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.
Kaj imajo te živali skupnega? Več, kot bi si mislili. Z več kot 5000 drugimi vrstami, se uvrščajo med sesalce oziroma v razred Mammalia. Vsi sesalci so vretenčarji, kar pomeni, da imajo hrbtenice. A sesalci se od ostalih vretenčarjev razlikujejo po številnih skupnih lastnostih. Mednje sodijo topla kri, dlakavost, sposobnost dihanja s pljuči in hranjenje mladičev z mlekom. Kljub podobnostim pa obstajajo med temi bitji tudi številne biološke razlike in ena najzanimivejših je način rojevanja. Začnimo z najbolje poznanimi, placentalnimi sesalci. Ta skupina zajema ljudi, mačke, pse, žirafe in celo sinjega kita, največjo žival na Zemlji. Posteljica, trdna plošča dobro prekrvavljenega tkiva, se pritrdi na steno maternice in vzdržuje razvijajoči zarodek. Posteljica ohranja mladiča pri življenju med nosečnostjo. Ker je direktno povezana z materinim ožiljem, pošilja hranila in kisik naravnost v telo mladiča prek popkovine ter odstranjuje njegove izločke. Placentalni sesalci lahko ostanejo v maternici veliko dlje od ostalih sesalcev. Mladi sinji kiti, na primer, preživijo v materi skoraj celo leto. Posteljica ohranja mladiča pri življenju vse do rojstva, ko se popkovina pretrga in mladičev lastni dihalni, krvožilni in izločalni sistemi prevzamejo njeno funkcijo. Novorojeni sinji kit, ki meri okrog 7 metrov, že lahko plava. Naslednjih 6 mesecev bo popil 225 litrov gostega, mastnega materinega mleka dnevno. Medtem lahko v Avstraliji najdemo drugo vrsto sesalcev - vrečarje. Mladiči vrečarjev so tako majhni in krhki, ko se rodijo, da morajo z razvojem nadaljevati v materini vreči. Vzemimo rod Dasyurus, ene najmanjših vrečarjev na svetu. Ob rojstvu tehtajo le 18 miligramov, kar ustreza 30 kristalom sladkorja. Kenguru, še en vrečar, povrže naenkrat enega samega mladiča v velikosti bombona. Mladič zleze navzdol po srednji izmed materinih treh vagin, nato pa mora splezati navzgor do vreče, kjer ob sesanju mleka preživi naslednjih 6-11 mesecev. Še ko mladi kenguru zapusti to varno zavetje, se bo vračal v vrečo sesati mleko. Včasih je le eden izmed treh mladičev, za katere mati skrbi. Kengurujka lahko pogosto vzdržuje enega v maternici, drugega pa v vreči. V neugodnih razmerah lahko kengurujke nosečnost začasno ustavijo. Ko pride do tega, lahko proizvajajo dve različni vrsti mleka, eno za novorojenca, drugo za starejšega mladiča. Beseda mammalia pomeni "od prsi," nekoliko neustrezno ime, saj kenguruji res proizvajajo mleko iz bradavic v svojih vrečah, vendar nimajo dojk. Isto velja za stokovce, tretjo in najnenavadnejšo vrsto sesalcev. Nekoč je obstajalo na stotine vrst stokovcev, danes pa jih je le še pet: štiri vrste kljunatih ježkov ter kljunaš. Ime so dobili po stoku ali kloaki, odprtini, ki jo uporabljajo tako za razmnoževanje, kot za izločanje in nošenje jajc. Tako kot ptice, plazilci, ribe, dinozavri in drugi, te vrste ležejo jajca, namesto da bi rodile žive mladiče. Njihova jajca imajo mehko lupino in ko se mladiči izležejo, sesajo mleko iz por na materinem telesu, dokler niso dovolj veliki, da se prehranjujejo sami. Čeprav nosijo jajca in imajo značilnosti, ki jih povezujemo z nesesalci, kot na primer kljunaševa plavalna kožica, kljun in strupena ost, ki jo imajo samci na nogah, so vseeno sesalci. To pa zato, ker imajo značilnosti, ki definirajo sesalce in so evolucijsko povezani z ostalimi člani razreda sesalcev. Placentalni sesalci, vrečarji in stokovci, vse te živali in njihove edinstvene metode rojevanja, še tako bizarne, so mnogo tisočletij uspešno prinašale novo življenje in raznolikost v kraljestvo sesalcev.