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From the spider in the corner of your house, to the moths in your attic, synanthropic species don't just live among us, they literally depend on us to live.

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Common house spiders
Purple martins
Northern dung beetles
Granary weevils
Webbing clothes moths
Anopheles albitarsis mosquitoes

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If you see a spider in your house, hopefully you don't reach for a shoe, you carefully catch the little fella and put it outside because THAT'S where SPIDERS belong! Well actually, the best thing you could leave it alone. Turns out, many of the spiders you find in your home have adapted to living in houses, and they might actually die if they're put out. They're what biologists call synanthropic species: organisms that live closely with humans and benefit from doing so. These animals aren't domesticated, but they still depend on humans to some degree, and some of them would straight up die out if we did. So you could say the six animals on this list love us a little too much. Meanwhile, our feelings on them vary. Considerably. We're not always as appreciative of helpful synanthropes as we should be, like those sweet little spiders.

 1. Common House Spider

The common house spider is found all over the world, though not because it's particularly hardy or particularly able to disperse; it's because it hitchhikes on our stuff. The species probably originated in South America, but we're not quite sure. They've been hanging around humans for so long that by the time scientists got around to describing them, they'd already made it as far as Germany. And pretty much, everywhere they live, they are almost totally dependent on us and our warm, cozy buildings. Like other species of house spider, they've gotten used to the very specific conditions inside human homes and other structures. Namely, a constant climate and, believe it or not, the limited access to food and water.

Only some house spiders can survive outside in warmer climates. Even then, they are almost exclusively found on man-made structures, like rubble piles and bridges, which give them sheltered, secure places to hide. In colder climates, they don't stand a chance. So, ushering them outside has the same effect as just squishing them.

But altruism isn't the only reason you should reconsider evicting your eight-legged roommate. Inside, these spiders can act as effective pest control. Collectively, all the world's spiders consume something like six hundred metric tons of insects every year. Some estimates suggest that a single spider can eat up to two thousand insects annually (though house spiders are probably less gluttonous than that, since indoor environments don't contain quite as much food). They do have an impact on common household bugs. Things like flies, mosquitoes, cockroaches; you know, bugs you like even less than spiders. And for the most part, they stay out of your way while they manage these pests for you.

 2. Purple Martin

In a way, you could say that purple martins like humans because humans like them. This large species of swallow lives primarily in the Eastern half of the United States. And it's not hard to see why people enjoy them: they're colourful, they have a pleasant song, and they're acrobatic fliers, which makes them really fun to watch. Plus, if that's not enough, they also help deal with pests. They like to eat invasive fire ants and insects that can damage crops. And they're thought to fend off crows and blackbirds, which have the annoying habit of digging up freshly planted seeds.

So people are pretty fond of them. So much so in fact, that throughout the Eastern US, it's customary to put out nest boxes and encourage the birds to hang around. Indigenous peoples may have also done this. And if that's true, then the practice has been going for a long time. Long enough, that martins have come to rely on those human provided homes. Before us, they made their nests in abandoned woodpecker holes and other hollows. But we've gotten rid of a lot of dead trees where they would have made such homes, and invasive birds like starlings and sparrows have moved into many of the natural sites that remain. So experts think that if human beings suddenly stopped providing them with nest boxes, there's a good chance the birds would die before they could revert back to their natural nesting habits - in the Eastern US anyway. Purple martins are also found West of the Rocky Mountains, and there, the birds haven't had people providing them nest sites for centuries. So they still nest the natural way, and they could probably ensure the species continues on, with or without us. 

 3. Dung Beetle 

Dung beetles may seem kind of gross, but they're helpful to have around because they do a job you likely do not want to do. They help break down large piles of animal waste, making them less offensive and less prolific. In case you're really curious, they do this by actually eating the poop, but not just any poop will do. Dung beetles are specifically looking for protein-rich sources of nitrogen, which comes from the gut wall of the pooper. So they're kind of picky about which piles they dine on.

Luckily enough for the people of Iceland, at some point in the past, the Northern dung beetle hitched a ride onto the island with human colonists and their livestock. Prior to that, these beetles lived mostly in the Northern parts of continental Europe and Great Britain. So weather wise, Iceland makes sense, but they can only breed in the dung of large mammals like cows and horses. Before humans lived there, Iceland didn't have any animals that fit their needs. Now, if domesticated animals were suddenly to vanish from Iceland, the beetles couldn't just move on to another animal's poop. They would disappear as well. And if the beetles weren't there, those giant poop piles would stick around a lot longer. So those are early Icelanders might not have meant to bring dung beetles with them, but it's kind of a good thing they did.

