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  • Thank you to KiwiCo for supporting PBS.

  • As a wildlife ecologist, I've spent hours following bear tracks around Nevada and New

  • York, hoping for a glimpse of these furry, four-legged, foragers.

  • Though, if we look across the whole animal kingdom, we'd find all kinds of bears.

  • Bears with four legs, eight legs, six legs, or no legs!

  • Some weighing over 1000 pounds, and others that are microscopic with very few, or even

  • no, organs at all

  • Even bears that can walk on paws or suction disks, fly, or sway in the ocean currents.

  • I'm talking about grizzly bears, but also water bears, bear moths, and pore bearers.

  • It's not always clear why such a diverse array of animals became associated with an

  • Old English or proto-German word for a brown creature...  [or why we thought that pore

  • bearer pun was a good idea!]

  • Like water bears look kinda like 8 legged grizzlies.

  • But bear moths spend most of their lives as fuzzy caterpillars, and pore- bear-ers is

  • a translation for the phylum Porifera that just contains sponges.

  • None of those look like bears!  

  • Turns out, “bearscome in all sorts of sizes and body plans, and vary a ton in how

  • they move

  • And we'll see that even though animals can look very, very different on their surface,

  • there are surprising similarities in how they've evolved to solve major problems -- like how

  • to support and move their bodies

  • I'm Rae Wynn-Grant, and this is Crash Course Zoology

  • INTRO

  • An animal's size and body plan shape their entire lives -- what they eat and how much,

  • how they move, where they live, and their place in their environment.

  • Like the grizzly and black bears I study carry their 1000 pounds of muscle and fur around

  • on four paws.

  • They spend most of the fall fattening up for the winter, and unlike most four-legged mammals,

  • they can stand up and sit like us!

  • But animals come in a huge range of sizes, and most animals are actually very small compared to us.

  • A human like me weighs about 59 kilograms, which is about 59,000 times more than a house spider.

  • And a blue whale can weigh 100 million times more than that spider.

  • Even the animals that get big as adults -- and bybig” I mean more than a couple inches

  • long -- spend a good part of their life being very small, undergoing drastic changes in

  • body shape and lifestyle as they grow.  

  • For instance, some fish, amphibians, reptiles, sponges, corals, and mollusks gain both weight

  • and length for their entire lives -- this is called indeterminate growth.

  • Some animals have periodic growth, alternating between growing fast and slow (or not at all).

  • Like animals with exoskeletons, or hard outer skeletons.

  • During molting when an animal sheds its old skin or shell, these animals actually grow

  • their new exoskeleton under their old one and then inflate it with fluid before it eventually

  • hardens into their new larger size!

  • Other animals experience predetermined growth and stop growing when they hit a more-or-less

  • maximum size.

  • But once any animal reaches a certain size, they hit some physical limitations.

  • Animals thicker than about 1 millimeter need extra plumbing like a cardiovascular system

  • to move oxygen and waste around their bodies.

  • Bigger animals also need to eat a lot more to feed their 1000s or millions or even trillions

  • of cells

  • And big animals need more structure like bones and muscles to support them as gravity pulls

  • on all their weight.

  • That's why the biggest animals, the whales, live in the ocean -- the water supports their

  • bodies instead of legs

  • Growing also means making new tissue, and animals have evolved a few different solutions.

  • Many clades, or groups of animals with a common ancestor, add more cells.

  • Other animals grow by making each cell bigger but keeping the same number of cells, a trait

  • called eutely.

  • Other animals like echinoderms use a weird process called maximal indirect development

  • and grow their adult form out of a special ball of cells that have been set-aside

  • The larval or immature form is mostly made up of cells that already have all their development

  • planned out for them, and a set amount of growing they're going to do.

  • But in the larvae, there's also a small amount of set-aside cells that take over once

  • the other cells get old and die off.

  • Then the ball of set-aside cells develop into the many cell types needed to create the adult

  • form.

