Thanks to Babbel, one of the top language
learning apps in the world, for supporting this SciShow video. As a SciShow viewer, you can use our link
to grow your language skills with Babbel for up to 60% off and a 20-day
money-back guarantee. Some parasites take a truly unnecessary-seeming
number of steps to live out their lives. Like, needing to bounce between three whole
host organisms just to reproduce. But it turns out, there is some logic to it. Logic that might even make you appreciate
parasites a little more. Maybe it has to do with upgrading your living
space, for example. And maybe it has to do with eating poo. [♪ INTRO] Let’s start with an example to set the scene. Take this parasitic flatworm here. It’s a little smudge of an animal that lives
in Southern California and infects snails. But if they only ever infected snails, they’d
go extinct. Their life cycle goes like this: First, the
eggs get eaten by horned snails. The eggs hatch inside the snail, infecting
it. Then, larvae leave the snail and swim around
until they find and infect a fish. They then have to do this ridiculous thing
where they literally mess with the fish’s brain until it gets
itself eaten by a bird like a heron. Then and only then can these little parasites
finally reproduce and begin the cycle all over again, starting
back with the snails. It’s the weirdest, grossest hero’s journey
ever. Why would such a complicated life cycle ever
evolve? You would think it wouldn’t work. But there must be something to it, because
a lot of parasites do this exact thing. But there is logic here. The easiest way to figure it out might be
to imagine a hypothetical parasite that just has one
host and talk about that. Let’s make it a flatworm, since a lot of
this research deals with them specifically. Let’s make it a big host, like a bird or
a deer or something like that. Big hosts are good because, well, they’re
big. Flatworm parasites that live inside bigger
hosts tend to be bigger. And that makes sense, right? You’ve got more living space to stretch
out. Bigger hosts tend to eat more calories per
day, so that’s more calories for the parasite. Plus, a lot of bigger animals are warm-blooded,
and it’s thought that parasites may be able to grow faster in warm-blooded
animals, since it’s warmer in there. Finally, bigger animals tend to live longer
than small ones, which means the parasites can live longer, too. Like, if you infect a fly, you’ve only got
the rest of that fly’s lifespan to make good. So big animals let parasites live longer,
grow bigger, and have more energy. And you know what that means? It means more baby parasites. Yayyyy! More babies than you might get if you lived in a cold, cramped, snail that died in a year. So this makes big animal hosts look really
good for parasites, from an evolutionary point of view. But there is a problem. In order to carry out their life cycle, parasite
offspring need to travel to new hosts. And while big animals may be great to infect, they’re also usually much rarer than small
ones. There might be, like, a million snails in
a swamp, but only a couple thousand birds and maybe
a dozen deer. So any given egg or larva might be waiting
a long time for a new big host. So long, babies. Furthermore, you may have to hope that a prospective
host eats poop, on purpose or accidentally. Poo is often how parasite eggs or young leave
the original host. So not only are your babies waiting for the
one big deer to come along, it’s also likely not going to be interested
in their way of getting in the door. So it’s almost a catch-22. Big hosts can help you produce a bazillion
babies, but it doesn’t do you any good to produce a bazillion babies if they all
die before they can find a new host. That’s where small hosts might be good. Small hosts, like snails, are more common and often more likely to eat a parasite’s
young. The parasite’s eggs and offspring are closer
to the size of their normal food, and many are not quite as picky as the big
hosts are about what they eat. Of course, as those babies grow, that big
host with all its benefits starts to look really good again, from an
evolutionary perspective. So to complete the cycle, you’d then need
to get back into the big host. This can be easy if the small host is prey
of the big host. Like if you’re going between a snail and
a frog, for example. And this is often how it works. Not always, though. There can be a missing step, what scientists
call a trophic vacuum, but that doesn’t mean the parasite is out
of luck. Enter the third host. Like, for example, how that flatworm from
earlier jumps from the snail to a fish first, with
the bird coming later. Using an intermediate host might turn one
virtually impossible jump into two reasonable ones. Plus, parasites can have some extra tricks
as well. That Californian flatworm actually does this
really weird thing where, after infecting the fish, its larvae actually
worm their way into the fish’s brain and make it shimmy and jerk at the water’s
surface, making the fish easier prey for the bird. Which, remember, is where the parasite wanted
to end up all along. As to the how of how this evolved, it may
have been a case of the parasite starting in one host and then evolving to survive in
either the host’s prey or predator. So if the host gets eaten, for example, the
parasite still has a shot. And it’s worth pointing out that parasites
aren’t planning all of this. Evolution works through mutations, luck, and the hard math of who gets to make babies
and who doesn’t. If it works, it works, even if it seems convoluted. There are some tradeoffs to jumping between
so many hosts. With every jump, the parasite risks getting
washed away by a current, or squished, or the next host developing some kind of resistance, or maybe they’d just run out of time before
a new host came along. Because of this, going above three hosts in
a life-cycle seems to be pretty rare. But as long as you don’t go over three,
these complicated life cycles do happen. Big hosts let them grow bigger and have a
lot of babies, but it’s easier for those babies to infect small hosts, and sometimes,
you need an extra leg up in between. Parasites often get a really bad reputation
in biology. But it’s worth admiring them and their evolution. And a lot of animals change their homes during
their lifetime. Frogs metamorphosize from living in water
to land, and hermit crabs swap shells. And if you’re a parasite, a host is just
a different ecosystem. And evolving to live in multiple ecosystem,
after all, is pretty impressive. This SciShow video is supported by Babbel, the #1 language learning app in the world. Babbel lets you learn any of their 14 available
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