So, if you’re a parasite, you can get a bunch of babies out into the world and still have a good chance of them finding a new host.
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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.
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. So, if you’re a parasite, you can get a bunch of babies out into the world and still have a good chance of them finding a new host. As babies grow, the benefits of the big host become increasingly attractive from an evolutionary perspective, and so the cycle of infection continues. If the small host is prey to the big host, this transition is easy, as seen in the example of a snail and a frog. However, there is a possibility of a missing step, known as a ’trophic vacuum’, where the parasite is left without a host. To make up for this, a third host may be used, like the flatworm that jumps from the snail to a fish first, before eventually reaching its intended destination, the bird. This could be the result of mutations, luck, and the competition to reproduce, and while there are some risks involved, such as being washed away, squished, or the host developing a resistance, the use of multiple hosts is still impressive.
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