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Hey, everyone. I am Jess Keating, and you're watching
Animalogic: Second Nature. [male announcer]
Understandings are first gained by seeing things at first hand,
watching, experiencing. Metamorphosis.
[Jess] Metamorphosis, when one thing becomes another.
[male announcer] Metamorphosis. When we talk about growth,
we usually think of growing up. Metamorphosis.
But the long road of growth is not just up.
If it were, it would mean that this little baby
would someday look like this. [♪♪♪]
This process is a truly mind-boggling phenomenon
shared by over 65% of all animal species on earth.
The majority of which are insects.
[male announcer] Insects develop from young to
adult in three different ways. They all start from eggs,
of course. Silverfish illustrate
the first method. The young are much like
the adult, except in size. There is no metamorphosis.
[Jess] So many insects undergo metamorphosis
that it would be easier to count those that don't
than those that do. Two common insects you may be
unlucky enough to see regularly that don't metamorphosize are
silverfish and bristletails. There are two broad categories
of metamorphosis, complete and incomplete.
[male announcer] In complete metamorphosis,
which means no basic change in structure from nymph to adult.
[Jess] Incomplete metamorphosis is when an insect changes
in several stages called instars or nymphs,
depending on the species. These changes are often
very gradual and some species can have
up to 15 nymphal stages. They start as eggs,
then they hatch, and at the end of each stage
they molt. During molting, the insects
separate from their cuticle, their equivalent to skin.
They begin to secrete materials that will form their new cuticle
while their old one eventually disintegrates
and they break out of it. They're soft at first,
but their new cuticle hardens up within a few hours. Duri
ng this process they gain new features. Cockroaches and crickets,
for example, gain wings through molting,
and their sexual organs only become functional
after their final molt. [male announcer] The young,
called a nymph, has no wings.
It gets wings in its final molt. Because of this
gradual development, adult insects that undergo
incomplete metamorphosis tend to look like they're babies
but with extra features. Like a Pokemon's final form,
except for maybe Magikarp. Complete metamorphosis
is where things start to get really interesting.
[male announcer] Complete metamorphosis
shows four distinct changes from egg to adult. The cycle starts when,
for example, an adult Mexican bean beetle
lays a cluster of eggs. Species that undergo
complete metamorphosis go through four distinct phases:
egg, larva, pupa, and adult. Each one of these life stages
serves a specific purpose. The goal of a larva is to eat
as much as possible so that it can grow and develop,
while the goal of an adult is to reproduce.
In fact, in some species the adults don't even eat.
They spend their entire adult lives trying to reproduce,
fueled by the energy stored by their larval stage.
[male announcer] Larvae hatch from the eggs.
The larval stage is mainly a feeding stage.
[Jess] Goliath beetles can live for up
to two years in the larval stage and get particularly swole
in preparation to pupate. [man] Now this one is 33 grams
exactly now. [Jess] The adults generally lay
their eggs near a food source so that when they hatch
the larvae can quickly begin to stuff themselves with as much food
as they can eat. As they grow they molt.
And once they've developed enough,
they pupate. [male announcer]
When all of the black widow's eggs have been eaten, the maggots pupate.
[Jess] During this stage, they develop into their
final form, adults. Some insects, like moths,
form cocoons out of silk or poop to protect themselves
as they go through this radical change.
Butterflies, on the other hand, don't form protective cocoons
and instead their exterior forms a hard chrysalid.
And like butterflies, they can be breathtaking.
Some even look like gold. [male announcer] The chrysalis
is a resting stage during which the animal changes
into a beautiful butterfly. During this stage,
the larvae release enzymes that dissolve their own tissue.
Once broken down, the adult structures
begin to form out of tiny clusters of cells called imaginal discs. Thes
e imaginal discs are cells that are
present when the larvae are born. They're attached to their
epidermis by a thin stalk and remain dormant
throughout their larval stage. But once awoken
during the pupil stage, they go to work.
What they're able to do is almost too fantastical
to be true. Once they're activated,
the cells begin to divide very quickly to make more cells, eventually building
entire structures, like wings, in just a few days.
Even more fantastical is the fact that these elements are
developed independently of one another
and then attached later. So a wing may be grown in one
area and then moved into place, somewhat like assembling a car.
You don't have to build each part where it's going to end up.
So a leg could be formed near the head
and then moved down to its thorax.
Even, even more fantastical is that some butterflies
can remember things from when they were caterpillars.
[male announcer] A caterpillar is a growing up stage
in the life of a moth or a butterfly.
[Jess] A group of scientists did an experiment
where they would emit a particular smell
and then administer a small shock to a caterpillar.
The caterpillar would learn to avoid the smell in the future.
But the surprising thing is that after the caterpillars went
through their metamorphosis, in which their bodies are completely dissolved
and reformed, when presented
with the same smell, the adult moths would freak out.
This suggests that parts of their nervous system
have continuity through their radical body change.
[male announcer] By the way, do you know how to tell
a butterfly from a moth? Well, the best way is to look closely at the
animal's antenne or feelers. Roughly 98% of all insects
undergo metamorphosis, but it wasn't always this way.