  • But the growth that probably seems the wildest to us humans is colonial growth when animals

  • get bigger by adding more complete, individual clones

  • These colonial animals, like Siphonophores and Bryozoans, are made up of tons of little

  • clones that work together, sort of like how a school of fish can coordinate and swim together

  • But even though animals can grow in completely different ways, a lot of them can look quite similar.

  • Basically, some body designs show up again and again.

  • Distantly related animals evolving similar traits independently is called convergent

  • evolution, and it usually happens because different lineages face similar problems in

  • their environment or take on similar ecological niches.

  • One of the most stunning examples of convergent evolution is carcinization, a process that

  • zoologist Lancelot A. Borradaile famously defined as "the many attempts of Nature to

  • evolve a crab.”

  • Let's go to the Thought Bubble.

  • It all started with the cycloids, a group of arthropods that lived from the Carboniferous

  • to the Cretaceous era.

  • They had that flat crabby shape, a small abdomen, and a bunch of walking legs just like today's crabs.

  • It wasn't until the early Jurassic period, tens of millions years after the first cycloids

  • were around, that the first of what we think of as real crabs -- members of infraorder

  • Brachyura -- showed up

  • A little after that is when the fake crabs started showing up, with things like this

  • early squat lobster all looking very crabby.

  • And the crab fad kept happening over the Mesozoic era -- now we have hermit crabs, hairy stone

  • crabs, horseshoe crabs, crab lice, and king crabs.

  • None of which are descended from Brachyurans and so none of which are actual crabs!

  • One way you can tell is that mostfake crabshave six walking legs instead of eight!

  • But why?

  • Probably becausecrabis a great body plan.

  • It's tough and adaptable to life on land or in water, and their flat and round bodies

  • fit into more places than a long lobster tail might.

  • So crab-shaped animals have more evolutionary fitness, which means they tend to survive

  • and pass on their genes more than non-crab shapes

  • The real kicker is that the cycloids -- the ones who first came up with the crab body

  • plan -- died out in the Cretaceous era, at a time when there were real crabs and fake

  • crabs all over the place.

  • One hypothesis is that the cycloids got outcompeted -- out-carcinized, out crabbed -- by both

  • the crabs andcrabswe know today!

  • Thanks Thought Bubble!

  • Convergent evolution pops up when a similar solution works in different environments for

  • different lineages.

  • Animals' bodies evolve to better suit a function -- even if it means turning into a crab.

  • Now it's important to remember that evolution has no set goals besides passing on genes

  • -- there's nobody plan plan.”

  • Often a simpler form can perform a function much better than a complex one!

  • Cephalization -- evolving a head -- and in some cases, decephalization -- or evolving

  • to not have a head -- are good examples of how sometimes simpler is better.

  • But like what is a head, really?

  • Heads collect the sense organs needed to perceive the world, the mouth, and the nerve cells

  • that coordinate them in the front of the animal, where they can react quickly to danger or prey.

  • We know that the ancestor of all the animals that can be divided into symmetrical halves,

  • which are called Bilaterians for their bilateral symmetry, had a head.

  • And most animals are bilaterians

  • But some animal groups have lost their heads -- literally -- because they became less useful.

  • Like bivalve mollusks, like clams, are bilaterians that stay rooted in one place and just filter

  • water through their mouths to catch bits of food.

  • Which you don't really need a dedicated head for, so thehead pieceslike the

  • central nervous system and sensory organs, are distributed around the clams' body.

  • Other animals with radial symmetry have bodies that are symmetric around a central point.

  • Most of these are echinoderms, and with the exception of sea cucumbers, they don't have

  • anything resembling a head.

  • But heads or no head, bilaterally or radially symmetrical, for all these forms to be possible,

  • they need some kind of structural support -- otherwise everything turns into a blob of cells.

  • Phylum Chordata solves this with a notochord, a flexible rod that supports their body as

  • embryos and sometimes as adults.

  • The notochord develops into the vertebral column, or spinal column, in vertebrates,

  • which is where they get their name.

