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Nearly every environment on Earth — and that includes animals — is home to some kind of parasite. Today we’re going to talk about parasites not just because they’re an essential part of their ecosystems, but also because by studying them we can learn more about their hosts and even their shared evolution and ecology. So stick with us, this one is going to get a little weird!

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#CrashCourse #Zoology #PBSNature
Allow me to introduce the best animal out there -- well, at least if you ask me.

The American black bear roams most of North America from Alaska to Mexico. As we watch our bear friend go about her day, we’re actually looking at two ecosystems -- the forest and the bear herself!

Let’s live a day in the life of an American black bear (and her parasites). An ecosystem is a community of organisms interacting with their physical environment. Like most wild animals, this bear is home to parasites, which are animals that evolved to live on or in other animals, causing their hosts harm.

On her body she has ectoparasites which spend their lives outside the bear. A whole bunch of lice, fleas, and ticks crawl around in the forest of the bear’s fur, sipping from the streams of blood that flow just below the surface. Occasionally a mosquito or even a vampire bat may visit when it gets dark out and maybe spread a disease if she’s really unlucky.

In her body are endoparasites who have evolved to spend their lives inside another living thing. Our bear might host some platyhelminthes, like tapeworms she got from eating infected fish or rabbits. These worms find their way to the nutrient packed intestine and the muscles where they latch on and suck up nutritious bear juices.

She might also have another wormy phyla, nematodes, in her intestines, or in rare cases other nematodes called lungworms. These are just some of the at least 31 parasite species that live in and on black bears. And we haven’t even touched on non-animal parasites like protozoa or viruses.

But as she goes about her day, the effects of her parasites are probably pretty mild and she can live a normal life. Most bears infected with parasites look and act perfectly fine, even when they have several uninvited guests -- as long as there aren't too many. Have a nice day Ms.

Bear...and “friends”! Parasites are an essential part of their ecosystem, whether that ecosystem is a forest, a stream, or the inside of a bear. And studying them teaches us not just about the parasite, but also its host and even their shared evolution and ecology.

So stick with me, it’s probably going to get pretty weird. I’m Rae Wynn-Grant, and this is Crash Course Zoology! Nearly every environment on Earth -- and that includes animals -- is home to some kind of parasite.

Some individual animals have a lot of parasites, and some carry very few. We call this parasite load, or the amount of parasites an individual carries in or on its body, which varies a lot between animals. Formally, parasitism is a symbiotic relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm.

The host provides the parasite with food, or safety, or a way to have offspring, or sometimes all three. And in return, the parasite does nice things like steal nutrients or wreck their host’s reproductive system. Like, they literally bite the hand that feeds them.

So the host isn’t really getting anything out of the deal. Which is why parasites are sometimes described as “predators that eat in units of less than one.” But parasites aren’t quite the same as predators who kill and eat other animals. For one, they’re usually much, much smaller than their hosts, and they usually try to keep their hosts alive.

They also interact with their hosts a lot more than a predator interacts with its prey, sometimes for years at a time! Parasites are super specialized and structurally adapted for their exploitative lifestyle. While a lot of parasites have lost traits, because they’ve adapted to rely on their host to fulfill some of their needs, they’re so much more than that.

Parasites are a super diverse group of animals, and there are lots of ways we can split them into categories. One way is by where they live -- so whether they’re endoparasites inside the body or ectoparasites outside the body, like we saw with our bear. We can also categorize parasites by how they find their host, though some animals fall into more than one category, or change their strategy over their life cycle.

Those ticks and lice that were on our black bear are directly transmitted parasites. They find their hosts on their own, like by waiting in tall grass and hopping on as the bear walked by. And directly transmitted parasites usually have a lot of the features we see in non-parasitic animals, like being able to move around on their own.

There are also trophically-transmitted parasites, like roundworms, that spread by getting eaten. A lot of these parasites use being eaten by one animal, called an intermediate host, to travel to its primary host, where it reproduces. Like the larvae could burrow into the muscles of a fish or rabbit, and begin growing when they sense the proteins and acid in the gut of their primary host, the bear, after being eaten.

And lots of bacteria and viruses, as well as parasitic worms called helminths are vector-transmitted parasites. They get into their proper hosts with the help of another organism called a vector, that transfers the disease-causing agent from one organism to another. A lot of the blood-feeding animals fall here too: Like the mosquitoes hovering around our bear could be vectors for the West Nile Virus, ticks can transfer the bacteria that causes.

