1
00:00:06,990 --> 00:00:09,802
What do these animals have in common?

2
00:00:09,802 --> 00:00:11,942
More than you might think.

3
00:00:11,942 --> 00:00:15,872
Along with over 5,000 other species,
they're mammals,

4
00:00:15,872 --> 00:00:18,853
or members of class mammalia.

5
00:00:18,853 --> 00:00:21,903
All mammals are vertebrates,
meaning they have backbones.

6
00:00:21,903 --> 00:00:24,411
But mammals are distinguished 
from other vertebrates

7
00:00:24,411 --> 00:00:27,283
by a number of shared features.

8
00:00:27,283 --> 00:00:28,953
That includes warm blood,

9
00:00:28,953 --> 00:00:31,173
body hair or fur,

10
00:00:31,173 --> 00:00:33,593
the ability to breathe using lungs,

11
00:00:33,593 --> 00:00:36,524
and nourishing their young with milk.

12
00:00:36,524 --> 00:00:38,264
But despite these similarities,

13
00:00:38,264 --> 00:00:41,552
these creatures also have 
many biological differences,

14
00:00:41,552 --> 00:00:44,823
and one of the most remarkable
is how they give birth.

15
00:00:44,823 --> 00:00:48,844
Let's start with the most familiar,
placental mammals.

16
00:00:48,844 --> 00:00:50,143
This group includes humans,

17
00:00:50,143 --> 00:00:50,864
cats,

18
00:00:50,864 --> 00:00:51,584
dogs,

19
00:00:51,584 --> 00:00:52,392
giraffes,

20
00:00:52,392 --> 00:00:56,354
and even the blue whale,
the biggest animal on Earth.

21
00:00:56,354 --> 00:00:59,433
Its placenta, a solid disk
of blood-rich tissue,

22
00:00:59,433 --> 00:01:03,984
attaches to the wall of the uterus
to support the developing embryo.

23
00:01:03,984 --> 00:01:07,384
The placenta is what keeps
the calf alive during pregnancy.

24
00:01:07,384 --> 00:01:09,814
Directly connected to 
the mother's blood supply,

25
00:01:09,814 --> 00:01:13,336
it funnels nutrients and oxygen 
straight into the calf's body

26
00:01:13,336 --> 00:01:15,194
via the umbilical cord,

27
00:01:15,194 --> 00:01:18,484
and also exports its waste.

28
00:01:18,484 --> 00:01:22,945
Placental mammals can spend far
longer inside the womb than other mammals.

29
00:01:22,945 --> 00:01:28,416
Baby blue whales, for instance, spend
almost a full year inside their mother.

30
00:01:28,416 --> 00:01:31,675
The placenta keeps the calf alive
right up until its birth,

31
00:01:31,675 --> 00:01:33,316
when the umbilical cord breaks

32
00:01:33,316 --> 00:01:35,305
and the newborn's own respiratory,

33
00:01:35,305 --> 00:01:36,606
circulatory,

34
00:01:36,606 --> 00:01:40,296
and waste disposal systems take over.

35
00:01:40,296 --> 00:01:45,199
Measuring about 23 feet,
a newborn calf is already able to swim.

36
00:01:45,199 --> 00:01:46,867
It will spend the next six months

37
00:01:46,867 --> 00:01:53,477
drinking 225 liters of 
its mothers thick, fatty milk per day.

38
00:01:53,477 --> 00:01:57,097
Meanwhile, in Australia,
you can find a second type of mammal -

39
00:01:57,097 --> 00:01:59,789
marsupials.

40
00:01:59,789 --> 00:02:02,968
Marsupial babies are so tiny and delicate
when they're born

41
00:02:02,968 --> 00:02:07,543
that they must continue developing
in the mother's pouch.

42
00:02:07,543 --> 00:02:11,368
Take the quoll, one of the world's
smallest marsupials,

43
00:02:11,368 --> 00:02:14,468
which weighs only 18 milligrams at birth,

44
00:02:14,468 --> 00:02:18,207
the equivalent of about 30 sugar grains.

45
00:02:18,207 --> 00:02:20,249
The kangaroo, another marsupial,

46
00:02:20,249 --> 00:02:24,497
gives birth to a single 
jelly bean-sized baby at a time.

