎盤類下的目約於100至85百萬年前分家,而現存大部分的科則於始新世中新世期間出現[63]。然而,目前並沒有發現生存於白堊紀的胎盤類哺乳動物化石[64],最早的胎盤類哺乳動物化石發現於古新世早期,於非鳥恐龍滅絕之後[64],被稱為Protungulatum donnae英语Protungulatum donnae[65],然而近年來的研究將牠們重新分類為非胎盤類的真獸下綱物種[66]。在基因與型態變化率的重新校準之後,普遍認為胎盤類哺乳動物起源於马斯特里赫特期,而現存大多的演化支則起源於古新世[67]

目前已知最古老的靈長目物種為生存於距今5千5百萬年前[68]阿喀琉斯基猴[68],體重僅有20至30克(0.71至1.06盎司),與人類的手掌大小相當[68]

解剖学结构编辑

特征编辑

现生的哺乳动物均生有汗腺,其中有的汗腺分化为可以产奶的器官以哺育幼崽。[69]然而,在对化石进行分类的时候,科学家需要参考哺乳动物的其他特征,因为腺体等软组织无法通过化石的形式保存下来。[70]

早期哺乳动物具有许多共同特征:

For the most part, these characteristics were not present in the Triassic ancestors of the mammals.[76] Nearly all mammaliaforms possess an epipubic bone, the exception being modern placentals.[77]

生物系统编辑

 
Raccoon lungs being inflated manually

The majority of mammals have seven cervical vertebrae (bones in the neck), including bats, giraffes, whales and humans. The exceptions are the manatee and the two-toed sloth, which have just six, and the three-toed sloth which has nine cervical vertebrae.[78] All mammalian brains possess a neocortex, a brain region unique to mammals.[79] Placental mammals have a corpus callosum, unlike monotremes and marsupials.[80]

The lungs of mammals are spongy and honeycombed. Breathing is mainly achieved with the diaphragm, which divides the thorax from the abdominal cavity, forming a dome convex to the thorax. Contraction of the diaphragm flattens the dome, increasing the volume of the lung cavity. Air enters through the oral and nasal cavities, and travels through the larynx, trachea and bronchi, and expands the alveoli. Relaxing the diaphragm has the opposite effect, decreasing the volume of the lung cavity, causing air to be pushed out of the lungs. During exercise, the abdominal wall contracts, increasing pressure on the diaphragm, which forces air out quicker and more forcefully. The rib cage英语rib cage is able to expand and contract the chest cavity through the action of other respiratory muscles. Consequently, air is sucked into or expelled out of the lungs, always moving down its pressure gradient.[81][82] This type of lung is known as a bellows lung due to its resemblance to blacksmith bellows.[82]

The mammalian heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers.[83] The heart has four valves, which separate its chambers and ensures blood flows in the correct direction through the heart (preventing backflow). After gas exchange in the pulmonary capillaries (blood vessels in the lungs), oxygen-rich blood returns to the left atrium via one of the four pulmonary veins. Blood flows nearly continuously back into the atrium, which acts as the receiving chamber, and from here through an opening into the left ventricle. Most blood flows passively into the heart while both the atria and ventricles are relaxed, but toward the end of the ventricular relaxation period英语diastole, the left atrium will contract, pumping blood into the ventricle. The heart also requires nutrients and oxygen found in blood like other muscles, and is supplied via coronary arteries.[84]

Didactic models英语Educational toy of a mammalian heart
 
Mammal skin: 1 — hair, 2 — epidermis, 3 — sebaceous gland, 4 — Arrector pili muscle, 5 — dermis, 6 — hair follicle, 7 — sweat gland, 8 (not labeled, the bottom layer)  — hypodermis, showing round adipocytes

The integumentary system (skin) is made up of three layers: the outermost epidermis, the dermis and the hypodermis. The epidermis is typically 10 to 30 cells thick; its main function is to provide a waterproof layer. Its outermost cells are constantly lost; its bottommost cells are constantly dividing and pushing upward. The middle layer, the dermis, is 15 to 40 times thicker than the epidermis. The dermis is made up of many components, such as bony structures and blood vessels. The hypodermis is made up of adipose tissue, which stores lipids and provides cushioning and insulation. The thickness of this layer varies widely from species to species;[85]:97 marine mammals require a thick hypodermis (blubber) for insulation, and right whales have the thickest blubber at 20英寸(51厘米).[86] Although other animals have features such as whiskers, feathers, setae英语setae, or cilia英语cilia (entomology) that superficially resemble it, no animals other than mammals have hair. It is a definitive characteristic of the class. Though some mammals have very little, careful examination reveals the characteristic, often in obscure parts of their bodies.[85]:61

The carnassials (teeth in the very back of the mouth) of the insectivorous aardwolf (left) vs. that of a gray wolf (right) which consumes large vertebrates

Herbivores have developed a diverse range of physical structures to facilitate the consumption of plant material英语Herbivore adaptations to plant defense. To break up intact plant tissues, mammals have developed teeth structures that reflect their feeding preferences. For instance, frugivore英语frugivores (animals that feed primarily on fruit) and herbivores that feed on soft foliage have low-crowned teeth specialized for grinding foliage and seeds. Grazing animals that tend to eat hard, silica-rich grasses, have high-crowned teeth, which are capable of grinding tough plant tissues and do not wear down as quickly as low-crowned teeth.[87] Most carnivorous mammals have carnassialiforme teeth (of varying length depending on diet), long canines and similar tooth replacement patterns.[88]

The stomach of Artiodactyls is divided into four sections: the rumen, the reticulum, the omasum and the abomasum (only ruminants have a rumen). After the plant material is consumed, it is mixed with saliva in the rumen and reticulum and separates into solid and liquid material. The solids lump together to form a bolus英语bolus (digestion) (or cud英语cud), and is regurgitated. When the bolus enters the mouth, the fluid is squeezed out with the tongue and swallowed again. Ingested food passes to the rumen and reticulum where cellulytic microbes (bacteria, protozoa and fungi) produce cellulase, which is needed to break down the cellulose in plants.[89] Perissodactyls, in contrast to the ruminants, store digested food that has left the stomach in an enlarged cecum, where it is fermented by bacteria.[90] Carnivora have a simple stomach adapted to digest primarily meat, as compared to the elaborate digestive systems of herbivorous animals, which are necessary to break down tough, complex plant fibers. The caecum is either absent or short and simple, and the large intestine is not sacculated英语sacculation or much wider than the small intestine.[91]

