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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.
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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.
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