The art of eating was passed down from the original animal ancestor to all its descendents,
but exactly what and how that ancestor ate remains mysterious.
Maybe it snagged tiny organisms that got stuck in its pores, like the Poriferans, the sponges.
Or swallowed their prey and then liquified it to circulate through their body with a
canal system like a nutritious smoothie, a la Ctenophores. Mmmm!
We can look to these early-diverging animals for clues. But we know from that metazoan
ancestor evolved all the feeding mechanisms and adaptations that we see in today’s animals.
Now there are animal-eaters, plant-and-fungi-eaters, and both-eaters. But also wood-eaters, bone-eaters,
and decaying things-eaters, each of which requires a suite of adaptations to turn food
into nutrients and energy.
Animals evolved to eat a lot of different things, even stuff that barely passes for
food, and it shapes our entire lives from what we look like to where we live. So grab
a snack because today’s episode is gonna make you hungry! Well, until we start talking
about what happens to food after you’ve digested it.
I’m Rae Wynn-Grant, and this is Crash Course: Zoology.
INTRO
Along with moving, sexually reproducing, and having multiple cells, eating is one of the
key traits we inherited from the original animal ancestor and part of the very definition
of what makes something an animal. Specifically, animals are ingestive heterotrophs which means
we engulf food with our bodies, and don’t make it from non-living sources.
Like I would love to gobble up some fettuccine alfredo right now. But my little desk succulent
would prefer to make its own food by absorbing sunlight, carbon dioxide, and a hint of water.
Of course, even though fettuccine alfredo is delicious, I admit it might not be for
everyone. So to better understand how animals live, we’re interested in two major things:
what animals eat, and how they eat it.
Both “what” and “how” have changed and expanded over time as more animals and
food sources have developed.
One of the oldest diets is carnivory, or eating other animals. Animal-eating animals -- like
those ctenophores we mentioned that liquify their prey -- probably evolved long before
plant-eating animals.
One piece of evidence for this is that if we consult the animal tree of life, the common
ancestors of many phyla are carnivores. Which means the animals in those groups likely came
from animal-eaters. Plant-eaters only show up within those phyla later.
In fact, animals could have been eating other animals for over 600 million years. Which
means carnivory developed many millions of years before plants even existed on land!
Though there could’ve been some plant-eaters in the oceans.
Today about 63% of animals are carnivores, according to a 2019 study by scientists at
the University of Arizona. Which might be surprising considering there are lots of plants
around, and plants don’t run away or fight back… usually.
Herbivory, or eating plants and fungi, requires a new set of adaptations to eek out calories
and to turn plant cells into animal cells. Like flat teeth and special organs called
gizzards to grind ‘em up; specialized gut bacteria to demolish the tough cellulose in
plant cell walls; and a lot of time to chew, re-chew, and digest huge volumes of food.
It’s easier to be a carnivore. Carnivores get a lot of energy from one serving, and
more easily get proteins and fats from their diet. And their intestines push food through
more quickly because they can get away with being less efficient at extracting nutrients.
About 32% of animals are herbivores according to the same 2019 study. And 3% are animals
like bears, crows, and us humans who use omnivory, or eating both plants and other animals.
Having a flexible diet sounds like a bright idea but it’s kind of a catch-22. Omnivores
have lots of food options, but they also have to maintain a lot more biological machinery
than animals that eat just one kind of thing.
But it’s the remaining 2% of animals that really push the definition of “food.”
Beavers, shipworms -- which are a type of clam! -- and insects like termites are xylophagers,
or wood-eaters, even though wood is a terrible food. It’s almost entirely made of hard-to-break-down
cellulose and has very few nutrients and calories.
So like more traditional herbivores, wood-eaters have extra adaptations to get their calories
like their own cellulose-busting proteins. And some wood-eaters cheat by sneaking other
plant parts and fungi into their diet.
But osteophagers take it a step farther and eat bone -- which is pretty amazing when you
consider that bone is basically biology’s best impression of a rock.
There’s a fair amount of nutrients in bone marrow, so animals like bone worms, a group
of deep-sea polychaetes, secrete a bone-dissolving acid out of their mouths to burrow into whale
bones, where they find fats and proteins. Even some herbivores like giraffes and cows
will chew on bones to add phosphate and calcium into their diet, minerals which are hard to
come by in plants.
[Well I’m not going to add “wood” and “bone” to my grocery list,] but termites,
bone worms, and others might’ve evolved such weird tastes because it can be advantageous
to figure out how to eat something no one else can. Even if it requires some bizarre
eating habits.
The animal menu has lots of options. But how do different animals stuff their faces or…
whatever they do to eat if they don’t have a face? Even within one ocean there’s a
huge variety in the way animals eat.
Many whales, fish, barnacles, shrimp, jellyfish and other animals large and small are filter-feeders,
which means they capture food suspended in water or air.
In the ocean, filter-feeders filter water, trapping relatively tiny bits of food in baleens,
modified gills, feeding baskets woven with legs, or if you’re a sponge, holes. Whether
they let the food come to them, or find places where tiny bits of food are plentiful, filter-feeders
don’t bother with hunting any one target in particular; they just gulp in the general
area and let the filters do their job.