 
Bovine kidney

The mammalian excretory system英语excretory system involves many components. Like most other land animals, mammals are ureotelic英语ureotelic, and convert ammonia into urea, which is done by the liver as part of the urea cycle.[92] Bilirubin, a waste product derived from blood cells, is passed through bile and urine with the help of enzymes excreted by the liver.[93] The passing of bilirubin via bile through the intestinal tract gives mammalian feces a distinctive brown coloration.[94] Distinctive features of the mammalian kidney include the presence of the renal pelvis and renal pyramid英语renal pyramids, and of a clearly distinguishable cortex and medulla英语renal medulla, which is due to the presence of elongated loops of Henle英语Loop of Henle. Only the mammalian kidney has a bean shape, although there are some exceptions, such as the multilobed reniculate kidney英语reniculate kidneys of pinnipeds, cetaceans and bears.[95][96] Most adult placental mammals have no remaining trace of the cloaca. In the embryo, the embryonic cloaca英语embryonic cloaca divides into a posterior region that becomes part of the anus, and an anterior region that has different fates depending on the sex of the individual: in females, it develops into the vestibule that receives the urethra and vagina, while in males it forms the entirety of the penile urethra英语penile urethra.[96] However, the tenrecs, golden moles, and some shrews retain a cloaca as adults.[97] In marsupials, the genital tract is separate from the anus, but a trace of the original cloaca does remain externally.[96] Monotremes, which translates from Greek into "single hole", have a true cloaca.[98]

发声结构编辑

 
A diagram of ultrasonic signals emitted by a bat, and the echo from a nearby object

As in all other tetrapods, mammals have a larynx that can quickly open and close to produce sounds, and a supralaryngeal vocal tract which filters this sound. The lungs and surrounding musculature provide the air stream and pressure required to phonate. The larynx controls the pitch and volume of sound, but the strength the lungs exert to exhale also contributes to volume. More primitive mammals, such as the echidna, can only hiss, as sound is achieved solely through exhaling through a partially closed larynx. Other mammals phonate using vocal folds, as opposed to the vocal cords seen in birds and reptiles. The movement or tenseness of the vocal folds can result in many sounds such as purr英语purring and screaming英语screaming. Mammals can change the position of the larynx, allowing them to breathe through the nose while swallowing through the mouth, and to form both oral and nasal sounds; nasal sounds, such as a dog whine, are generally soft sounds, and oral sounds, such as a dog bark, are generally loud.[99]

Beluga whale echolocation sounds

Some mammals have a large larynx and thus a low-pitched voice, namely the hammer-headed bat (Hypsignathus monstrosus) where the larynx can take up the entirety of the thoracic cavity while pushing the lungs, heart, and trachea into the abdomen.[100] Large vocal pads can also lower the pitch, as in the low-pitched roars of big cats.[101] The production of infrasound is possible in some mammals such as the African elephant (Loxodonta spp.) and baleen whales.[102][103] Small mammals with small larynxes have the ability to produce ultrasound, which can be detected by modifications to the middle ear and cochlea. Ultrasound is inaudible to birds and reptiles, which might have been important during the Mesozoic, when birds and reptiles were the dominant predators. This private channel is used by some rodents in, for example, mother-to-pup communication, and by bats when echolocating. Toothed whales also use echolocation, but, as opposed to the vocal membrane that extends upward from the vocal folds, they have a melon to manipulate sounds. Some mammals, namely the primates, have air sacs attached to the larynx, which may function to lower the resonances or increase the volume of sound.[99]

The vocal production system is controlled by the cranial nerve nuclei英语cranial nerve nucleus in the brain, and supplied by the recurrent laryngeal nerve and the superior laryngeal nerve英语superior laryngeal nerve, branches of the vagus nerve. The vocal tract is supplied by the hypoglossal nerve and facial nerves. Electrical stimulation of the periaqueductal gray (PEG) region of the mammalian midbrain elicit vocalizations. The ability to learn new vocalizations is only exemplified in humans, seals, cetaceans, elephants and possibly bats; in humans, this is the result of a direct connection between the motor cortex英语motor cortex, which controls movement, and the motor neuron英语motor neurons in the spinal cord.[99]

皮毛编辑

 
豪豬利用身上的棘刺英语spine (zoology)進行防衛

哺乳動物皮毛的主要功能為體溫調節,其他功能則包括防衛、感測環境、防水與偽裝[104],不同形式的皮毛具有不同的稱呼及功能[85]:99

  • 定期毛髮(Definitive)– 當達到一定長度後即會脫落
  • 感覺毛(Vibrissae)– 用於感測周遭環境。
  • 毛皮(Pelage)– 包括護毛、絨毛與芒毛英语awn hair
  • 棘刺英语spine (zoology)(Spine)– 堅硬的毛髮,用於防衛(例如豪豬背上的長刺)。
  • 剛毛(Bristle)– 主要用於展示視覺訊號(例如獅子的鬃毛)。
  • 柔毛英语Vellus hair(Velli)– 用於保暖,主要常見於新生的哺乳動物。
  • 羊毛(Wool])– 長而柔軟的卷曲毛髮。

体温調節编辑

毛髮長度與體溫調節無關:例如,部分棲息於熱帶地區的哺乳動物,如樹懶,具有與極區哺乳動物等長度的毛髮,然而並不具有相當保暖的能力;反而,部分僅具有短毛的熱帶哺乳動物,保暖能力卻與極區哺乳動物相當。毛髮生長的密度可以影響保暖的能力,尤其棲息於極區的哺乳動物都舉有相當高密度的毛髮,例如麝牛除了具有30 cm(12英寸)長的護毛外,還具有高密度的絨毛,能有效鎖住並減少體熱的散失,讓牠們能於−40 °C(−40 °F)的環境下生存[85]:162–163。有些棲息於沙漠的哺乳動物,如駱駝,則透過高密度的毛來將外在環境的熱度隔絕於外,確保個體不會過熱;在夏天時,駱駝的毛髮溫度可達70 °C(158 °F),而皮膚溫度則能維持40 °C(104 °F)[85]:188水生哺乳動物英语Aquatic mammal也具有高密度的毛髮,透過將水隔絕於外保持皮膚乾燥的方式來減少體熱散失[85]:162–163

 
A leopard's disruptively colored英语disruptive coloration coat provides camouflage for this ambush predator英语ambush predator.

