What do these animals have in common? More than you might think. Along with over 5,000 other species, they're mammals, or members of class mammalia. All mammals are vertebrates, meaning they have backbones. But mammals are distinguished from other vertebrates by a number of shared features. That includes warm blood, body hair or fur, the ability to breathe using lungs, and nourishing their young with milk. But despite these similarities, these creatures also have many biological differences, and one of the most remarkable is how they give birth. Let's start with the most familiar, placental mammals. This group includes humans, cats, dogs, giraffes, and even the blue whale, the biggest animal on Earth. Its placenta, a solid disk of blood-rich tissue, attaches to the wall of the uterus to support the developing embryo. The placenta is what keeps the calf alive during pregnancy. Directly connected to the mother's blood supply, it funnels nutrients and oxygen straight into the calf's body via the umbilical cord, and also exports its waste. Placental mammals can spend far longer inside the womb than other mammals. Baby blue whales, for instance, spend almost a full year inside their mother. The placenta keeps the calf alive right up until its birth, when the umbilical cord breaks and the newborn's own respiratory, circulatory, and waste disposal systems take over. Measuring about 23 feet, a newborn calf is already able to swim. It will spend the next six months drinking 225 liters of its mothers thick, fatty milk per day. Meanwhile, in Australia, you can find a second type of mammal - marsupials. Marsupial babies are so tiny and delicate when they're born that they must continue developing in the mother's pouch. Take the quoll, one of the world's smallest marsupials, which weighs only 18 milligrams at birth, the equivalent of about 30 sugar grains. The kangaroo, another marsupial, gives birth to a single jelly bean-sized baby at a time. The baby crawls down the middle of the mother's three vaginas, then must climb up to the pouch, where she spends the next 6-11 months suckling. Even after the baby kangaroo leaves this warm haven, she'll return to suckle milk. Sometimes, she's just one of three babies her mother is caring for. A female kangaroo can often simultaneously support one inside her uterus and another in her pouch. In unfavorable conditions, female kangaroos can pause their pregnancies. When that happens, she's able to produce two different kinds of milk, one for her newborn, and one for her older joey. The word mammalia means of the breast, which is a bit of a misnomer because while kangaroos do produce milk from nipples in their pouches, they don't actually have breasts. Nor do monotremes, the third and arguably strangest example of mammalian birth. There were once hundreds of monotreme species, but there are only five left: four species of echidnas and the duck-billed platypus. The name monotreme means one hole referring to the single orifice they use for reproduction, excretion, and egg-laying. Like birds, reptiles, fish, dinosaurs, and others, these species lay eggs instead of giving birth to live young. Their eggs are soft-shelled, and when their babies hatch, they suckle milk from pores on their mother's body until they're large enough to feed themselves. Despite laying eggs and other adaptations that we associate more with non-mammals, like the duck-bill platypus's webbed feet, bill, and the venomous spur males have on their feet, they are, in fact, mammals. That's because they share the defining characteristics of mammalia and are evolutionarily linked to the rest of the class. Whether placental, marsupial, or monotreme, each of these creatures and its unique birthing methods, however bizarre, have succeeded for many millennia in bringing new life and diversity into the mammal kingdom.
這些動物的共通處是甚麼? 比你想的還要多 除了有五千多種種類外 牠們是哺乳類動物 或是隸屬哺乳綱目 所有的哺乳動物都是脊椎動物 這代表牠們都有脊骨 但哺乳動物跟其他脊椎動物的區分法 來自若干個相同特徵 包括溫血 毛髮或毛皮 使用肺呼吸的能力 以母乳哺育稚子 儘管有這些相似處 這些生物仍存在著許多生物差異 其中一項令人驚異的差異在生產方式 讓我們從大家最熟悉的 胎盤哺乳類(真獸類)動物談起 這類組包括:人類 貓、狗、長頸鹿 甚至藍鯨——世界上最大的哺乳類動物 哺乳動物的胎盤—— 紮實的碟狀充血組織 附著在子宮壁上 提供發育中的胚胎養分 藍鯨寶寶在孕期靠胎盤維生 從母體血液直接獲得養分 運輸養分與氧氣直達藍鯨寶寶體內 臍帶就是運輸通道 臍帶也排出代謝廢物 胎盤哺乳類比其他哺乳類在子宮內待得更久 以藍鯨寶寶為例 在媽媽體內幾乎待滿一年 出生前胎盤持續維持胎兒的生命 當臍帶斷落 新生兒的呼吸系統 循環系統 和代謝系統會取代臍帶 大約23英尺長 新生藍鯨寶寶已經會游泳了 藍鯨寶寶接下來大約六個月 每天會喝下225公升、醇濃全脂的母乳 同時在澳洲,你會發現第二種哺乳動物 有袋類哺乳動物 有袋類寶寶剛出生時小巧嬌弱 必須繼續在媽媽的育兒袋內茁壯 舉例來說:袋鼬—— 世界上最小的有袋類動物之一 出生時只有18毫克重 相當於30顆糖粒的重量 另一種有袋動物—袋鼠 一次只產下一隻 果凍豆糖大小的袋鼠寶寶 袋鼠寶寶爬到媽媽三個陰道中間 然後必須爬到育兒袋 在接下來的六到十一個月 在育兒袋裡吸奶 儘管袋鼠寶寶離開這個溫暖的天堂 仍然會回到育兒袋吸奶 有時牠只是袋鼠媽媽照顧的 三寶中的其中一寶 母袋鼠通常能同時一邊提供養分 給子宮裡的寶寶 一邊在育兒袋中哺乳另一寶寶 在不佳的環境下 母袋鼠會滯育 子宮休眠下,牠能夠產出兩種母乳 一種供給新生的二寶 另一種供給大寶 mammalia 這個字的意思 是「來自乳房」 似乎有點用詞不當 袋鼠媽媽的確 自育兒袋裡的奶頭產出母奶 但牠們並沒有乳房 單孔目哺乳動物也沒有乳房 第三種,也可能是 最怪異的哺乳動物例子 世界上曾經有數百種單孔目動物 現今只剩下五種 針鼴科四種與鴨嘴獸一種 “monotreme”這個字的意思是「單孔」 意指單孔目哺乳動物 以同一泄殖腔繁衍 排泄 以及產卵 類似鳥類、爬蟲類、魚類 恐龍和其他生物 這些物種不是產下胎兒而是產卵 牠們的殼質地薄軟 孵化後,寶寶們 會從媽媽身上的毛孔吸允母乳 到牠們會自己覓食為止 儘管我們把產卵和其他的演化調適 連結到非哺乳類較多 就像鴨嘴獸的蹼 喙 雄鴨嘴獸腳上的毒刺 事實上,牠們是哺乳動物 因為牠們與哺乳動物有一樣的界定特徵 也與其他的哺乳綱動物有演化關聯 不論是胎盤哺乳類 有袋哺乳類或單孔目類 這些生物擁有獨特的哺育方式 不管多麼地奇特 數千年來已成功地賦予新生命 和生物多樣性 到這個哺乳動物王國