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There are animals who can swim, walk, burrow, or fly. There are plant-eaters and animal-eaters, and...everything-eaters!

Animals that spawn eggs, win their mates in fierce contests, or partner for life. From that first animal ancestor evolved a staggering variety of species over hundreds of millions of years. Today in 2021 we know of about 1.5 million different animals out there, but there are still so many to discover.  Before we dive into the wild diversity of how animals function, behave, and interact with each other and their environment, let's start with understanding just one.

The animal that best represents all of Metazoa, the animal kingdom.  I’m Rae Wynn-Grant, and this is Crash Course Zoology. INTRO. Zoologists want to know what’s an average animal and what’s a rare animal because it helps us understand what the life of most animals is like and make sense of all the wild variations out there.

Now before we pick the “most animal” animal, we have to decide how we’re going to judge. In statistics we talk about the mean, median, and mode as ways to decide on the average of something, and... I think we can try something similar with animals.  Maybe to find the “mean” animal we try to average out the features of all the animals.

Like add up the number of legs and eyes and divide by the number of animals we included. But then we’d end up with something that doesn't match reality at all, like an animal with a body plan for 3 legs. And that wouldn’t tell us anything about how real animals live.  Or, we could try to find the median animal by ordering them from the first to diverge from other animals to latest.

Then, we could pick one that diverged in the middle. But that’s also hard to do because we don’t know exactly when every animal group diverged. So the easiest move that will still give us insight into how most metazoans live seems to be figuring out the “mode” or most common type of animal.  First, we need to figure out how many different species, or different types of animals, there are.

In fact, generations of zoologists have tried to calculate the total number of species on. Earth, or the global species richness. One technique calculates a diversity ratio, or how abundant one group of species is compared to another.

Like how many beetles there are compared to types of trees.  Counting all the species in an area would be exhausting and probably would take forever. So instead zoologists make really accurate counts of a group of species that they know well, and assume that information is representative of all the animals in the area.  In 1982, an entomologist, or a zoologist who focuses on insects, named Terry Erwin from what’s now the Smithsonian National Museum of Natural History used the diversity ratio technique to estimate that there were 30 million species…of just tropical arthropods. So 30 million species of insects, crustaceans, arachnids...but mostly beetles.

Let’s go to the Thought Bubble. Trudging through the Panamanian forest, Erwin smoked out his quarry from the canopy with the aid of a backpack insecticide cannon. Which doesn’t sound great for the beetles, but the reality of studying animals is that, sometimes, you also study dead animals and it’s often up to the zoologist to decide what she’s ok with, though there are some regulations.

After meticulously examining each arthropod back in his US lab, Erwin estimated over 1,200 beetle species lived in each tree that he sampled from. Then he did the math.  Erwin estimated 13.5% of beetles would be host-specific. This meant about 162 of the original 1,200 species of beetle would only live on the type of tree he sampled.  A hectare of tropical forest can have 40 to 100 species of tree, so Erwin decided his hypothetical hectare would have 70 species on average.  That would mean there would be 11,410 host-specific beetles, plus the remaining 1,038 beetle species that are willing to live in any old tree.

Altogether that’s over 12,448 species of beetles in a single hectare of forest!  Since 40% of arthropods are beetles, Erwin then estimated there would be 31,120 species of arthropods per hectare. He bumped that up by a third to account for species on the forest floor that avoided his pesticide fog to get 41,389 total arthropods.  This was just one hectare of one forest, but Erwin then extrapolated his formula to include other tropical forests. Which is how Erwin concluded there were 30 million species of arthropods in the tropics alone!

Whoo. Thanks, Thought Bubble. And since the 1980s, Erwin’s estimate has been recalculated to make various improvements.

Like not all forest communities are the same -- not all trees host 162 unique species, and not all forests have 70 types of tree.  Aside from diversity ratios, zoologists also use global trends in where species tend to live called macroecological patterns to estimate global species richness. Like that more species live in the tropics than at the poles.  We can also explore the species-area relationship to calculate the global species richness. Like larger areas have more species because there are more unique ecological roles to fill.  And geographical features like mountains, rivers, or oceans can also affect species numbers by isolating populations from each other.