 4. Granary Weevil 

Unless you are very very lucky, you have almost certainly done battle with granary weevils. Even if you think you've been very very lucky, that may be because you're not looking close enough. Granary weevils are tiny and easy to miss. Adults are a mere three to six millimetres long, though at least they are dark in colour. The larvae are even harder to spot. They look kind of like the grains of rice they are often found living in.

And their presence in our dried grains is no coincidence: thanks to evolution, that's where they live now. Like the common house spider, granary weevils can be found all over the globe. They probably evolved on the Eastern slopes of the Himalayas, but they've been with humans for a long time and, somewhere along the way, they adapted to living with us. So much that they can't survive without us anymore. Like, they lost the ability to fly. Their wings evolved into hard, fused structures that help protect them from being bumped around inside large bags of food. And without wings, riding along in bags of rice and dried food stuff is the only way they can move around from place to place. So they have become entirely dependent upon us for their dispersal. That also means that your weevil problem didn't happen like, in your house. The eggs and larvae actually arrived tucked away inside that bag of rice you bought. So...yum. And if humans weren't constantly acted like unwitting weevil chauffeurs, these bugs would probably die out. In fact, some species of granary weevil have never been found outside of human food storage areas. 

 5. Webbing Clothes Moth

If you're fond of wool, you are probably too well acquainted with the webbing clothes moth. Even if you've never seen one, you've probably seen their handiwork. The moths themselves are kind of innocent looking though. At four to nine millimeters, they're a bit larger than a granary weevil, but not much, and they're a pleasant buff colour with a tuft of red hair on their heads, kind of like flying, sweater-eating version of Eddie Redmayne. Webbing clothes moths are famous for destroying wool but they will eat any animal fiber, including silk, leather, fur and feathers. And you might think they've had a long history with humans, since they're so good at making neat little holes in your sweaters and scarves, but they've actually only been hanging out in humans homes since the invention of indoor heating. So, sometime around the latter half of the eighteenth century.

Originally, this moth is thought to hail from Central or Southern Africa, where the climate is warm and dry, and human homes or colder climates weren't really its kind of place - at first, but then we started heating our houses artificially and the bugs saw an opportunity. Today, webbing clothes moths living outside of Africa are rarely found outdoors, and when they are, they're probably there by accident. They either prefer living in homes, or have changed some aspects of their biology to make it so they have to. So, perhaps the most effective way to rid a house of these pests would be to shut off the heat. Except, you can't do that because the moths ate all of your sweaters. 

 6. Anopheles Albitarsis

The mosquito Anopheles Albitarsis probably became a synanthrope opportunistically. While other mosquitoes are still breeding in stagnant pools like a bunch of suckers, these South American mosquitoes seems to be developing a preference for irrigated lands, especially rice paddies. Researchers studying mosquitoes on a Brazilian rice farm had a hard time finding them in nearby forest habitats, suggesting they either prefer man made wetlands, or don't survive as well outside of them - which could make sense. This habitat not only provides a place to breed, it also ensures the animals have plenty of food nearby.

And by food, I mean, of course, our blood. In case you need a refresher, female mosquitoes use the protein in blood to make their eggs. So without blood to drink, they do not do so well. Now, this particular species isn't fussy about what kind of animal blood it imbibes, so it's not specializing on us, but large populations of humans probably look like a nice banquet to them. And partly because of that, they now stick to waterways we create. Which is a big problem. As if stealing our blood wasn't annoying enough, these mosquitoes can also carry the parasites that cause malaria. In fact, in some parts of Brazil, they are the primary malaria vector. Not only are they taking advantage of habitats we've created, they're also sickening, and sometimes killing, the people who create those habitats. And that is just the kind of synanthropic relationship we do not need.


As you can see, synanthropes run the gamut from helpful pest eaters to dangerous disease spreaders, but what they have in common is that they are incredible opportunists. They didn't need to be domesticated to somehow figure out that throwing their lot in with humans was a great survival strategy. We've unwittingly provided them with comfortable, warm places to live, plenty of food, and you know, if they get bored, lots of good Youtube videos to watch. And because of that, they will likely be a part of our lives for a long time.

Thanks for watching this episode of SciShow! If you want to learn more about how our species has influenced other organisms, you might want to check out our episode on animals that have evolved at hyper-speed. And to catch every episode of Scishow, be sure to subscribe and ring that notification bell like it's 2014. 

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