  • All other animals are invertebrates, and they have several different types of support -- which

  • is part of the reason they don't form just one phylum.

  • In general, skeletons are frameworks that support, shape, and protect soft tissues.

  • When you think of a skeleton, you probably picture an endoskeleton, an internal support

  • structure made of mineralized tissues.

  • Vertebrates have rigid endoskeletons made of bone, which gets its hardness from large

  • amounts of calcium phosphate

  • Invertebrates have endoskeletons made from other materials.

  • Like echinoderms such as sea urchins have endoskeletons made from fused plates called

  • ossicles, which are made of calcite

  • Even sponges have an endoskeleton made of the flexible protein spongin and spicule crystals.

  • But some sponges also secrete an outer skeleton from cells on their skin, which leads us to

  • exoskeletons -- skeletons that sit outside the rest of the body!  

  • Mineralized exoskeletons show up in at least 18 clades, including some sponges, echinoderms,

  • corals, and mollusks

  • Other animals based their exoskeletons on long chains of sugar molecules, called polysaccharides

  • Arthropods like insects, crustaceans, and arachnids use chitin to make their skeletons

  • The third type of exoskeleton is actually made of water.

  • Which sounds rather...flimsy.

  • But hydroskeletons work because water is incompressible -- you can't realistically squeeze it into

  • a smaller volume like you could a marshmallow

  • So as long as animals can contain water in a tube or sac, they've got the makings of

  • a stable structure

  • Invertebrates like worms and jellyfish use hydroskeletons to support their very flexible bodies.

  • It's an especially great adaptation for living deep in the ocean

  • Endo-, exo-, and hydroskeletons -- plus heads when animals have them -- are what give animals

  • their shape.

  • But animals aren't statues like you'd see in a museum -- they move, and how animals

  • move also influences how they look.

  • Some animals move with the help of their environment -- spiders cartwheel down sand dunes, and

  • Velella velella, a jellyfish-like colony of animals, use a sail to catch the breeze.

  • These animals needed to evolve the right instincts and structures to take advantage of their surroundings.

  • Other animals move under their own power with the help of cilia and muscles.

  • Cilia and flagella are hair- or tail shaped parts of cells that beat in co-ordinated waves

  • to paddle microscopic animals forward.

  • And how these tissues connect with the skeleton influences how an animal moves

  • Moving an entire skeleton at once is harder, because they're usually rigid and heavy.

  • Most animals solve this by turning their skeletons into a bunch of levers that pivot around joints

  • as pairs of muscles contract and relax

  • Even animals with hydroskeletons use muscles to control fluid pressure and bend their body

  • Animals move their bodies in all sorts of ways, balancing where they want to go, how

  • quickly, and how much energy it'll take.

  • In fact, there's a whole field of zoology called biomechanics that's interested in

  • how mechanical principles guide how animals are shaped and move

  • But all this moving and growing takes a tremendous amount of energy, and we'll talk more about

  • where animals find that energy in our next episode.

  • Evolution is a wild journey that brings us so many different animals with a huge array

  • of bodies and sizes.

  • That is, until everything turns into a crab.

  • Thank you to KiwiCo for supporting PBS.

  • KiwiCo's mission is to inspire kids to see themselves as makers by providing them with

  • the tools and a foundation to become creative problem-solvers and critical thinkers.

  • The crates include everything you'll need in the box and cover a wide variety of topics

  • from month to month like art, science, engineering, and geography.

  • Inside you'll find the project materials of course, a blueprint (which are the instructions

  • written for kids), and a magazine containing lots of additional content and experiments

  • Go to kiwico.com/crashcourse or click the link in the description for more information

  • Thanks for watching this episode of Crash Course Zoology which was produced by Complexly

  • in partnership with PBS and NATURE.

  • It is shot on the Team Sandoval Pierce stage at Porchlight Studios in Santa Barbara, California

  • and made with the help of all these nice people.

  • If you'd like to help keep Crash Course free for everyone, forever, you can join our community on Patreon.

Thank you to KiwiCo for supporting PBS.

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