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Every year, millions of animals undertake amazing journeys.  They migrate to avoid the cold, to give birth, or to find food, and to do it, they can travel hundreds of kilometers over all kinds of terrain.  Now, you've probably already heard about things like geese heading south for the winter or salmon swimming from oceans to rivers to lay eggs, but these feats only represent a tiny fraction of how animals migrate.  There are some seriously impressive stories out there, and they come from all kinds of creatures, some big and familiar, and others so small you can barely see them.  Here are six of our favorites.

Caribou are found in North America, Europe, Asia, and Greenland, and if you're looking at this picture and thinking that's a reindeer, you're right.  They're the same thing, although the name changes depending on where you live and whether the animal is domesticated.   Regardless of its name, though, some subspecies of caribou are serious migraters.  In fact, one of them, called Grant's caribou, holds the world record for longest migration over land.

Each year, they take a journey that can cover more than 4,000 kilometers, which is about the width of the entire United States, and this adventure can span many types of terrain, taking them through ice fields or over mountains.  Like some animals, caribou do this partly to reach their calving grounds near the coast where they've given birth for generations, but they also do it to find more high quality vegetation and even avoid pesky mosquitoes and other biting insects.  In fact, when the insects are at their worst, these caribou actually travel the fastest and who can blame them?

Even though the caribou migrates the farthest over land, its journey is nothing compared to the arctic tern's.  The arctic tern is a small seabird and it holds the record for having the longest migration distance ever recorded.  When winter arrives in the Northern Hemisphere, the birds fly from the Arctic Circle in the northernmost part of the globe towards the Antarctic Circle in the southernmost part.  Now that's already a long distance, about 15,000 kilometers, but the birds can travel up to five times farther than that by taking a meandering route.

That long trek sounds exhausting, but it might be a smart decision for the birds.  One hypothesis suggests that traveling so far south helps the terns follow the bright summer sunlight.  That would allow them to see their aquatic prey more clearly and they could take advantage of the calmer summer breezes.  As for why they don't just fly straight to the Antarctic?  Well, that also seems to be about food.  In a 2009 study published in the Proceedings of the National Academy of Sciences, researchers attached lightweight geolocators to terns and tracked their migration routes.  They found that along the way, the terns traveled over food-rich regions of the ocean, flying along with the winds to help them keep up their energy.  In other words, the meandering route allows them to get in some good meals and not run themselves into the ground or fly themselves into the ocean.  You get it.

It makes sense that birds and mammals would migrate in search of more favorable conditions, but this next story takes place in oceans and lakes with a much smaller creature: zooplankton.  These are tiny aquatic organisms about as thick as a dime and they usually drift along with the current since they're weak swimmers, but when they have to move, they move.  In fact, zooplankton could possibly hold two world records for migration: the record for the smallest migrating animal and the most numerous one.  

Every day, up to billions of these animals participate in what's called diel vertical migration.  During the day, they migrate downwards into darker waters, but at night, they go upwards towards the surface.  Some evidence for this comes from studies of zooplankton in their natural habitats, like a 2000 paper published in a German journal about freshwater biology.  

In the study, scientists found that allowing artificial light to shine on a lake at night significantly reduced the number of one type of zooplankton hanging out there, and that makes sense since that light mimics the sunlight normally found during the day.  Other studies have found similar results with other types of zooplankton too.  Some researchers suggest the animals do this to avoid predators that hunt with their sense of sight.  By sticking to the darker, deeper waters during the day, there's a smaller chance they'll be spotted and eaten, but they can't avoid their foes like this forever.  After all, there's not a lot of food in those deep waters.

So at night, once the bright sunlight goes away, they might migrate to the surface to get their meals.

Next up, here's another world record holder, the bar-headed goose.  If you live in the US, you're probably familiar with Canada geese, which hang out in warmer climates during the winter, but the bar-headed goose in Central Asia is arguably way cooler.  They're listed with Guinness World Records as the highest migrant.  To enter and exit their breeding grounds in Tibet, these birds can fly more than 7,000 meters above the ground.  That's only about 2,000 meters lower than where commercial jets hang out.  