体色编辑

Mammalian coats are colored for a variety of reasons, the major selective pressures including camouflage, sexual selection, communication, and thermoregulation. Coloration in both the hair and skin of mammals is mainly determined by the type and amount of melanin; eumelanins for brown and black colors and pheomelanin for a range of yellow to reddish-brown colors, giving mammals an earth tone英语earth tone.[105][106] Some mammals, like the mandrill, have more vibrant colors due to structural coloration.[107] Many sloths appear green because their fur hosts green algae; this may be a symbiotic relation that affords camouflage to the sloths.[108]

Camouflage is a powerful influence in a large number of mammals, as it helps to conceal individuals from predators or prey.[109] In arctic and subarctic mammals such as the arctic fox (Alopex lagopus), collared lemming (Dicrostonyx groenlandicus), stoat (Mustela erminea), and snowshoe hare (Lepus americanus), seasonal color change英语seasonal polyphenism between brown in summer and white in winter is driven largely by camouflage.[110] Some arboreal mammals, notably primates and marsupials, have shades of violet, green, or blue skin on parts of their bodies, indicating some distinct advantage in their largely arboreal英语arboreal habitat due to convergent evolution.[107]

Aposematism, warning off possible predators, is the most likely explanation of the black-and-white pelage of many mammals which are able to defend themselves, such as in the foul-smelling skunk英语skunk and the powerful and aggressive honey badger.[111] Coat color is sometimes sexually dimorphic, as in many primate species英语Sexual dimorphism in non-human primates.[112] Differences in female and male coat color may indicate nutrition and hormone levels, important in mate selection.[113] Coat color may influence the ability to retain heat, depending on how much light is reflected. Mammals with a darker colored coat can absorb more heat from solar radiation, and stay warmer, and some smaller mammals, such as vole英语voles, have darker fur in the winter. The white, pigmentless fur of arctic mammals, such as the polar bear, may reflect more solar radiation directly onto the skin.[85]:166–167[104] The dazzling black-and-white striping of zebras appear to provide some protection from biting flies.[114]

生殖系统编辑

 
Goat kids stay with their mother until they are weaned.

In male placentals, the penis is used both for urination and copulation. Depending on the species, an erection may be fueled by blood flow into vascular, spongy tissue or by muscular action. A penis may be contained in a prepuce英语Penile sheath when not erect, and some placentals also have a penis bone (baculum).[115] Marsupials typically have forked penises,[116] while the echidna penis generally has four heads with only two functioning.[117] The testes of most mammals descend into the scrotum which is typically posterior to the penis but is often anterior in marsupials. Female mammals generally have a clitoris, labia majora and labia minora on the outside, while the internal system contains paired oviduct英语oviducts, 1-2 uteri, 1-2 cervices and a vagina. Marsupials have two lateral vaginas and a medial vagina. The "vagina" of monotremes is better understood as a "urogenital sinus". The uterine systems of placental mammals can vary between a duplex, were there are two uteri and cervices which open into the vagina, a bipartite, were two uterine horn英语uterine horns have a single cervix that connects to the vagina, a bicornuate, which consists where two uterine horns that are connected distally but separate medially creating a Y-shape, and a simplex, which has a single uterus.[118][119][85]:220–221, 247

 
Matschie's tree-kangaroo英语Matschie's tree-kangaroo with young in pouch

The ancestral condition for mammal reproduction is the birthing of relatively undeveloped, either through direct vivipary or a short period as soft-shelled eggs. This is likely due to the fact that the torso could not expand due to the presence of epipubic bones英语epipubic bones. The oldest demonstration of this reproductive style is with Kayentatherium, which produced undeveloped perinate英语perinates, but at much higher litter sizes than any modern mammal, 38 specimens.[120] Most modern mammals are viviparous, giving birth to live young. However, the five species of monotreme, the platypus and the four species of echidna, lay eggs. The monotremes have a sex determination system different from that of most other mammals.[121] In particular, the sex chromosome英语sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.[122]

Viviparous mammals are in the subclass Theria; those living today are in the marsupial and placental infraclasses. Marsupials have a short gestation英语gestation period, typically shorter than its estrous cycle and gives birth to an undeveloped newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. This is the plesiomorphic英语Symplesiomorphy condition among viviparous mammals; the presence of epipubic bones in all non-placental mammals prevents the expansion of the torso needed for full pregnancy.[77] Even non-placental eutherians probably reproduced this way.[123] The placentals give birth to relatively complete and developed young, usually after long gestation periods.[124] They get their name from the placenta, which connects the developing fetus to the uterine wall to allow nutrient uptake.[125] In placental mammals, the epipubic is either completely lost or converted into the baculum; allowing the torso to be able to expand and thus birth developed offspring.[120]

The mammary glands of mammals are specialized to produce milk, the primary source of nutrition for newborns. The monotremes branched early from other mammals and do not have the nipples seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.[126] Compared to placental mammals, the milk of marsupials changes greatly in both production rate and in nutrient composition, due to the underdeveloped young. In addition, the mammary glands have more autonomy allowing them to supply separate milks to young at different development stages.[127] Lactose is the main sugar in placental mammal milk while monotreme and marsupial milk is dominated by oligosaccharides.[128] Weaning is the process in which a mammal becomes less dependent on their mother's milk and more on solid food.[129]

温血状态编辑

Nearly all mammals are endothermic英语endothermy ("warm-blooded"). Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in weather and climates too cold for ectotherm英语ectothermic ("cold-blooded") reptiles and insects. Endothermy requires plenty of food energy, so mammals eat more food per unit of body weight than most reptiles.[130] Small insectivorous mammals eat prodigious amounts for their size. A rare exception, the naked mole-rat produces little metabolic heat, so it is considered an operational poikilotherm.[131] Birds are also endothermic, so endothermy is not unique to mammals.[132]