For millions of years, ametabolic animals,
or animals that don't metamorphosize,
reign supreme. These early insects hatched
as miniature versions of the adults,
but that all started to change These new insects
hatched with bodies that were radically different
than their adult forms. Not only did
they look different, but they behaved
differently too. They ate different food and
occupied different niches. Soon they started
to change their shape to best exploit their resources.
For example, some newborns began to look more like worms
in order to dig better and find meals underground,
while the adults became larger and evolved tools
for defence against predators, better vision for flying,
colour vision to find the tastiest food and so on.
Since insect generations are so short,
new adaptations can evolve very quickly.
Eggs begin to hatch earlier and earlier.
These prenymphs got better and better at fending for themselves
and didn't need to stay in the egg as long.
These became known as larvae. Though despite how adept these
new larvae were at finding food and surviving inside the egg,
they still lack the sexual organs they needed to
reproduce. Instead of a slow, gradual transformation,
gaining sexual organs as they matured,
these insects underwent brief periods of accelerated change.
What we now know as pupation. With their various life stages
able to dominate different niches,
ametabolic insects who could only specialize
in one area and had to compete with
their own young for survival were pushed out of the market.
Insects aren't alone. Amphibians and some fish
also undergo metamorphosis. [Frog chirping]
[Jess] Frogs, salamanders, and all other amphibians
can live on land, but they lay their eggs in the water.
[male announcer] The adult toad spends most of his life on land,
but when spring comes he returns to the pond
for the mating season. [Jess] When the eggs hatch,
the babies emerge with external gills
and some fish-like features like fins and a lateral line,
an organ that detects vibrations in the water
and changes in water pressure. Frogs and toads
begin as tadpoles, and as they grow, their legs
appear under their gill sacs and their mouth become larger.
They start to transition from vegetarian to carnivore
and their intestines lose 75% of their length as a result.
They lose their lateral line and instead develop binocular vision
which gives them depth perception.
Their tails stay around a bit longer than the rest
but ultimately are lost. Depending on the species,
this transition from legless vegetarian swimmers to hopping
carnivorous amphibians can take between two and
eight months to complete. [♪♪♪]
[Jess] Some sala manders,
like the axolotl, don't undergo a full metamorphosis
and stay in their aquatic form instead,
retaining their external gills throughout their lives.
Amphibian metamorphosis is controlled by a single hormone
called thyroxine, and we still really don't know
how it works. This single hormone
tells tail muscles to die, tells limbs to grow,
tells intestines to shrink, and makes lateral lines
completely disappear. All that from a single hormone.
I'd like t o know more about
different kinds of hormones. [Jess] Some species of fish
also undergo metamorphosis controlled by thyroxine,
though not nearly as many fish
metamorphosize as insects as their environment
doesn't change all that much. That said, the fish
that do change, change their strategies
as they mature. Flatfish are born symmetrical
and swim around like regular fish.
But as they age, their eyes slowly start migrating
to one side of their head so that they can eventually
stop swimming and spend the rest of their
lives on the ocean floor. Eels have four stages
of metamorphosis and their larvae are hands
down the most mesmerizing larvae in the animal kingdom.
In some species, they can stay at this stage
for over two years, looking for nutrients as they grow and develop.
Salmon undergo metamorphosis because they are anadromous.
[male announcer] They battle their way upstream
against swift currents seeking quiet waters
where they may spawn. [Jess] Their lives begin
in fresh water, then they go out to sea,
and finally they return to fresh water to spawn and die.
They start out as alevins. During this stage,
they survive off a yolk sac that is attached to them
out of the egg. When they finish the yolk,
they swim to the surface, fill their swim bladders
with oxygen and begin to look for food.
In this stage, they're called frys.
As the frys migrate to the ocean,
they get larger, grow scales, and turn silver.
They mature in the ocean before returning to the stream
from once they came. They stop eating and begin to
make the struggle upstream. Once they reach
their destination, the males develop a hooked nose
to fight each other for mating rights.
After they spawn, they die. Metamorphosis
is how most things on this planet get by. Yet, despite this,
it's far too fantastical to describe it as common.
[male announcer] The astonishing change
that takes place in this phase of the life cycle is known as metamorphosis.
I'm new around here, so I thought I would
take a really quick second to introduce myself a little bit more. I'm Jess Keating, and yes,
I get "just kidding" all the time.
I'm a zoologist, illustrator, and author.
I've written eight children's books so far
with five more on the way, all about science, nature,
animals, and badass women in S.T.E.M.
And I'm so happy to be hosting Animalogic: Second Nature
along with Danielle. Once question I get
all the time is, what's the secret to writing?
The bad news is there is no secret.
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So what should we talk about next?
Please let us know in the comments down below,
and do not forget to subscribe for new episodes of
Animalogic: Second Nature every other week.
Thank you so much for watching. [female announcer]
The principles of growth and development
we've seen in this film apply to all human beings.
Now let's begin our discussion by answering the first question
from the preview committee. Who can tell us when
human growth begins? [♪♪♪]