Lyme disease, and so on. Which brings us to another way we can categorize parasites: how they get energy. All the mosquitoes, ticks, and leeches that nibble at their host without killing it outright are showcasing micropredation.

Usually they don’t do too much damage by themselves, but sometimes numbers or mismatched ecologies can make micropredators become “capital P” predators, like we saw with sea lamprey back in Episode 4. Invertebrates, who you've probably noticed by now are waaay more hardcore than us chordates might use parasitic castration. This is when a parasite wrecks its host’s ability to reproduce so that the parasite can suck up all that energy that would have gone to making eggs or finding a mate.

Animals normally use a lot of their resources to reproduce, so that’s an even bigger free lunch buffet for the parasite! But I guess at least the host gets to go on living? Which isn’t always the case.

Parasitoids, just end the actual life of their host, which is a pretty bold move for a parasite. Most parasitoids are insects like many wasps, that lay their eggs on host larvae. Sometimes the larvae develop inside the host, sometimes outside.

Sometimes the host is immobilized once it’s infected, and sometimes it keeps going along its business until the larvae kill it. Either way, the parasitoid sucks energy from its host until there’s nothing left. And don’t think parasites are safe from other parasites!

There are parasitoids that attack the larvae of other parasites which is called hyperparasitism. One example is the cabbage white butterfly, which is attacked by two species of parasitoid wasps. The larvae of those wasps are attacked by another wasp.

So you get a moth infected with a parasite that’s itself infected with another parasite. It’s basically like a parasite turducken situation. Since parasites and their hosts share such a close relationship, we might wonder how evolutionarily related they are.

Let’s say we wanted to make a taxonomic sandwich where the host is one piece of bread and the parasite is the other, and the evolutionary time between them is the filling. Like take E. perplexa the parasite and E. perplexa the host. Their sandwich is...just two pieces of bread!

Encarsia perplexa are parasitoid wasps who are like self-parasites. E. perplexa usually parasitizes the eggs of citrus blackflies, but when there’s no male wasps around, unmated females will lay unfertilized eggs inside larvae of their own species. True to form, the new baby E. perplexa cannibalizes its cousin to grow up.

Since these wasps have a haplodiploid sex determination system, where sex depends on the number of chromosome sets an offspring has, unfertilized eggs always become males. So now there’s males to solve that fertilization problem. Turns out you can’t even trust your (evolutionary) relatives.

These wasps are adelphoparasites which target animals that are very closely related to themselves, often belonging to the same family or even genus! Talk about terrible in-laws. In general though, hosts and their parasites are usually not closely related.

For one, there are just so many parasites out there -- it’s a lifestyle that’s evolved literally over a hundred times! Which is because there are so many niches, or ecological places, that they can take. Each species of animal is its own ecosystem, and each tissue or cell type a unique habitat.

Parasitism is smart, evolutionarily speaking, because you freeload off someone else’s hard work to get food, have babies, move around, or find a home. And with natural selection being as selective as it is, animals can use every little advantage they can get. Though “freeloading” might be kinda harsh.

Parasites get a bad reputation, but they’re really important parts of their ecosystems. For example parasites are a fundamental part of the food web, and for some animals, parasites are even a part of their regular diet, like how opossums love to eat ticks. But parasites depend on their hosts for survival, which sometimes put them on the wrong side of conservation efforts.

Like the California condor mite went extinct in the late 1980s when the 22 remaining California condors were brought into captivity and deloused for a breeding program. In saving condors, we doomed the mites, an example of what’s called conservation-induced extinction. With the benefit of hindsight, we’ve realized we lost a lot of information about not only the mites, but the condors as well.

The mite biohackers probably knew some things about the condor immune system we don’t, for example. And having parasites in a population isn’t bad -- it at least means that the hosts are also present. And since a lot of parasites use multiple hosts, they’re a good indicator of how the whole ecosystem is doing.

So scientists are starting to use parasite abundance to track the effects of pollution or other forms of environmental degradation and are pushing for more parasite conservation! And lastly, parasites are also really important in the context of evolution. They're an agent of natural selection since parasite infected animals have a harder time passing down their genes!

And studying how parasites interact with their hosts might give us some hints about how things happened millions of years ago like how complex social relationships between animals may have evolved from animals picking parasites off of each other’s skin or fur. So studying parasites could change everything we know about animals! Parasites are mysterious, but next episode we’ll dive deep into some of the most enduring zoological mysteries, and how sometimes more information means more complicated questions.

Thanks for watching this episode of Crash Course Zoology which was produced by Complexly in partnership with PBS and NATURE. It’s shot on the Team Sandoval Pierce stage 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.