生命周期编辑

Among mammals, species maximum lifespan varies significantly (for example the shrew has a lifespan of two years, whereas the oldest bowhead whale is recorded to be 211 years).[133] Although the underlying basis for these lifespan differences is still uncertain, numerous studies indicate that the ability to repair DNA damage is an important determinant of mammalian lifespan. In a 1974 study by Hart and Setlow,[134] it was found that DNA excision repair capability increased systematically with species lifespan among seven mammalian species. Species lifespan was observed to be robustly correlated with the capacity to recognize DNA double-strand breaks as well as the level of the DNA repair protein Ku80英语Ku80.[133] In a study of the cells from sixteen mammalian species, genes employed in DNA repair were found to be up-regulated英语Downregulation and upregulation in the longer-lived species.[135] The cellular level of the DNA repair enzyme poly ADP ribose polymerase英语poly ADP ribose polymerase was found to correlate with species lifespan in a study of 13 mammalian species.[136] Three additional studies of a variety of mammalian species also reported a correlation between species lifespan and DNA repair capability.[137][138][139]

运动编辑

陆生动物编辑

 
Running gait. Photographs by Eadweard Muybridge, 1887.

Most vertebrates—the amphibians, the reptiles and some mammals such as humans and bears—are plantigrade, walking on the whole of the underside of the foot. Many mammals, such as cats and dogs, are digitigrade英语digitigrade, walking on their toes, the greater stride length allowing more speed. Digitigrade mammals are also often adept at quiet movement.[140] Some animals such as horses are unguligrade, walking on the tips of their toes. This even further increases their stride length and thus their speed.[141] A few mammals, namely the great apes, are also known to walk on their knuckles英语Knuckle-walking, at least for their front legs. Giant anteaters[142] and platypuses[143] are also knuckle-walkers. Some mammals are bipeds英语bipedalism, using only two limbs for locomotion, which can be seen in, for example, humans and the great apes. Bipedal species have a larger field of vision than quadrupeds, conserve more energy and have the ability to manipulate objects with their hands, which aids in foraging. Instead of walking, some bipeds hop, such as kangaroos and kangaroo rats.[144][145]

Animals will use different gaits for different speeds, terrain and situations. For example, horses show four natural gaits, the slowest horse gait英语horse gait is the walk英语Horse gait, then there are three faster gaits which, from slowest to fastest, are the trot英语Trot (horse gait), the canter英语canter and the gallop英语Horse gait. Animals may also have unusual gaits that are used occasionally, such as for moving sideways or backwards. For example, the main human gaits英语gait (human) are bipedal walking and running, but they employ many other gaits occasionally, including a four-legged crawl英语crawling (human) in tight spaces.[146] Mammals show a vast range of gait英语gaits, the order that they place and lift their appendages in locomotion. Gaits can be grouped into categories according to their patterns of support sequence. For quadrupeds, there are three main categories: walking gaits, running gaits and leaping gaits英语leaping gaits.[147] Walking is the most common gait, where some feet are on the ground at any given time, and found in almost all legged animals. Running is considered to occur when at some points in the stride all feet are off the ground in a moment of suspension.[146]

林栖动物编辑

 
Gibbons are very good brachiators英语Brachiation because their elongated limbs enable them to easily swing and grasp on to branches.

Arboreal animals frequently have elongated limbs that help them cross gaps, reach fruit or other resources, test the firmness of support ahead and, in some cases, to brachiate英语brachiation (swing between trees).[148] Many arboreal species, such as tree porcupines, silky anteaters, spider monkeys, and possums, use prehensile tail英语prehensile tails to grasp branches. In the spider monkey, the tip of the tail has either a bare patch or adhesive pad, which provides increased friction. Claws can be used to interact with rough substrates and reorient the direction of forces the animal applies. This is what allows squirrels to climb tree trunks that are so large to be essentially flat from the perspective of such a small animal. However, claws can interfere with an animal's ability to grasp very small branches, as they may wrap too far around and prick the animal's own paw. Frictional gripping is used by primates, relying upon hairless fingertips. Squeezing the branch between the fingertips generates frictional force that holds the animal's hand to the branch. However, this type of grip depends upon the angle of the frictional force, thus upon the diameter of the branch, with larger branches resulting in reduced gripping ability. To control descent, especially down large diameter branches, some arboreal animals such as squirrels have evolved highly mobile ankle joints that permit rotating the foot into a 'reversed' posture. This allows the claws to hook into the rough surface of the bark, opposing the force of gravity. Small size provides many advantages to arboreal species: such as increasing the relative size of branches to the animal, lower center of mass, increased stability, lower mass (allowing movement on smaller branches) and the ability to move through more cluttered habitat.[148] Size relating to weight affects gliding animals such as the sugar glider.[149] Some species of primate, bat and all species of sloth achieve passive stability by hanging beneath the branch. Both pitching and tipping become irrelevant, as the only method of failure would be losing their grip.[148]

飛行动物?编辑

Slow-motion and normal speed of Egyptian fruit bat英语Egyptian fruit bats flying

Bats are the only mammals that can truly fly. They fly through the air at a constant speed by moving their wings up and down (usually with some fore-aft movement as well). Because the animal is in motion, there is some airflow relative to its body which, combined with the velocity of the wings, generates a faster airflow moving over the wing. This generates a lift force vector pointing forwards and upwards, and a drag force vector pointing rearwards and upwards. The upwards components of these counteract gravity, keeping the body in the air, while the forward component provides thrust to counteract both the drag from the wing and from the body as a whole.[150]

The wings of bats are much thinner and consist of more bones than those of birds, allowing bats to maneuver more accurately and fly with more lift and less drag.[151][152] By folding the wings inwards towards their body on the upstroke, they use 35% less energy during flight than birds.[153] The membranes are delicate, ripping easily; however, the tissue of the bat's membrane is able to regrow, such that small tears can heal quickly.[154] The surface of their wings is equipped with touch-sensitive receptors on small bumps called Merkel cell英语Merkel cells, also found on human fingertips. These sensitive areas are different in bats, as each bump has a tiny hair in the center, making it even more sensitive and allowing the bat to detect and collect information about the air flowing over its wings, and to fly more efficiently by changing the shape of its wings in response.[155]

