WEBVTT

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What do these animals have in common?

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More than you might think.

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Along with over 5,000 other species,
they're mammals,

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or members of class mammalia.

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All mammals are vertebrates,
meaning they have backbones.

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But mammals are distinguished 
from other vertebrates

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by a number of shared features.

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That includes warm blood,

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body hair or fur,

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the ability to breathe using lungs,

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and nourishing their young with milk.

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But despite these similarities,

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these creatures also have 
many biological differences,

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and one of the most remarkable
is how they give birth.

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Let's start with the most familiar,
placental mammals.

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This group includes humans,

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cats,

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dogs,

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giraffes,

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and even the blue whale,
the biggest animal on Earth.

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Its placenta, a solid disk
of blood-rich tissue,

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attaches to the wall of the uterus
to support the developing embryo.

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The placenta is what keeps
the calf alive during pregnancy.

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Directly connected to 
the mother's blood supply,

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it funnels nutrients and oxygen 
straight into the calf's body

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via the umbilical cord,

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and also exports its waste.

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Placental mammals can spend far
longer inside the womb than other mammals.

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Baby blue whales, for instance, spend
almost a full year inside their mother.

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The placenta keeps the calf alive
right up until its birth,

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when the umbilical cord breaks

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and the newborn's own respiratory,

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circulatory,

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and waste disposal systems take over.

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Measuring about 23 feet,
a newborn calf is already able to swim.

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It will spend the next six months

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drinking 225 liters of 
its mothers thick, fatty milk per day.

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Meanwhile, in Australia,
you can find a second type of mammal -

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marsupials.

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Marsupial babies are so tiny and delicate
when they're born

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that they must continue developing
in the mother's pouch.

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Take the quoll, one of the world's
smallest marsupials,

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which weighs only 18 milligrams at birth,

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the equivalent of about 30 sugar grains.

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The kangaroo, another marsupial,

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gives birth to a single 
jelly bean-sized baby at a time.

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The baby crawls down the middle
of the mother's three vaginas,

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then must climb up to the pouch,

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where she spends 
the next 6-11 months suckling.

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Even after the baby kangaroo leaves
this warm haven,

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she'll return to suckle milk.

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Sometimes, she's just one of three babies
her mother is caring for.

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A female kangaroo can often simultaneously
support one inside her uterus

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and another in her pouch.

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In unfavorable conditions,

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female kangaroos can pause
their pregnancies.

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When that happens, she's able
to produce two different kinds of milk,

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one for her newborn,

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and one for her older joey.

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The word mammalia means of the breast,

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which is a bit of a misnomer

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because while kangaroos do produce
milk from nipples in their pouches,

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they don't actually have breasts.

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Nor do monotremes, the third and arguably
strangest example of mammalian birth.

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There were once hundreds 
of monotreme species,

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but there are only five left:

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four species of echidnas
and the duck-billed platypus.

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The name monotreme means one hole

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referring to the single orifice they use
for reproduction,

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excretion,

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and egg-laying.

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Like birds,

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reptiles,

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fish,

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dinosaurs,

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and others,

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these species lay eggs instead
of giving birth to live young.

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Their eggs are soft-shelled,

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and when their babies hatch, they suckle
milk from pores on their mother's body

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until they're large enough 
to feed themselves.

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Despite laying eggs and other adaptations
that we associate more with non-mammals,

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like the duck-bill platypus's webbed feet,

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bill,

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and the venomous spur 
males have on their feet,

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they are, in fact, mammals.

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That's because they share the defining
characteristics of mammalia

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and are evolutionarily linked
to the rest of the class.

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Whether placental,

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marsupial,

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or monotreme,

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each of these creatures and its unique
birthing methods, however bizarre,

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have succeeded for many millennia
in bringing new life and diversity

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into the mammal kingdom.