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In most cases, the way species spread from place to place is relatively easy to understand. Like, squirrel is born.

Squirrel grows up. Squirrel decides they're sick of living in the same old tree where they've always lived. So they wander to the next habitat over, and before you know it, the entire continent is covered with squirrels.

It's not always that simple, though. Sometimes, species end up in places we wouldn't necessarily expect to see them. For instance, groups of the same or very similar species can end up on opposite parts of the globe, separated by hundreds of kilometers or entire oceans.

When two species that share a common ancestor have a lot of geographical distance between them, that's called disjunct distribution. And although it's puzzled us for decades, we're finally starting to understand some of the many reasons species show up in the most unlikely places. First, the obvious: Humans are long distance travelers and habitual migrants, and we don't always think about what we might be dragging around with us.

So we're a major driver of disjunct distributions. This isn't new: We've been doing this for thousands of years. Take the story of the Eurasian pygmy shrew, for example.

Pygmy shrews live in a lot of European countries and regions, from Britain to Iberia, to Italy and the Balkans. They also live in Ireland — which is a bit strange because. Ireland is an island, and it's not where pygmy shrews evolved.

For a long time, people just assumed that shrews got there thanks to some kind of temporary land bridge between Ireland and Britain. But recent genetic evidence shows that the Irish shrews actually have the same lineage as shrews that live in Andorra, more than a thousand kilometers to the south. If there had been a land bridge, you'd expect to see shrews descended from British populations as well.

But the fact that they're descended from animals in Andorra suggests that the shrews were probably brought to Ireland on boats… by humans. Not on purpose, though. See, pygmy shrews don't generally burrow, but they might take shelter inside piles of hay, where they can be relatively safe and also have easy access to the spiders and other arthropods they eat.

And when humans brought their livestock to Ireland, they also brought hay for the animals… so the pygmy shrews hiding inside it probably just hitched a ride. This sounds like a story that could have happened last week, but researchers think this happened a long time ago — either when humans first came to Ireland 5,000-10,000 years ago, or after early trade routes were established. So, we've been spreading invasive species for thousands of years… which is both fascinating and maybe a little sobering, too.

Like we mentioned earlier, though, plenty of disjunct distributions don't involve humans — including some caused by range fragmentation. This is when multiple groups live in the same basic area, but with a sizable gap between them. And it explains the homes of the screaming hairy armadillo.

Like other armadillos, this species has armor, but it also has a lot of hair. And it screams when threatened... so there's the complete picture for you. Screaming hairy armadillos live in the arid and semiarid regions in parts of Argentina, Bolivia, and Paraguay.

But the range is split into two distinct sections, with 500 kilometers between them. That's range fragmentation at its finest. One hypothesis about how this happened is that this species moved toward the Atlantic coast back when the climate there was dry.

See, screaming hairy armadillos are adapted to living in dry places. They love loose, sandy soil, and they can go a long time without water. And during the Pleistocene — between 2.6 million and about 12,000 years ago — glaciation kept the climate pretty dry and comfortable for them.

But at the end of that era, when the glaciers began to retreat, the climate turned humid. As a result, the intermediate population of armadillos became extinct — while the smaller population continued to thrive on the armadillo-friendly, shelly soils of the Atlantic coast. Now, beyond helping us solve mysteries, range fragmentation can also teach us how vulnerable species are to changes in climate.

Like, if the climate changes faster than an animal can adapt, we might see that reflected in how the animals are distributed. And also, sometimes, when we look at that information in the context of geologic history — like in this case — we can even get clues about what the climate was like in the distant past.

Next: Most people think of marsupials as an Australian thing. Kangaroos, wallabies, koalas — these animals all belong to a unique group of mammals that live on the continent of Australia. But you can also find marsupials across the ocean. Like, the common North American opossum is a marsupial, and dozens of opossum species also live in South America.

In fact, scientists think that marsupials evolved in North America, then spread south. So how did they end up in Australia?! Well, during the late Precambrian — around 600 million years ago —.

South America, Australia, and Antarctica were all a part of a supercontinent called Gondwana. Residents of Gondwana could comfortably travel from one place to another without fins or wings, which was handy if your goal was to spread your genes across the globe. So after marsupials arrived on the shores of Antarctica — back when it was green and had actual forests — they went by foot to Australia.

Then, Australia and the other continents we know today eventually broke away and drifted to their current locations. The marsupials' range literally split up. And over time, the animals evolved into the species we're familiar with.