穴居动物与土棲动物编辑

Semi-fossorial wombat (left) vs. fully fossorial eastern mole (right)

A fossorial (from Latin fossor, meaning "digger") is an animal adapted to digging which lives primarily, but not solely, underground. Some examples are badger英语badgers, and naked mole-rats. Many rodent species are also considered fossorial because they live in burrows for most but not all of the day. Species that live exclusively underground are subterranean, and those with limited adaptations to a fossorial lifestyle sub-fossorial. Some organisms are fossorial to aid in temperature regulation while others use the underground habitat for protection from predators or for food storage英语food storage.[156]

Fossorial mammals have a fusiform body, thickest at the shoulders and tapering off at the tail and nose. Unable to see in the dark burrows, most have degenerated eyes, but degeneration varies between species; pocket gophers, for example, are only semi-fossorial and have very small yet functional eyes, in the fully fossorial marsupial mole英语marsupial mole the eyes are degenerated and useless, talpa moles have vestigial eyes and the cape golden mole英语cape golden mole has a layer of skin covering the eyes. External ears flaps are also very small or absent. Truly fossorial mammals have short, stout legs as strength is more important than speed to a burrowing mammal, but semi-fossorial mammals have cursorial英语cursorial legs. The front paws are broad and have strong claws to help in loosening dirt while excavating burrows, and the back paws have webbing, as well as claws, which aids in throwing loosened dirt backwards. Most have large incisors to prevent dirt from flying into their mouth.[157]

Many fossorial mammals such as shrews, hedgehogs, and moles were classified under the now obsolete order Insectivora.[158]

水生动物编辑

Fully aquatic mammals, the cetaceans and sirenians, have lost their legs and have a tail fin to propel themselves through the water. Flipper movement is continuous. Whales swim by moving their tail fin and lower body up and down, propelling themselves through vertical movement, while their flippers are mainly used for steering. Their skeletal anatomy allows them to be fast swimmers. Most species have a dorsal fin英语dorsal fin to prevent themselves from turning upside-down in the water.[159][160] The flukes of sirenians are raised up and down in long strokes to move the animal forward, and can be twisted to turn. The forelimbs are paddle-like flippers which aid in turning and slowing.[161]

Semi-aquatic英语List of semiaquatic tetrapods mammals, like pinnipeds, have two pairs of flippers on the front and back, the fore-flippers and hind-flippers. The elbows and ankles are enclosed within the body.[162][163] Pinnipeds have several adaptions for reducing drag. In addition to their streamlined bodies, they have smooth networks of muscle bundles英语Muscle fascicle in their skin that may increase laminar flow and make it easier for them to slip through water. They also lack arrector pili, so their fur can be streamlined as they swim.[164] They rely on their fore-flippers for locomotion in a wing-like manner similar to penguins and sea turtles.[165] Fore-flipper movement is not continuous, and the animal glides between each stroke.[163] Compared to terrestrial carnivorans, the fore-limbs are reduced in length, which gives the locomotor muscles at the shoulder and elbow joints greater mechanical advantage;[162] the hind-flippers serve as stabilizers.[164] Other semi-aquatic mammals include beavers, hippopotamuses, otters and platypuses.[166] Hippos are very large semi-aquatic mammals, and their barrel-shaped bodies have graviportal skeletal structures,[167] adapted to carrying their enormous weight, and their specific gravity英语specific gravity allows them to sink and move along the bottom of a river.[168]


行为编辑

沟通与发声编辑

 
Vervet monkey英语Vervet monkeys use at least four distinct alarm calls英语alarm signal for different predators.[169]

Many mammals communicate by vocalizing. Vocal communication serves many purposes, including in mating rituals, as warning calls英语alarm signal,[170] to indicate food sources, and for social purposes. Males often call during mating rituals to ward off other males and to attract females, as in the roaring英语roar (vocalization) of lions and red deer.[171] The songs of the humpback whale may be signals to females;[172] they have different dialects in different regions of the ocean.[173] Social vocalizations include the territorial calls of gibbons, and the use of frequency in greater spear-nosed bat英语greater spear-nosed bats to distinguish between groups.[174] The vervet monkey英语vervet monkey gives a distinct alarm call for each of at least four different predators, and the reactions of other monkeys vary according to the call. For example, if an alarm call signals a python, the monkeys climb into the trees, whereas the eagle alarm causes monkeys to seek a hiding place on the ground.[169] Prairie dogs similarly have complex calls that signal the type, size, and speed of an approaching predator.[175] Elephants communicate socially with a variety of sounds including snorting, screaming, trumpeting, roaring and rumbling. Some of the rumbling calls are infrasonic, below the hearing range of humans, and can be heard by other elephants up to 6英里(9.7公里) away at still times near sunrise and sunset.[176]

Orca calling including occasional echolocation clicks

Mammals signal by a variety of means. Many give visual anti-predator signals, as when deer and gazelle stot, honestly indicating their fit condition and their ability to escape,[177][178] or when white-tailed deer and other prey mammals flag with conspicuous tail markings when alarmed, informing the predator that it has been detected.[179] Many mammals make use of scent-marking, sometimes possibly to help defend territory, but probably with a range of functions both within and between species.[180][181][182] Microbats and toothed whales including oceanic dolphins vocalize both socially and in echolocation英语Animal echolocation.[183][184][185]

哺育编辑

A short-beaked echidna foraging for insects.

