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Today we're going to take a closer look at brains, how animals use them, and how some animals have even evolved to lose them! It turns out a brain (and intelligence more broadly) isn't easy to define, but what we do know for sure is that brains have evolved over time in response to the challenges in an animals' environment. And what we'll find is that sometimes it's much smarter to have a tiny, simple brain than a big, complicated one!

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#CrashCourse #Zoology #PBSNature
There are lots of big questions out there that we just don’t know the answer to.

Like how we store and retrieve memories, or why we sleep and dream. But alongside those complex issues, there are some deceptively simple questions thousands of years of science and observation haven’t been able to answer yet.

Like, what is a brain???!?!?!? We humans love to focus on our brain power and our ability to use complex tools and languages, solve problems, and grapple with deep philosophical questions. But those might not be important tasks for other animals.

And we might think of big-brained animals like primates and dolphins as being the smart ones. But you don’t need a big brain to do a lot of smart things like recognize your family or trick your prey. And we’ll even see how some animals literally lost their minds, opting for a simpler but no less evolved lifestyle.

I’m Rae Wynn-Grant, and this is Crash Course Zoology. Brains look different across the vast wildness of metazoa, but for the most part animals have some sort of nervous system which coordinates the actions of the other systems in the body. And we can think of a brain as the mission control center of the nervous system.

Brains are organs, or groups of tissue or cells that all do the same thing, that are made up of big bunches of nerve cells called neurons. Neurons send and receive information from other cells using electrical signals and process that information in order to respond in some way, like to move. And they have two main parts that help them with all that information: cell bodies and tails called axons.

Groups of neuron cell bodies that work together as a unit are called ganglia. And bunches and bunches of interconnected ganglia -- plus a few other things to hold everything together -- are what make up brains. The nerve cells in ganglia send instructions from the brain to other cells, like to tell muscles to contract.

Though how a bunch of ganglia becomes a cooperative network of millions of cells is still a mystery! On the other end, a bunch of neuron axons is called a nerve, and nerves can reach throughout the body, in order to send information to the brain when they sense things like pain, light, or sound. And for the vast majority of animals we know about in 2021, brains -- or at least some kind of nervous system -- are pretty fundamental to two key animal traits: multicellularity and movement.

A nervous system keeps the sometimes millions and millions of cells in touch, and coordinates their actions to digest food or wag a tail. Just like a skeleton, nervous systems are part of the scaffolding that builds an animal body. But since brains are… squishy… it’s hard to get a lot of information about how they developed from fossils -- squishy stuff usually gets broken down long before fossils form.

So instead we can examine early diverging clades, which are groups of animals that last shared a common ancestor with other animals a very long time ago, as a window into what early nervous systems might’ve looked like. The simplest -- but also the most mysterious -- nervous system belongs to Phylum Porifera: the sponges. Sponges don’t have neurons, yet somehow they can still coordinate their cells to squeeze water through their bodies, or close up to shield themselves from predators.

Instead, we think Porifera cells can send electrical and chemical signals to each other...somehow. We’re still working on how. Then between whatever not-really-a-nervous-system sponges have going on and animals with heads full of brains more like ours, we have animals like Ctenophores and Cnidarians -- the phyla of comb jellies and jellyfish, respectively.

Like sponges, neither comb jellies nor jellyfish have a brainy mission control in their head. But they do have a diffuse network of ganglia and nerves spread throughout their body, called a neural net. While a neural net seems like the next evolutionary step up from nerve-less sponges, remember we’re still trying to figure out the animal family tree.

So if once and only once upon a time the last common ancestor of all metazoans evolved a nervous system, that would mean all animals inherited the same basic nervous system. So studying Ctenophores could help us understand early stages of nervous system evolution. For example, it would mean that ancient sponges probably had a more complex nervous system like Ctenophores in the past, but lost it at some point.

Or maybe nervous systems evolved twice: once in the lineage containing cnidarians and bilaterians, and once in ctenophores. Which is a compelling theory because the ctenophore nervous system is just so strange. Like they don’t have the genes or proteins to make most of the neurotransmitters, the chemical signals that neurons send to each other, that all other animals use, like dopamine and serotonin.

And if Ctenophores came up with a nervous system all on their own, it leads to some interesting questions about who were the first animals to diverge from the rest. Which until recently we were pretty sure were the sponges. So finding an answer could reshape the entire animal family tree!

We do know that central brains, bundles of ganglia stuffed into a head, show up on the bilaterian branch of the tree, which are all the animals with mirror image symmetry. All but 4 of the 35 phyla are on this branch, so there’s a huge variety of bilaterian brains! Like arthropods are bilaterians that pack a lot of brain power into a tiny space.