More recently, zoologists are turning to a new technique called DNA barcoding to make their estimates of species richness. Comparing bits of DNA can identify unknown animals or even check if what zoologists think is one species isn’t actually two or three. So using all our different methods from diversity ratios and macroecological patterns to DNA barcoding, in 2021 we’ve counted 1.5 million animal species, and estimate that the global species richness is 8 to 10 million.

And those numbers will change with new information and techniques.  Next, to decide on the “most animal” animal, we have to look across all these millions and millions of animals and figure out which traits are rare and which are common. Like being able to fly sounds so cool, but flight is actually pretty common.  To keep track of all the different traits, remember we use binomial nomenclature and break animals up into a similarity hierarchy starting by grouping animals of the same type together as a species.  At the very top of the hierarchy is the entire animal kingdom, but just below that zoologists collect species in large groups, called phyla or phylum if we’re just talking about one group, based on their evolutionary history and their body plan.  A phylum is like a genre of animals -- they share some key characteristics, but each lineage within a phylum is a little bit different.  Our mode animal probably belongs to the most successful phylum -- which for us means the phylum that’s made it all the way to the top of the Billboard Top 40(-ish) of Evolution and has lots of different species with a wide variety of traits. Diverse phyla are like the mainstream hits: they’re unlikely to go extinct because they have so many species spread across different habitats and niches.

And phyla with just a few species are the hidden indie gems -- much less successful, and much more vulnerable to getting wiped out.  The Top phyla aren’t who you’d think!  Like so far we know there are 65,000 species of Chordates, or animals with a flexible rod to support their body called a notochord, which includes fishes, amphibians, mammals, reptiles, and birds.   That sounds like a lot, but remember the global species richness is between 8 and 10 million. So they’re the pop music of the animal kingdom -- super famous and popular, but only about  0.03% of all animals.  And there are 35-ish different phyla, so despite what we might see and hear, most other animals are… well… non-chordates!  Phylum Arthropoda includes over 1.1 million different species, making them the true rockstars of evolution. Named for their jointed limbs, arthropods have a segmented body covered in a hard exoskeleton and include insects along with crustaceans, milli- and centi-pedes, horseshoe crabs, spiders, scorpions, and other chelicerates.  So to narrow it down and get into what our mode animal looks like, eats, and where it lives, let’s live a day in the life of the “most animal” animal!  Allow me to introduce you to the humble, uh, "mode animal."  As insects are the biggest group of arthropods, our mode animal moves through life on six legs with three body segments, compound eyes, antennae, and a hard exoskeleton.

That still just paints a vague insect-shaped picture,  so the biggest Order within Class Insecta is Coleoptera, the beetles. Beetles diverged a very long time ago, with the earliest fossils being 300 million years old, so they’ve had a lot of time to split off into over 386,000 estimated species, representing about one fourth of all known animals. But all these beetle cousins share a few key body parts, so our mode animal definitely has front wings that form a protective shell.  To find our average friend, one need only lift up the nearest rock or sift through the sand as beetles can live almost anywhere.

With every new habitat comes a slew of specialized niches that beetles have evolved to fill. 
 And with so many different habitats, beetles have grown to eat all sorts of things-  so there’s lots of local delicacies out there to help lots of young larvae store up energy. Our mode animal will be an average joe in a wildly diverse family photo, like a carpet beetle squeezed in between a weighty Titan beetle and a cheery ladybug. But today, our friend is special because it's so average.

Have a great day little beetle! But just because beetles are popular now doesn't mean they always will be! As zoologists pay more attention to things like nematodes and parasitic wasps, maybe one of them will unseat the beetle as the mode animal as we discover more and more species and revise our estimates.   Next time we’ll continue exploring what it means to be an animal and how they’ve evolved into so many different shapes and sizes.  Thanks to Great Courses Plus for Supporting PBS.  The Great Courses Plus is a subscription on-demand video learning service with lectures and courses from professors from top universities and institutions.

Through your subscription, you get access to a library of lectures about anything that interests you...science, math, history, literature, or even how to cook, play chess, or become a better photographer.  One new course I recommend to help round out your knowledge of the biosphere is Plant Science:. An introduction to Botany hosted by Dr. Catherine Kleier.

In this course you’ll learn how plants are different (but sometimes still surprisingly similar) to animals, you’ll learn about what came first - pollen or the pollinator - and the story of how animals and flowers coevolved, and you’ll even explore a day (and year) in the lives of plants!  To learn more, click on the link in the description below to start your trial today. 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.

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