没有一只帝王蝶能飞完整趟迁徙：这段路要四代接力才能走完

There's a butterfly that makes a long migration across North America every year.

The strange thing is that no single butterfly ever completes the whole journey from start to finish.

This butterfly is the monarch: orange wings with black stripes, each one weighing only about a gram.

Let's start with what it eats.

North America has a wild plant called milkweed; its leaves contain a toxin that makes people and many animals nauseous and sick.

Yet the monarch's whole life depends on this poisonous plant.

Females lay hundreds of eggs, spread out on the undersides of milkweed leaves.

A few days later, the larvae hatch and eat only milkweed leaves.

As they grow, the larvae molt several times, going from a few millimeters to about five centimeters long.

At the same time, the toxin slowly builds up in their bodies.

Later, the larvae become pupae and then butterflies, and the toxin stays in their bodies the whole time.

So the monarch's bright orange and black is actually a warning.

An associate professor at Beijing Forestry University says blue jays that have eaten a monarch avoid orange-and-black butterflies 97% of the time.

One American writer has even called the monarch a genius chemist.

But what monarchs are most famous for is their migration.

Every spring, in the forests of central Mexico, millions of monarchs rest in layers on the trunks of firs and pines.

As temperatures rise, they wake up and start flying north.

Along the way they can fly over a hundred kilometers a day — and in good weather with rising air currents, even three or four hundred.

As they fly, they sip nectar to refuel, and in the evening they rest in the woods.

On the way they mate; then the males die, and the females find milkweed, lay their eggs, and die too.

About a month later, a new generation emerges as butterflies and carries on north, repeating the same process.

After a relay of three or four generations, monarchs finally reach the northern US and southern Canada — almost five thousand kilometers from where they started.

When autumn comes and temperatures drop, the last generation born in the north is completely different from the ones before.

Their reproductive systems pause; they don't mate or lay eggs, and they save all their energy.

That way they can live seven to nine months, while earlier generations usually live only a month or so.

Some call this generation Methuselah, after a man in the Bible who lived to 969.

These long-lived monarchs start flying south.

They use the sun to find their direction and can also sense Earth's magnetic field; over two months they fly four thousand kilometers back to Mexico.

Remember, they've never been to Mexico — it's where their ancestors set out from.

Once in Mexico, they pack together in layers on the tree trunks again, huddling for warmth through the winter.

Four months or so later, when it warms up, they wake and become the first leg of the next journey.

Many animals return to where they were born — Chinese sturgeon go back to the upper Yangtze to spawn, and green sea turtles return to the beaches where they hatched.

But monarchs are different: no monarch can return to the place it was truly born, and none can complete the whole journey.

Generation after generation, this migration relay has gone on for tens of thousands of years.

If you knew you could never finish something in your lifetime, would you still start it?