For example, insects have a brain in their head, pack more ganglia under and behind their esophagus, and have a few more distributed throughout their body. Other bilaterians like Chordates and Cephalopods, tend to be larger, and can have much larger brains. At least 80% of brain size is determined by an animal’s body size, but that last 20% that comes from things like sex, age, and genetics can tip the scales unexpectedly.

So to compare brains, zoologists can use something called the encephalization quotient or EQ, which is how much bigger or smaller an animal’s brain is compared to what we expect for its body size. An animal with a brain that’s the expected size has an EQ of 1. A mouse has an EQ of 0.5, meaning it’s brain is only half the size that we’d predict.

Dolphin’s have brains about 4 times as big as we’d expect, so their EQ is 4. Of course, we devised the system, and we calculated a species’ predicted brain size using data just from fellow mammals. So maybe it’s not surprising it makes us look good -- humans have over 7 times more brains than we’d predict given our body size!

But other phyla, which have brains that might be wired in fundamentally different ways, could be playing by different brain-to-body-size rules. Like octopuses have a large central brain, but two-thirds of their neurons are in their super-smart arms! Intelligence, which to us humans means being able to acquire knowledge and skills and do something with them, is tricky to talk about and even trickier to measure -- especially in non-human animals.

Often when we hear about non-human animals being smart, it’s because they can do things that humans do, like paint, open a jar, or solve a puzzle. But lot’s of other non-human animals are smart in their own way and have ample brainpower to do all the things they need to do to survive and thrive. So if we had two animals -- say like an elephant and honeybee -- who’s smarter?

Let’s go to the Thought Bubble! Instead of focusing on human-like tasks, we could test something that both humans and most non-human animals would agree is important: being able to tell close relatives from strangers and adjust your behavior. Under this criteria, a lot of animals pass the test.

Both elephants and honeybees can tell some individuals from others, which is helpful for managing a hive or herd. So this round is a draw. In the second round, we could compare brain size.

Obviously a honeybee’s brain is literally a lot smaller than an elephant’s. But a honeybee's brain is about 4% of their total body mass, whereas an elephant’s is only about 0.18%!! Brain size can be tricky though, because it includes neurons, but also other stuff like water.

So in round three we can compare the smarts of elephants and honeybees by testing their “neurological processing power” -- which is basically counting how many neurons they have. More neurons means more processing power or potential for “intelligence.” A honeybee has 960,000 neurons, while our elephant has 257 billion neurons, so the elephant easily takes this round. Generally, counting neurons works pretty well until we realize bigger brains usually also naturally have more neurons.

In our fourth and final round, we can refine our counts and compare the number of neurons an animal has relative to how big it is. When we do that, honeybees have 200 times as many brain cells, relative to body size, as our elephant with billions of neurons. So who’s really the smart one?

Both the honeybee and the elephant can get around, feed themselves, and manage a complicated social life. The elephant has a bigger cognitive engine overall, but the bee’s engine is much bigger for its body. Thanks Thought Bubble!

Measuring intelligence in animals is hard, because we have to grapple with the fact that intelligence means different things to different animals, and we have to design tests that match. Some animal lineages even seem to have given brains a try, and then decided they weren’t all that great, losing them over time. Like sea urchins, sea stars, and other echinoderms .

Echinoderms of today’s world have radial symmetry, which means they’re symmetric around a central point. But we know they evolved from bilaterally symmetrical animals. And most bilaterians have heads, which tend to get filled with some sort of brain.

So ancient echinoderms probably had brains, but modern ones have simple neural nets running from each arm to their mouth in the center. A less complex nervous system was all that ancestor needed for their filter-feeding lifestyle. And for animals that don’t need to do a lot of thinking or other complex tasks, having a big brain isn’t only pointless, it can be a liability!

Brains are incredibly active organs and need fuel to function. Our brains are only about 2% of our body weight, but use about 20% of our energy. That’s a lot of energy to sink into an organ you might not need.

Like every other part of an animal, brains and nervous systems evolve over time in response to the challenges in an animal’s environment and lifestyle. And sometimes it’s much smarter to have a tiny, simple brain than a big, complicated one. Next episode, we’ll look into one of the senses that the brain uses to gather information about an animal’s environment… see you there!

It’s sight. We’re going to talk about sight. Thanks for watching this episode of Crash Course Zoology which was produced by Complexly in partnership with PBS and NATURE.

It’s shot on the Team Sandoval Pierce stage 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.