0:00:06.990,0:00:09.802 What do these animals have in common? 0:00:09.802,0:00:11.942 More than you might think. 0:00:11.942,0:00:15.872 Along with over 5,000 other species,[br]they're mammals, 0:00:15.872,0:00:18.853 or members of class mammalia. 0:00:18.853,0:00:21.903 All mammals are vertebrates,[br]meaning they have backbones. 0:00:21.903,0:00:24.411 But mammals are distinguished [br]from other vertebrates 0:00:24.411,0:00:27.283 by a number of shared features. 0:00:27.283,0:00:28.953 That includes warm blood, 0:00:28.953,0:00:31.173 body hair or fur, 0:00:31.173,0:00:33.593 the ability to breath using lungs, 0:00:33.593,0:00:36.524 and nourishing their young with milk. 0:00:36.524,0:00:38.264 But despite these similarities, 0:00:38.264,0:00:41.552 these creatures also have [br]many biological differences, 0:00:41.552,0:00:44.823 and one of the most remarkable[br]is how they give birth. 0:00:44.823,0:00:48.844 Let's start with the most familiar,[br]placental mammals. 0:00:48.844,0:00:50.143 This group includes humans, 0:00:50.143,0:00:50.864 cats, 0:00:50.864,0:00:51.584 dogs, 0:00:51.584,0:00:52.392 giraffes, 0:00:52.392,0:00:56.354 and even the blue whale,[br]the biggest animal on Earth. 0:00:56.354,0:00:59.433 Its placenta, a solid disk[br]of blood-rich tissue, 0:00:59.433,0:01:03.984 attaches to the wall of the uterus[br]to support the developing embryo. 0:01:03.984,0:01:07.384 The placenta is what keeps[br]the calf alive during pregnancy. 0:01:07.384,0:01:09.814 Directly connected to [br]the mother's blood supply, 0:01:09.814,0:01:13.336 it funnels nutrients and oxygen [br]straight into the calf's body 0:01:13.336,0:01:15.194 via the umbilical cord, 0:01:15.194,0:01:18.484 and also exports its waste. 0:01:18.484,0:01:22.945 Placental mammals can spend far[br]longer inside the womb than other mammals. 0:01:22.945,0:01:28.416 Baby blue whales, for instance, spend[br]almost a full year inside their mother. 0:01:28.416,0:01:31.675 The placenta keeps the calf alive[br]right up until its birth, 0:01:31.675,0:01:33.316 when the umbilical cord breaks 0:01:33.316,0:01:35.305 and the newborn's own respiratory, 0:01:35.305,0:01:36.606 circulatory, 0:01:36.606,0:01:40.296 and waste disposal systems take over. 0:01:40.296,0:01:45.199 Measuring about 23 feet,[br]a newborn calf is already able to swim. 0:01:45.199,0:01:46.867 It will spend the next six months 0:01:46.867,0:01:53.477 drinking 225 liters of [br]its mothers thick, fatty milk per day. 0:01:53.477,0:01:57.097 Meanwhile in Australia,[br]you can find a second type of mammal - 0:01:57.097,0:01:59.789 marsupials. 0:01:59.789,0:02:02.968 Marsupial babies are so tiny and delicate[br]when they're born 0:02:02.968,0:02:07.543 that they must continue developing[br]in the mother's pouch. 0:02:07.543,0:02:11.368 Take the quoll, one of the world's[br]smallest marsupials, 0:02:11.368,0:02:14.468 which weighs only 18 milligrams at birth, 0:02:14.468,0:02:18.207 the equivalent of about 30 sugar grains. 0:02:18.207,0:02:20.249 The kangaroo, another marsupial, 0:02:20.249,0:02:24.497 gives birth to a single [br]jelly bean-sized baby at a time. 0:02:24.497,0:02:27.969 The baby crawls down the middle[br]of the mother's three vaginas, 0:02:27.969,0:02:29.979 then must climb up to the pouch, 0:02:29.979,0:02:33.799 where she spends [br]the next 6-11 months suckling. 0:02:33.799,0:02:36.509 Even after the baby kangaroo leaves[br]this warm haven, 0:02:36.509,0:02:38.898 she'll return to suckle milk. 0:02:38.898,0:02:43.380 Sometimes, she's just one of three babies[br]her mother is caring for. 0:02:43.380,0:02:47.949 A female kangaroo can often simultaneously[br]support one inside her uterus 0:02:47.949,0:02:50.360 and another in her pouch. 0:02:50.360,0:02:52.209 In unfavorable conditions, 0:02:52.209,0:02:55.699 female kangaroos can pause[br]their pregnancies. 0:02:55.699,0:02:59.390 When that happens, she's able[br]to produce two different kinds of milk, 0:02:59.390,0:03:00.652 one for her newborn, 0:03:00.652,0:03:03.691 and one for her older joey. 0:03:03.691,0:03:06.840 The word mammalia means of the breast, 0:03:06.840,0:03:08.211 which is a bit of a misnomer 0:03:08.211,0:03:11.611 because while kangaroos do produce[br]milk from nipples in their pouches, 0:03:11.611,0:03:14.351 they don't actually have breasts. 0:03:14.351,0:03:21.391 Nor do monotremes, the third and arguably[br]strangest example of mammalian birth. 0:03:21.391,0:03:24.301 There were once hundreds [br]of monotreme species, 0:03:24.301,0:03:26.892 but there are only five left: 0:03:26.892,0:03:31.571 four species of echidnas[br]and the duck-billed platypus. 0:03:31.571,0:03:34.412 The name monotreme means one hole 0:03:34.412,0:03:37.182 referring to the single orifice they use[br]for reproduction, 0:03:37.182,0:03:38.242 excretion, 0:03:38.242,0:03:40.152 and egg-laying. 0:03:40.152,0:03:40.952 Like birds, 0:03:40.952,0:03:41.812 reptiles, 0:03:41.812,0:03:42.663 fish, 0:03:42.663,0:03:43.613 dinosaurs, 0:03:43.613,0:03:44.613 and others, 0:03:44.613,0:03:49.053 these species lay eggs instead[br]of giving birth to live young. 0:03:49.053,0:03:51.492 Their eggs are soft-shelled, 0:03:51.492,0:03:56.005 and when their babies hatch, they suckle[br]milk from pores on their mother's body 0:03:56.005,0:03:59.694 until they're large enough [br]to feed themselves. 0:03:59.694,0:04:03.994 Despite laying eggs and other adaptations[br]that we associate more with non-mammals, 0:04:03.994,0:04:06.507 like the duck-bill platypus's webbed feet, 0:04:06.507,0:04:07.364 bill, 0:04:07.364,0:04:10.174 and venomous spur [br]males have on their feet, 0:04:10.174,0:04:13.103 they are in fact mammals. 0:04:13.103,0:04:16.794 That's because they share the defining[br]characteristics of mammalia 0:04:16.794,0:04:20.504 and are evolutionarily linked[br]to the rest of the class. 0:04:20.504,0:04:21.575 Whether placental, 0:04:21.575,0:04:22.544 marsupial, 0:04:22.544,0:04:23.695 or monotreme, 0:04:23.695,0:04:27.380 each of these creatures and its unique[br]birthing methods, however bizarre, 0:04:27.380,0:04:32.007 have succeeded from many millennia[br]in bringing new life and diversity 0:04:32.007,0:04:34.032 into the mammal kingdom.