Overall, geographic forces work pretty slowly, so for a long time, it wasn't obvious how such similar animals could end up on opposite ends of the globe. But as we've learned more about how continents drift and discovered related fossils around the world, we've been able to piece it together. Sometimes, species end up with weird distributions not because anything changed about Earth, but because of their own biology — like with mangrove trees.

Mangrove trees may have evolved somewhere in the Indo West Pacific, or near the Tethys Sea, an ancient body of water that stretched from present-day Turkey to Indochina. But now, they exist on roughly 75% of the world's tropical coastlines. Range splitting and continental drift can explain some of that, but the tree's affinity for long-distance travel also has something to do with its seeds.

Mangrove trees grow in habitats that are regularly flooded by seawater. So they've had to develop unique ways to spread their seeds around. Many plants have evolved seeds that can drift on the wind, or be carried by pollinators.

But mangroves seeds are buoyant. This isn't unique to them, and the reason why is simple:. If you're going to disperse in water, you need to be able to float.

But, as a very nice side effect, the seeds that are good at being buoyant also tend to be good at traveling long distances. So, you could view this as a sort of accidental side-effect of the adaptation that mangroves needed to reproduce in a challenging habitat. Even though natural selection doesn't really favor broad distribution directly, buoyancy allowed the plant to spread its genes to faraway places and thrive all over the world.

This next example is one of our favorites. Because not only has this animal ended up in distinct habitats because of its own biology: It's relied on the biology of other species. Killifish are colorful, cute fish known for ending up in weird places, including temporary ponds and isolated bodies of water.

And worldwide, they can be found on every continent except Australia and Antarctica. Overall, these things are tough. Some species lay eggs that can survive long periods in completely dry conditions, which explains how they end up in seasonal ponds.

But what's even more impressive is how at least some killifish appear to have arrived in new habitats. Scientists have suspected that these fish may sometimes resort to, shall we say, unusual dispersal techniques. The existence of remote populations in bodies of water at high altitudes, for example, were kind of hard to explain.

Then, in 2019, a group of researchers in Brazil officially discovered that killifish can get around in the most unpleasant way. I'll just say it: They get pooped out. Because some killifish eggs can survive being eaten.

When the researchers fed killifish eggs to coscoroba swans, they found that at least a few of them came out the other end relatively intact. That's partly because of that thick membrane that helps protect the eggs from harsh conditions, and partly because the birds have inefficient digestive systems. So the eggs can travel long distances inside the creature that ate them and then get dropped into some random pond, where they might actually hatch.

We've suspected for a long time that birds are responsible for dispersing seeds — blackberries probably have the odd and scattered range that they do because of birds. But this was some of the first evidence that fish can also be dispersed this way. And so far, it looks like killifish aren't the only ones.

In June 2020, another paper came out showing that the same thing can happen with mallard ducks and carp eggs. So if you're wondering how fish got into your neighborhood pond… that might explain it. Finally, of all the examples we've talked about, this one might be the cutest.

It involves iguanas, which are found in two main areas: in the Americas from Florida to Southern Brazil, and also in the Caribbean. It's pretty obvious that iguanas crossed the ocean at some point, because there really aren't any other good explanations for their presence on so many isolated islands. What's less clear is how they did it.

Iguanas are good swimmers, but they're not Michael Phelps or anything. Still, they got to those islands somehow — and one prevailing hypothesis is that they rafted there. You know, by floating across the sea on something like a boat.

Before you get too many pictures in your head of iguanas building dugout canoes with their sharp claws, the “rafts” were probably built by much more natural forces. For instance, in 1995, a bunch of iguanas arrived on the Caribbean island of Anguilla on a large mat of uprooted trees, which could have been made during something like a hurricane. That doesn't prove that's how all iguanas traveled around the world, but it did demonstrate that this is possible.

And even beyond iguanas, this form of distribution could explain a lot of things about how animals arrived on certain landmasses. The ancestors of lemurs, for example, probably got to Madagascar in much the same way, which sounds like a Pixar short waiting to happen. The general idea is that large mats of debris produced by hurricanes could easily transport several members of one species, and eventually, you'd end up with a whole, viable population.

So the way animals spread across the globe isn't always as simple as Squirrel A moves to Point B and so on and so forth. Animals also get distributed in amazing, complicated ways, and understanding how that happens can help us learn a lot about evolutionary history. And to some degree, those disjunct distributions can even help us understand how the world we live in has changed over millions of years.

Thanks for watching this episode of SciShow. If you want to learn more about animals living in weird places, we've got another episode you might like. It's about some animals who are living their absolute best lives in cities, and you can watch it after this. [♪ OUTRO].