But there’s a lot more to eat in the ocean than just tiny food particles. Filter-feeding
is a pretty simple strategy, so we see a huge variety of animals doing it, but to eat larger
and more specialized food items, animals needed to evolve a more specialized structure. Namely,
a head.
Heads can be optional for filter-feeders, but predators like octopuses out there killing
and consuming another organism to absorb its nutrients need a head. They combine the sense
organs that find food and the weaponry that clinches the food in one convenient package.
Dolphins, comb jellies, bobbit worms and other animal-eating carnivores are also definitely
predators, but you don’t have to necessarily be a carnivore to be a predator. Even dugongs,
sometimes called sea cows, and other herbivores can be predators too. They’re plant-predators
if they eat and kill the whole plant or eat things that are future whole plants, like
seeds.
And then there are animals that eat dead stuff. These are scavengers who wait for something
else to do the killing before they dig into the carcass, and detritivores, who eat bits
of decaying plants and animals (and their poop). And these garbage-eaters are really
important, because they recycle nutrients through their animal communities so everyone
can feed again.
How animals eat can intersect in many ways with what they eat. Like predators can be
herbivores, and scavengers can be carnivores or bone-eating osteophagers. And you might
start out eating one thing one way and completely change over time -- whether it takes millions
of years or just a lifetime.
One mode of feeding we haven’t talked about yet is parasitism, which is a special kind
of predation. Entomologist E. O. Wilson put it best, calling parasites "predators that
eat prey in units of less than one.”
Just hitch a ride on (or in) your host and nibble at them forever.
We’ll get you hooked on parasites in a later episode, but as a little appetizer let’s
experience a day in the life(cycle) of a parasitic, filter-feeding, blood-sucker!
Allow me to introduce the sea lamprey, or Petromyzon marinus, a creature so evolutionarily
ancient it diverged from other fish before they’d developed jaws (and fish have had
jaws for a long time: about 500 million years).
Until about 80 years ago, it was impossible to find a sea lamprey here, in Lake Michigan.
But this lamprey’s enterprising great-great-great grandparents found their way inland as human
workers widened and deepened shipping canals in the Great Lakes region.
And ever since, generations of sea lamprey like this one have feasted on trout, whitefish,
and other small-ish lake fishes, latching onto them with their sucker mouth filled with
hooks, like ancient vampires. This sea lamprey is so large compared to the lake-fish, and
causes so much blood loss and damage that they act more like a predator than a parasite
in the Great Lakes ecosystem.
But once this sea lamprey has grown large and become a mature adult by feasting on nutrient-rich
fish blood, she’ll migrate upstream to lay eggs. She won’t live to see her larvae grow
up, or make sure they get enough to eat.
But lamprey larvae can take care of themselves. They’re filter-feeders, snacking on tiny
plants, microbes, and other junk that they trap in mucous in their throat. Eventually
these lamprey larvae develop into parasitic juveniles, moving downstream in search of
the big, tasty prey that nourished their parents years before.
Their whole digestive system shuts down for months as they turn into a vampire and shift
from a detrivore to a parasitic carnivore. Once that’s done, they can really take advantage
of the benefits of feasting on flesh as they mature and get ready to have their own larvae.
It’s the circle of (lamprey) life!
Sea lamprey show how easily animals can flip between filter-feeder, parasite, and predator
based on their life stage and ecological context. They also show what and how an animal eats
-- whether it’s tiny scraps or the finest fish in the Great Lakes -- has a huge influence
on other parts of an animal's life, like how it looks and acts.
But regardless of how they get it, all animals have to digest or break down the engulfed
food in small molecules that can be absorbed by the body. And then, well... it’s poop,
we’re talking about poop.
After the food is digested, waste excretion, which is like taking out the biological trash,
is essential. Otherwise all the bits that animals can’t eat build up, some of which
are quite toxic.
Simple, tiny animals, like Placozoans and Rhombozoans, get rid of their carbon dioxide
and ammonia waste by just pushing it out through their cell membranes and into the environment.
More complex animals have a lot more cells, so they have dedicated systems for liquid
waste excretion -- which can be stuff like uric acid, urea, or ammonia depending on the
animal’s diet and biology.
Then there’s the solid stuff, which can be made up of things that didn’t get completely
digested, like cellulose, mucous, and bacteria. We have to get rid of it at some point, but
some animals vary the timing.
Like eyelash mites don’t poop at all. Instead, they store all their waste in their bodies
until they die, which is after only a few weeks. While this may seem like a horrible
(and gross) adaptation, a few weeks is enough for the mites to grow and reproduce, and they
don’t need to use any energy to grow an excretory system.
And they’re super successful -- basically every human has little poop-filled mites on
their skin.
So just like there’s a range of what and how animals eat, there’s a whole lotta ways
for them to get rid of it too.
So to some extent, we are what we eat. Eating is a fundamental trait to being an animal
that’s been passed down from the earliest metazoans. And the food animals choose and
how they engulf it dictate their role in their environment and even how successful they -- and
their descendents -- will be.
Next time, we’ll talk about a body system that makes you feel hungry, and the head honcho
that coordinates just about everything else in an animal body … the nervous system and
brains!
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.