47
00:02:24,497 --> 00:02:27,969
The baby crawls down the middle
of the mother's three vaginas,

48
00:02:27,969 --> 00:02:29,979
then must climb up to the pouch,

49
00:02:29,979 --> 00:02:33,799
where she spends 
the next 6-11 months suckling.

50
00:02:33,799 --> 00:02:36,509
Even after the baby kangaroo leaves
this warm haven,

51
00:02:36,509 --> 00:02:38,898
she'll return to suckle milk.

52
00:02:38,898 --> 00:02:43,380
Sometimes, she's just one of three babies
her mother is caring for.

53
00:02:43,380 --> 00:02:47,949
A female kangaroo can often simultaneously
support one inside her uterus

54
00:02:47,949 --> 00:02:50,360
and another in her pouch.

55
00:02:50,360 --> 00:02:52,209
In unfavorable conditions,

56
00:02:52,209 --> 00:02:55,699
female kangaroos can pause
their pregnancies.

57
00:02:55,699 --> 00:02:59,390
When that happens, she's able
to produce two different kinds of milk,

58
00:02:59,390 --> 00:03:00,652
one for her newborn,

59
00:03:00,652 --> 00:03:03,691
and one for her older joey.

60
00:03:03,691 --> 00:03:06,840
The word mammalia means of the breast,

61
00:03:06,840 --> 00:03:08,211
which is a bit of a misnomer

62
00:03:08,211 --> 00:03:11,611
because while kangaroos do produce
milk from nipples in their pouches,

63
00:03:11,611 --> 00:03:14,351
they don't actually have breasts.

64
00:03:14,351 --> 00:03:21,391
Nor do monotremes, the third and arguably
strangest example of mammalian birth.

65
00:03:21,391 --> 00:03:24,301
There were once hundreds 
of monotreme species,

66
00:03:24,301 --> 00:03:26,892
but there are only five left:

67
00:03:26,892 --> 00:03:31,571
four species of echidnas
and the duck-billed platypus.

68
00:03:31,571 --> 00:03:34,412
The name monotreme means one hole

69
00:03:34,412 --> 00:03:37,182
referring to the single orifice they use
for reproduction,

70
00:03:37,182 --> 00:03:38,242
excretion,

71
00:03:38,242 --> 00:03:40,152
and egg-laying.

72
00:03:40,152 --> 00:03:40,952
Like birds,

73
00:03:40,952 --> 00:03:41,812
reptiles,

74
00:03:41,812 --> 00:03:42,663
fish,

75
00:03:42,663 --> 00:03:43,613
dinosaurs,

76
00:03:43,613 --> 00:03:44,613
and others,

77
00:03:44,613 --> 00:03:49,053
these species lay eggs instead
of giving birth to live young.

78
00:03:49,053 --> 00:03:51,492
Their eggs are soft-shelled,

79
00:03:51,492 --> 00:03:56,005
and when their babies hatch, they suckle
milk from pores on their mother's body

80
00:03:56,005 --> 00:03:59,694
until they're large enough 
to feed themselves.

81
00:03:59,694 --> 00:04:03,994
Despite laying eggs and other adaptations
that we associate more with non-mammals,

82
00:04:03,994 --> 00:04:06,507
like the duck-bill platypus's webbed feet,

83
00:04:06,507 --> 00:04:07,364
bill,

84
00:04:07,364 --> 00:04:10,174
and the venomous spur 
males have on their feet,

85
00:04:10,174 --> 00:04:13,103
they are, in fact, mammals.

86
00:04:13,103 --> 00:04:16,794
That's because they share the defining
characteristics of mammalia

87
00:04:16,794 --> 00:04:20,504
and are evolutionarily linked
to the rest of the class.

88
00:04:20,504 --> 00:04:21,575
Whether placental,

89
00:04:21,575 --> 00:04:22,544
marsupial,

90
00:04:22,544 --> 00:04:23,695
or monotreme,

91
00:04:23,695 --> 00:04:27,380
each of these creatures and its unique
birthing methods, however bizarre,

92
00:04:27,380 --> 00:04:32,007
have succeeded for many millennia
in bringing new life and diversity

93
00:04:32,007 --> 00:04:34,032
into the mammal kingdom.