| 6s | What do these animals have in common? |
| 9s | More than you might think. |
| 11s | Along with over 5,000 other species, they're mammals, |
| 15s | or members of class mammalia. |
| 18s | All mammals are vertebrates, meaning they have backbones. |
| 21s | But mammals are distinguished from other vertebrates |
| 24s | by a number of shared features. |
| 27s | That includes warm blood, |
| 28s | |
| 31s | the ability to breathe using lungs, |
| 33s | and nourishing their young with milk. |
| 36s | But despite these similarities, |
| 38s | these creatures also have many biological differences, |
| 41s | and one of the most remarkable is how they give birth. |
| 44s | Let's start with the most familiar, placental mammals. |
| 48s | This group includes humans, |
| 50s | |
| 50s | |
| 51s | |
| 52s | and even the blue whale, the biggest animal on Earth. |
| 56s | Its placenta, a solid disk of blood-rich tissue, |
| 59s | attaches to the wall of the uterus to support the developing embryo. |
| 63s | The placenta is what keeps the calf alive during pregnancy. |
| 67s | Directly connected to the mother's blood supply, |
| 69s | it funnels nutrients and oxygen straight into the calf's body |
| 73s | |
| 75s | and also exports its waste. |
| 78s | Placental mammals can spend far longer inside the womb than other mammals. |
| 82s | Baby blue whales, for instance, spend almost a full year inside their mother. |
| 88s | The placenta keeps the calf alive right up until its birth, |
| 91s | when the umbilical cord breaks |
| 93s | and the newborn's own respiratory, |
| 95s | |
| 96s | and waste disposal systems take over. |
| 100s | Measuring about 23 feet, a newborn calf is already able to swim. |
| 105s | It will spend the next six months |
| 106s | drinking 225 liters of its mothers thick, fatty milk per day. |
| 113s | Meanwhile, in Australia, you can find a second type of mammal - |
| 117s | |
| 119s | Marsupial babies are so tiny and delicate when they're born |
| 122s | that they must continue developing in the mother's pouch. |
| 127s | Take the quoll, one of the world's smallest marsupials, |
| 131s | which weighs only 18 milligrams at birth, |
| 134s | the equivalent of about 30 sugar grains. |
| 138s | The kangaroo, another marsupial, |
| 140s | gives birth to a single jelly bean-sized baby at a time. |
| 144s | The baby crawls down the middle of the mother's three vaginas, |
| 147s | then must climb up to the pouch, |
| 149s | where she spends the next 6-11 months suckling. |
| 153s | Even after the baby kangaroo leaves this warm haven, |
| 156s | she'll return to suckle milk. |
| 158s | Sometimes, she's just one of three babies her mother is caring for. |
| 163s | A female kangaroo can often simultaneously support one inside her uterus |
| 167s | and another in her pouch. |
| 170s | In unfavorable conditions, |
| 172s | female kangaroos can pause their pregnancies. |
| 175s | When that happens, she's able to produce two different kinds of milk, |
| 179s | |
| 180s | and one for her older joey. |
| 183s | The word mammalia means of the breast, |
| 186s | which is a bit of a misnomer |
| 188s | because while kangaroos do produce milk from nipples in their pouches, |
| 191s | they don't actually have breasts. |
| 194s | Nor do monotremes, the third and arguably strangest example of mammalian birth. |
| 201s | There were once hundreds of monotreme species, |
| 204s | but there are only five left: |
| 206s | four species of echidnas and the duck-billed platypus. |
| 211s | The name monotreme means one hole |
| 214s | referring to the single orifice they use for reproduction, |
| 217s | |
| 218s | |
| 220s | |
| 220s | |
| 221s | |
| 222s | |
| 223s | |
| 224s | these species lay eggs instead of giving birth to live young. |
| 229s | Their eggs are soft-shelled, |
| 231s | and when their babies hatch, they suckle milk from pores on their mother's body |
| 236s | until they're large enough to feed themselves. |
| 239s | Despite laying eggs and other adaptations that we associate more with non-mammals, |
| 243s | like the duck-bill platypus's webbed feet, |
| 246s | |
| 247s | and the venomous spur males have on their feet, |
| 250s | they are, in fact, mammals. |
| 253s | That's because they share the defining characteristics of mammalia |
| 256s | and are evolutionarily linked to the rest of the class. |
| 260s | |
| 261s | |
| 262s | |
| 263s | each of these creatures and its unique birthing methods, however bizarre, |
| 267s | have succeeded for many millennia in bringing new life and diversity |
| 272s | |