To maintain a high constant body temperature is energy expensive—mammals therefore need a nutritious and plentiful diet. While the earliest mammals were probably predators, different species have since adapted to meet their dietary requirements in a variety of ways. Some eat other animals—this is a carnivorous diet (and includes insectivorous diets). Other mammals, called herbivores, eat plants, which contain complex carbohydrates such as cellulose. An herbivorous diet includes subtypes such as granivory英语granivory (seed eating), folivory (leaf eating), frugivory英语frugivory (fruit eating), nectarivory英语nectarivory (nectar eating), gummivory英语gummivory (gum eating) and mycophagy英语mycophagy (fungus eating). The digestive tract of an herbivore is host to bacteria that ferment these complex substances, and make them available for digestion, which are either housed in the multichambered stomach or in a large cecum.[89] Some mammals are coprophagous, consuming feces to absorb the nutrients not digested when the food was first ingested.[85]:131–137 An omnivore eats both prey and plants. Carnivorous mammals have a simple digestive tract because the proteins, lipids and minerals found in meat require little in the way of specialized digestion. Exceptions to this include baleen whales who also house gut flora in a multi-chambered stomach, like terrestrial herbivores.[186]

The size of an animal is also a factor in determining diet type (Allen's rule). Since small mammals have a high ratio of heat-losing surface area to heat-generating volume, they tend to have high energy requirements and a high metabolic rate. Mammals that weigh less than about 18盎司(510克;1.1英磅) are mostly insectivorous because they cannot tolerate the slow, complex digestive process of an herbivore. Larger animals, on the other hand, generate more heat and less of this heat is lost. They can therefore tolerate either a slower collection process (carnivores that feed on larger vertebrates) or a slower digestive process (herbivores).[187] Furthermore, mammals that weigh more than 18盎司(510克;1.1英磅) usually cannot collect enough insects during their waking hours to sustain themselves. The only large insectivorous mammals are those that feed on huge colonies of insects (ants or termites).[188]

The hypocarnivorous英语hypocarnivore American black bear (Ursus americanus) vs. the hypercarnivorous polar bear (Ursus maritimus)[189]

Some mammals are omnivores and display varying degrees of carnivory and herbivory, generally leaning in favor of one more than the other. Since plants and meat are digested differently, there is a preference for one over the other, as in bears where some species may be mostly carnivorous and others mostly herbivorous.[190] They are grouped into three categories: mesocarnivory英语mesocarnivore (50–70% meat), hypercarnivory (70% and greater of meat), and hypocarnivory英语hypocarnivore (50% or less of meat). The dentition of hypocarnivores consists of dull, triangular carnassial teeth meant for grinding food. Hypercarnivores, however, have conical teeth and sharp carnassials meant for slashing, and in some cases strong jaws for bone-crushing, as in the case of hyenas, allowing them to consume bones; some extinct groups, notably the Machairodontinae, had saber-shaped canines英语maxillary canine.[189]

Some physiological carnivores consume plant matter and some physiological herbivores consume meat. From a behavioral aspect, this would make them omnivores, but from the physiological standpoint, this may be due to zoopharmacognosy英语zoopharmacognosy. Physiologically, animals must be able to obtain both energy and nutrients from plant and animal materials to be considered omnivorous. Thus, such animals are still able to be classified as carnivores and herbivores when they are just obtaining nutrients from materials originating from sources that do not seemingly complement their classification.[191] For example, it is well documented that some ungulates such as giraffes, camels, and cattle, will gnaw on bones to consume particular minerals and nutrients.[192] Also, cats, which are generally regarded as obligate carnivores, occasionally eat grass to regurgitate indigestible material (such as hairballs), aid with hemoglobin production, and as a laxative.[193]

Many mammals, in the absence of sufficient food requirements in an environment, suppress their metabolism and conserve energy in a process known as hibernation.[194] In the period preceding hibernation, larger mammals, such as bears, become polyphagic to increase fat stores, whereas smaller mammals prefer to collect and stash food.[195] The slowing of the metabolism is accompanied by a decreased heart and respiratory rate, as well as a drop in internal temperatures, which can be around ambient temperature in some cases. For example, the internal temperatures of hibernating arctic ground squirrel英语arctic ground squirrels can drop to −2.9 °C(26.8 °F), however the head and neck always stay above 0 °C(32 °F).[196] A few mammals in hot environments aestivate英语aestivate in times of drought or extreme heat, namely the fat-tailed dwarf lemur英语fat-tailed dwarf lemur (Cheirogaleus medius).[197]


智力编辑

In intelligent mammals, such as primates, the cerebrum is larger relative to the rest of the brain. Intelligence itself is not easy to define, but indications of intelligence include the ability to learn, matched with behavioral flexibility. Rats英语Rat IQ, for example, are considered to be highly intelligent, as they can learn and perform new tasks, an ability that may be important when they first colonize a fresh habitat. In some mammals, food gathering appears to be related to intelligence: a deer feeding on plants has a brain smaller than a cat, which must think to outwit its prey.[188]

 
A bonobo fishing for termites with a stick

Tool use by animals英语Tool use by animals may indicate different levels of learning and cognition. The sea otter英语Tool use by sea otters uses rocks as essential and regular parts of its foraging behaviour (smashing abalone from rocks or breaking open shells), with some populations spending 21% of their time making tools.[198] Other tool use, such as chimpanzees using twigs to "fish" for termites, may be developed by watching others use tools and may even be a true example of animal teaching.[199] Tools may even be used in solving puzzles in which the animal appears to experience a "Eureka moment"英语Eureka effect.[200] Other mammals that do not use tools, such as dogs, can also experience a Eureka moment.[201]

Brain size was previously considered a major indicator of the intelligence of an animal. Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass英语Brain-to-body mass ratio may increase the amount of brain mass available for more complex cognitive tasks. Allometric analysis indicates that mammalian brain size scales at approximately the 23 or 34 exponent of the body mass. Comparison of a particular animal's brain size with the expected brain size based on such allometric analysis provides an encephalisation quotient that can be used as another indication of animal intelligence.[202] Sperm whales have the largest brain mass of any animal on earth, averaging 8,000立方厘米(490立方英寸) and 7.8公斤(17英磅) in mature males.[203]

Self-awareness英语Self-awareness appears to be a sign of abstract thinking. Self-awareness, although not well-defined, is believed to be a precursor to more advanced processes such as metacognitive reasoning. The traditional method for measuring this is the mirror test, which determines if an animal possesses the ability of self-recognition.[204] Mammals that have passed the mirror test include Asian elephants (some pass, some do not);[205] chimpanzees;[206] bonobos;[207] orangutans;[208] humans, from 18 months (mirror stage英语mirror stage);[209] bottlenose dolphins[a][210] killer whales;[211] and false killer whales.[211]

社会结构编辑

 
Female elephants live in stable groups, along with their offspring.
 
