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Plenty of animals migrate,
moving from place to place in search of food,
warmer weather, or safe places to reproduce.
And lots of animals pull off incredibly long treks .
But the animals we’re going to talk about today,
while they don’t take the longest trips ever,
go farther relative to their body size than just about anything else.
They’re all under about 10 centimeters in size,
and they pull off some seriously impressive long distance travel.
And they’ve got some tricks up their sleeves to make it happen.
Up first, ruby-throated hummingbirds.
Ruby-throated hummingbirds migrate from the southern United States to Mexico --
a trip that puts the Gulf of Mexico squarely between them
and a winter home filled with flowers to dine on.
But these birds only have a body size of 8 centimeters,
so the 800-kilometer trip is 10 million times their body length!
If a human of average height, about 170 centimeters tall,
were to travel that kind of relative distance,
it would be equivalent to traveling 17,000 kilometers.
Which would be like walking from the northern to the southern tip of Africa --
and back again.
For the hummingbirds, this is a non-stop flight.
There’s nowhere for them to take a layover.
Because while they could travel around the Gulf, they opt for the shorter direct trip instead.
It takes them nearly 20 hours of non-stop flying --
and they burn up nearly half their body weight in fuel.
Small birds tend to have high metabolic rates, and hummingbirds run some of the highest. Under
normal circumstances, if a hummingbird can’t find enough food and calories during the day,
they risk starving overnight.
So to prepare for their snack-free 20-hour marathon,
ruby-throated hummingbirds pack on as much extra energy as they can,
increasing their body weight until their body composition is nearly 40% fat.
Which is a lot.
Other animals that have that much body fat include whales and seals,
whose insulating blubber keeps them warm, and hibernating bears who are inactive for months
-- not hours.
Technically, this isn’t even the longest migration undertaken by hummingbirds.
The rufous hummingbird migrates from Mexico to Canada along the west coast of North America.
That trip averages 6400 kilometers --
or 80 million times their body length --
but flying over land means they have chances to stop for food and rest.
Now hoverflies --
a group of flies that superficially resemble bees --
don’t migrate as far as the hummingbirds.
But their tiny size makes the scale of their migration all the more impressive.
Hoverflies migrate across the English Channel between the UK and mainland Europe.
At its shortest, that’s a distance of about 250 kilometers.
Which might not sound like too far, but for a centimeter-long insect,
that distance is 25 million times their body length.
For our average human, that would be like traveling 42,500 kilometers,
or a full lap around the Earth and change -- just to get some food!
In the spring and fall, between 1 and 4 billion hoverflies make this seasonal roundtrip.
Now they’re not the strongest fliers.
And near ground level, the winds are blowing the wrong direction to assist them in their journey.
And this happens in both directions --
the winds change with the seasons, which just seems a little unfair.
But by flying up at least 150 meters into the atmosphere --
and occasionally as high as 750 meters --
hoverflies tap into tailwinds that blow them in the right direction.
And being so tiny, they can save energy and just let the wind carry them.
This trick of using high altitude air currents is actually pretty common in migratory insects.
But it wasn’t until advances in radar in the 1970s that scientists
were really able to track them as they moved through the atmosphere.
That’s when we learned that a lot of insects migrate --
and have a serious ecological impact. And we pretty much had no idea.
In fact, these international hoverfly travelers are incredibly important for pollinating crops
in the UK, where they spend the summer. They also eat up pesky aphids, protecting the plants.
And video footage from 2017 detected migratory hoverflies in North America as well, where they’re
likely also providing this important agricultural service as they travel along the West Coast.
Now monarch butterflies, with their yearly trek
across the US to their overwintering sites in Mexico,
might be the most well-known migratory insects.
Depending on where in North America they start out,
the migration south can be nearly 4800 kilometers.
Given that the butterflies are only about 3.5 centimeters long,
that distance is 135 million times their body length.
That’s like our average human traveling 229,500 kilometers --
equivalent to going around the Earth over five and a half times.
Now, not all North American monarchs migrate.
Those that live in the warm southeastern US, for example, stay put all year.
But the migratory monarchs have quite the challenge ahead of them.
Like hoverflies, monarchs take advantage of air currents to save energy as they fly.
But traveling across nearly an entire continent requires some navigation skills, too.
And researchers believe that monarch butterflies make use of not one
but two internal compass mechanisms to orient themselves.
The first one uses the Sun.
While traveling, monarchs monitor the position
of the Sun in the sky as it rises in the east and sets in the west.
If they are headed south, they must keep the Sun on their left in the morning,
but on their right in the afternoon.