They don't do it just for funsies, though.  They have to reach these altitudes because their migration path takes them over the Himalayas, one of the tallest mountain ranges in the world.  At this point, it's not super clear why they take such an extreme route, though.  Scientists suggest it could give them an advantage over other animals or it could be because they've been flying this way for millions of years, so maybe they had to adjust as the Himalayas got taller.  In any case, what scientists do know is that geese have evolved special adaptations to deal with the low oxygen levels on their flights.

For example, they have a special type of protein in their blood that lets them quickly absorb oxygen at high altitudes.  They also have more small blood vessels around their cells, which improves how oxygen gets transported to their muscles.  In a 2019 paper, researchers even reported that the birds lower their metabolisms and change the way they move their wings to conserve as much oxygen as possible, so scientists will keep investigating why they migrate at such serious heights but one way or another, they're pretty great at it.

Now, emperor penguins aren't exactly obscure.  They're featured in everything from primetime documentaries to children's movies, because, well have you seen them?  They're great.  These birds live on the frigid Antarctic side of the world and you can often pick them out by the yellow feathers around their necks.  Unlike some other birds, they don't migrate across continents and oceans, but their journeys are no less impressive.  In their quest to reach their breeding grounds, they travel across Antarctica in the winter, a decision so extreme that it ultimately forces some of them to stop eating for weeks.  

During the three months of summer, emperor penguins live by the ocean, where they can hunt and feed on prey like fish and squids, but as soon as the waters begin to freeze and the ice thickens up, they start to march inward towards their breeding grounds.

These breeding sites are called rookeries, and they can be found at the base of icy cliffs.  The cliffs provide some shelter from the weather but once the penguins arrive, not all of them stay there for the whole season.  After the females lay their eggs, they head back to the sea to eat, leaving the males to care for the eggs until they hatch, and unfortunately, that means the poor males get no fish.  Instead, they have to survive off of fat stores for the better part of the winter.  

Somehow, though, it's a strategy that works for them.  The birds even take advantage of these cliffs for as long as possible.  Usually, they're there for months, because once their chicks hatch, they have to wait for their waterproof feathers to grow before they can make their first trip to the ocean.

Our final example brings us far from the Antarctic to a tropical lake on the remote Pacific island of Palau.  In that lake live millions of squishy umbrella shaped creatures called golden jellyfish.  Among other things, they're really fond of the Sun.  The tissues in the jellyfish's bodies contain algae called zooxanthellae and the two organisms have a seemingly happy partnership.  

The jellyfish make sure that the algae receive ample sunlight so they can undergo photosynthesis, and byproducts of the algae's photosynthesis provide energy for the jellyfish.  This symbiotic relationship is what drives the jellyfish's daily migration.  As the Sun crawls across the sky, the jellyfish migrate horizontally to maximize the amount of sunlight their algae receive, and at night, the jellyfish migrate downward vertically so that the algae can receive other necessary nutrients.

So mostly, these animals aren't migrating for themselves, but for their partners, which is kind of cute.  Of course, it does have one last advantage for the jellyfish.  By migrating away from the lakeshore, these animals can avoid their main enemies, the anemones.  As with many creatures, not all golden jellyfish migration is the same.  Their movements, routes, and even speed may vary depending on where they live, but the basic strategy is still pretty cool.

These six stories show how amazing animal migrations can be, but somehow, they still haven't scratched the surface of all the diversity here.  From reptiles to insects, there are plenty of organisms we couldn't cover, each with adventures of their own.

It just goes to show that something that might feel ordinary is more fascinating than you'd think.  You just have to look closely.  One of the many reasons scientists have been able to learn about these migrations is because they're good at what you might call scientific thinking.  They have a strong understanding of the laws of nature and how their evidence fits within that framework, so scientific thinking is a great way to understand the world around you, and if you want to learn more about how to do it, you can check out the scientific thinking course from Brilliant.

It's packed with puzzles and requires no pre-requisite courses, so if you've never tried Brilliant before, this might be a good fit, and when you're done, you can also check out one of their many other science, engineering, computer science, or math courses.  To sign up or to learn more, head over to brilliant.org/scishow.  If you're one of the first 200 people to sign up at that link, you'll get 20% off of an annual premium subscription.

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