Dolphins are known for their intelligence and travel in groups

Eusociality is the highest level of social organization. These societies have an overlap of adult generations, the division of reproductive labor and cooperative caring of young. Usually insects, such as bees, ants and termites, have eusocial behavior, but it is demonstrated in two rodent species: the naked mole-rat[212] and the Damaraland mole-rat英语Damaraland mole-rat.[213]

Presociality is when animals exhibit more than just sexual interactions with members of the same species, but fall short of qualifying as eusocial. That is, presocial animals can display communal living, cooperative care of young, or primitive division of reproductive labor, but they do not display all of the three essential traits of eusocial animals. Humans and some species of Callitrichidae (marmoset英语marmosets and tamarins) are unique among primates in their degree of cooperative care of young.[214] Harry Harlow set up an experiment with rhesus monkeys, presocial primates, in 1958; the results from this study showed that social encounters are necessary in order for the young monkeys to develop both mentally and sexually.[215]

A fission-fusion society英语fission-fusion society is a society that changes frequently in its size and composition, making up a permanent social group called the "parent group". Permanent social networks consist of all individual members of a community and often varies to track changes in their environment. In a fission–fusion society, the main parent group can fracture (fission) into smaller stable subgroups or individuals to adapt to environmental英语Social environment or social circumstances. For example, a number of males may break off from the main group in order to hunt or forage for food during the day, but at night they may return to join (fusion) the primary group to share food and partake in other activities. Many mammals exhibit this, such as primates (for example orangutans and spider monkeys),[216] elephants,[217] spotted hyenas,[218] lions,[219] and dolphins.[220]

Solitary animals defend a territory and avoid social interactions with the members of its species, except during breeding season. This is to avoid resource competition, as two individuals of the same species would occupy the same niche, and to prevent depletion of food.[221] A solitary animal, while foraging, can also be less conspicuous to predators or prey.[222]

 
Red kangaroos "boxing" for dominance英语dominance hierarchy

In a hierarchy英语dominance hierarchy, individuals are either dominant or submissive. A despotic hierarchy is where one individual is dominant while the others are submissive, as in wolves and lemurs,[223] and a pecking order is a linear ranking of individuals where there is a top individual and a bottom individual. Pecking orders may also be ranked by sex, where the lowest individual of a sex has a higher ranking than the top individual of the other sex, as in hyenas.[224] Dominant individuals, or alphas, have a high chance of reproductive success, especially in harems英语harem (zoology) where one or a few males (resident males) have exclusive breeding rights to females in a group.[225] Non-resident males can also be accepted in harems, but some species, such as the common vampire bat (Desmodus rotundus), may be more strict.[226]

Some mammals are perfectly monogamous英语Monogamy in animals, meaning that they mate for life英语pair bond and take no other partners (even after the original mate's death), as with wolves, Eurasian beavers, and otters.[227][228] There are three types of polygamy: either one or multiple dominant males have breeding rights (polygyny英语polygyny in animals), multiple males that females mate with (polyandry), or multiple males have exclusive relations with multiple females (polygynandry). It is much more common for polygynous mating to happen, which, excluding leks, are estimated to occur in up to 90% of mammals.[229] Lek mating occurs when males congregate around females and try to attract them with various courtship display英语courtship displays and vocalizations, as in harbor seals.[230]

All higher mammals (excluding monotremes) share two major adaptations for care of the young: live birth and lactation. These imply a group-wide choice of a degree of parental care. They may build nests and dig burrows to raise their young in, or feed and guard them often for a prolonged period of time. Many mammals are K-selected, and invest more time and energy into their young than do r-selected animals. When two animals mate, they both share an interest in the success of the offspring, though often to different extremes. Mammalian females exhibit some degree of maternal aggression, another example of parental care, which may be targeted against other females of the species or the young of other females; however, some mammals may "aunt" the infants of other females, and care for them. Mammalian males may play a role in child rearing, as with tenrecs, however this varies species to species, even within the same genus. For example, the males of the southern pig-tailed macaque (Macaca nemestrina) do not participate in child care, whereas the males of the Japanese macaque (M. fuscata) do.[231]

人类与其他哺乳动物编辑

人类文化中的哺乳动物编辑

 
Upper Paleolithic cave painting of a variety of large mammals, Lascaux, c. 17,300 years old

Non-human mammals play a wide variety of roles in human culture. They are the most popular of pets, with tens of millions of dogs, cats and other animals including rabbits and mice kept by families around the world.[232][233][234] Mammals such as mammoths, horses and deer are among the earliest subjects of art, being found in Upper Paleolithic cave paintings such as at Lascaux.[235] Major artists such as Albrecht Dürer, George Stubbs英语George Stubbs and Edwin Landseer英语Edwin Landseer are known for their portraits of mammals.[236] Many species of mammals have been hunted for sport and for food; deer and wild boar are especially popular as game animals.[237][238][239] Mammals such as horses and dogs英语greyhound racing are widely raced for sport, often combined with betting on the outcome.[240][241] There is a tension between the role of animals as companions to humans, and their existence as individuals with rights of their own.[242] Mammals further play a wide variety of roles in literature,[243][244][245] film,[246] mythology, and religion.[247][248][249]

对哺乳动物的利用编辑

 
Cattle have been kept for milk for thousands of years.

Domestic mammals form a large part of the livestock raised for meat across the world. They include (2009) around 1.4 billion cattle, 1 billion sheep, 1 billion domestic pigs,[250][251] and (1985) over 700 million rabbits.[252] Working domestic animals including cattle and horses have been used for work and transport from the origins of agriculture, their numbers declining with the arrival of mechanised transport and agricultural machinery. In 2004 they still provided some 80% of the power for the mainly small farms in the third world, and some 20% of the world's transport, again mainly in rural areas. In mountainous regions unsuitable for wheeled vehicles, pack animal英语pack animals continue to transport goods.[253] Mammal skins provide leather for shoes, clothing and upholstery英语upholstery.[254] Wool from mammals including sheep, goats and alpacas has been used for centuries for clothing.[255][256] Mammals serve a major role in science as experimental animals, both in fundamental biological research, such as in genetics,[257] and in the development of new medicines, which must be tested exhaustively to demonstrate their safety英语Pharmacovigilance.[258] Millions of mammals, especially mice and rats, are used in experiments each year.[259] A knockout mouse is a genetically modified mouse英语genetically modified mouse with an inactivated gene, replaced or disrupted with an artificial piece of DNA. They enable the study of sequenced genes whose functions are unknown.[260] A small percentage of the mammals are non-human primates, used in research for their similarity to humans.[261][262][263]