And the butterfly’s biological clock helps to compensate for
the sunrise-to-sunset shift in the position of the Sun,
and tells the insect which direction is which.
Light cues from both their eyes
and their antennae are crucial for this clock-compass combo to work properly.
The second compass likely acts as a backup in cases
where the weather is overcast and the butterflies can’t rely on sunlight.
This one is a magnetic compass,
like the one that’s been observed in some birds.
Basically, they can detect differences in the angle of Earth’s magnetic
field as they travel from north to south, again using their antennae,
and orient themselves in the right direction.
And they can use it to point them either north or south.
Which is important,
most migratory insects simply don’t live long enough to make a two-way trip
-- a different generation goes back the other way.
But some monarchs are actually tough enough to make the full round trip migration --
so they need the tools to get home again.
Now for a somewhat less charismatic insect, but one that’s just as great a traveler.
The desert locust is a notorious pest, one whose massive swarms can devastate crops.
Under normal circumstances, the desert locust is just another grasshopper.
But when their numbers get crowded, their behavior switches to traveling swarm-mode.
They can regularly travel 5000 kilometers across Africa and parts of the Middle East,
and they’ve even crossed the Atlantic Ocean --
a distance of some 10,000 kilometers,
and a trip that wrecked fields in the Caribbean and South America in 1988.
For these grasshoppers, which are about 7 centimeters long,
their travel distance can be anywhere from about 70 to 140 million times their body size.
For a human, this would be comparable to traveling 238,000 kilometers at the longest.
That’ll get you more than halfway to the Moon.
Even when they aren’t accidentally picked up by strong Atlantic-spanning winds,
locust swarms typically make use of air currents to quickly cover thousands of kilometers.
When they have to do the flying themselves, though, they load up on carbohydrate-rich foods,
much like a human endurance athlete would.
Locusts store the carbohydrates as energy-rich glycogen
and burn this fuel to power their wing muscles.
And this is precisely why locust plagues can be so devastating for farmers:
our favorite crops, like wheat and corn, are carb goldmines.
Add the fact that an adult locust can easily eat its weight in food daily,
multiply that by a few tens of millions of locusts per swarm, and you’ve got a recipe for disaster.
Our last tiny record holder really lives up to its name.
The wandering glider, also called the globe wanderer, is a species of dragonfly
that is found all over the world -- on every continent except Antarctica.
In 2012, researchers discovered that wandering gliders
they found on the eastern coast of Africa had arrived there from northern India.
The little dragonflies would have had to have crossed the Arabian Sea,
and possibly the Himalayas, to reach the African shores.
That’s a distance of 16,000 kilometers, or a whopping 320 million times
its 5-centimeter body length. And a good portion of that distance is over open water.
If a human were to travel that kind of distance, it would be equivalent to 544,000 kilometers.
So you’d walk not just to the Moon, but halfway back, as well.
Like many of our tiny migrants, these dragonflies use wind to travel efficiently.
The wandering glider has relatively large wings to help it glide,
and it makes use of air currents higher than 1000 meters up in the atmosphere,
much higher than the hoverflies.
Many species tend to live in separate populations across their range,
not always interacting with members of other populations.
But researchers think there is only one population of wandering gliders.
A 2016 study found pretty regular gene flow occurring between
all the different locations where the gliders are found,
suggesting they’re traveling far enough to mate with one another pretty much wherever they live.
So the researchers proposed that the species is best considered
to be a single population of globally mingling individuals.
Unlike the round-tripping monarchs, though,
a single wandering glider can’t pull off a complete tour of the globe.
Successive generations each travel a leg of the journey,
so you can think of this worldwide
population as participating in a continent-spanning relay race --
with no defined finish line.
Now, most migrating animals do so in order to find food,
or to find breeding grounds full of food for their offspring.
This makes most migrations seasonal.
Wandering gliders require freshwater pools to breed,
so their migration tracks rainfall.
Rainfall does vary seasonally, but instead of the there-and-back-again of migration patterns,
these dragonflies have taken to wandering.
They follow those high-altitude wind currents --
even across oceans --
and snack on tiny airborne organisms while they glide wherever the wind takes them.
But because insects are small and short-lived,
it’s still tricky to track the movements of a single individual.
Scientists haven’t yet been able to figure out all the wandering gliders’ migratory paths,
so there’s probably more we don’t know yet.
So the next time you think it’s a long way to the grocery store,
just remember these globe-trotting creatures.
At least no one’s asking you to walk to the Moon and back!
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