Charles Darwin, Jared Diamond and others have noted the importance of domesticated mammals in the Neolithic development of agriculture and of civilization, causing farmers to replace hunter-gatherers around the world.[b][265] This transition from hunting and gathering to herding flocks英语pastoralism and growing crops was a major step in human history. The new agricultural economies, based on domesticated mammals, caused "radical restructuring of human societies, worldwide alterations in biodiversity, and significant changes in the Earth's landforms and its atmosphere... momentous outcomes".[266]

杂交编辑

A true quagga, 1870 (left) vs. a bred-back quagga英语Quagga Project, 2014 (right)

Hybrids are offspring resulting from the breeding of two genetically distinct individuals, which usually will result in a high degree of heterozygosity, though hybrid and heterozygous are not synonymous. The deliberate or accidental hybridizing of two or more species of closely related animals through captive breeding is a human activity which has been in existence for millennia and has grown for economic purposes.[267] Hybrids between different subspecies within a species (such as between the Bengal tiger and Siberian tiger) are known as intra-specific hybrids. Hybrids between different species within the same genus (such as between lions and tigers) are known as interspecific hybrids or crosses. Hybrids between different genera (such as between sheep and goats) are known as intergeneric hybrids.[268] Natural hybrids will occur in hybrid zone英语hybrid zones, where two populations of species within the same genera or species living in the same or adjacent areas will interbreed with each other. Some hybrids have been recognized as species, such as the red wolf (though this is controversial).[269]

Artificial selection, the deliberate selective breeding of domestic animals, is being used to breed back英语breeding back recently extinct animals in an attempt to achieve an animal breed with a phenotype that resembles that extinct wildtype ancestor. A breeding-back (intraspecific) hybrid may be very similar to the extinct wildtype in appearance, ecological niche and to some extent genetics, but the initial gene pool of that wild type is lost forever with its extinction. As a result, bred-back breeds are at best vague look-alikes of extinct wildtypes, as Heck cattle英语Heck cattle are of the aurochs.[270]

Purebred英语Purebred wild species evolved to a specific ecology can be threatened with extinction[271] through the process of genetic pollution, the uncontrolled hybridization, introgression genetic swamping which leads to homogenization or out-competition from the heterosic hybrid species.[272] When new populations are imported or selectively bred by people, or when habitat modification brings previously isolated species into contact, extinction in some species, especially rare varieties, is possible.[273] Interbreeding can swamp the rarer gene pool and create hybrids, depleting the purebred gene pool. For example, the endangered wild water buffalo is most threatened with extinction by genetic pollution from the domestic water buffalo. Such extinctions are not always apparent from a morphological standpoint. Some degree of gene flow is a normal evolutionary process, nevertheless, hybridization threatens the existence of rare species.[274][275]

威胁编辑

 
Biodiversity of large mammal species per continent before and after humans arrived there

The loss of species from ecological communities, defaunation英语defaunation, is primarily driven by human activity.[276] This has resulted in empty forest英语empty forests, ecological communities depleted of large vertebrates.[277][278] In the Quaternary extinction event, the mass die-off of megafaunal variety coincided with the appearance of humans, suggesting a human influence. One hypothesis is that humans hunted large mammals, such as the woolly mammoth, into extinction.[279][280] The 2019 Global Assessment Report on Biodiversity and Ecosystem Services英语Global Assessment Report on Biodiversity and Ecosystem Services by IPBES states that the total biomass of wild mammals has declined by 82 percent since the beginning of human civilization.[281][282]

Various species are predicted to become extinct in the near future英语List of critically endangered species,[283] among them the rhinoceros,[284] primates,[285] pangolins,[286] and giraffes.[287] Hunting alone threatens hundreds of mammalian species around the world.[288][289] Scientists claim that the growing demand for meat is contributing to biodiversity loss英语biodiversity loss as this is a significant driver of deforestation and habitat destruction; species-rich habitats, such as significant portions of the Amazon rainforest, are being converted to agricultural land for meat production.[290][291][292] According to the World Wildlife Fund's 2016 Living Planet Index英语Living Planet Index, global wildlife populations have declined 58% since 1970, primarily due to habitat destruction, over-hunting and pollution. They project that if current trends continue, 67% of wildlife could disappear by 2020.[293][294] Another influence is over-hunting and poaching英语Species affected by poaching, which can reduce the overall population of game animals,[295] especially those located near villages,[296] as in the case of peccaries.[297] The effects of poaching can especially be seen in the ivory trade英语ivory trade with African elephants.[來源請求] Marine mammals are at risk from entanglement from fishing gear, notably cetaceans英语Cetacean bycatch, with discard mortalities ranging from 65,000 to 86,000 individuals annually.[298]

Attention is being given to endangered species globally, notably through the Convention on Biological Diversity, otherwise known as the Rio Accord, which includes 189 signatory countries that are focused on identifying endangered species and habitats.[299] Another notable conservation organization is the IUCN, which has a membership of over 1,200 governmental and non-governmental organizations.[300]

Recent extinctions英语List of recently extinct mammals can be directly attributed to human influences.[301][276] The IUCN characterizes 'recent' extinction as those that have occurred past the cut-off point of 1500,[302] and around 80 mammal species have gone extinct since that time and 2015.[303] Some species, such as the Père David's deer[304] are extinct in the wild, and survive solely in captive populations. Other species, such as the Florida panther, are ecologically extinct英语Ecological extinction, surviving in such low numbers that they essentially have no impact on the ecosystem.[305]:318 Other populations are only locally extinct (extirpated), still existing elsewhere, but reduced in distribution,[305]:75–77 as with the extinction of gray whales in the Atlantic.[306]

注解编辑

  1. ^ Decreased latency to approach the mirror, repetitious head circling and close viewing of the marked areas were considered signs of self-recognition since they do not have arms and cannot touch the marked areas.[210]
  2. ^ Diamond discussed this matter further in his 1997 book Guns, Germs, and Steel.[264]

参见编辑

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