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

Let's start with a question: if a road were too long for you to finish in your whole lifetime, would you still set out?

There's a butterfly in North America that answers that question every year.

It's the monarch — orange wings with black stripes, each one weighing only about a gram.

Every year monarchs make a round-trip migration of almost nine thousand kilometers, yet no single monarch ever completes the trip from start to finish.

To understand this, we have to start with a poisonous weed.

The plant is called milkweed; it contains substances called cardiac glycosides, and people and many animals who eat it by mistake get nauseous and throw up.

Yet the monarch's whole life depends on it.

When breeding, females lay hundreds of eggs, scattered across the undersides of milkweed leaves.

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

After several molts, the tiny larvae, just a few millimeters long, grow into caterpillars about five centimeters long, while the milkweed toxin builds up bit by bit in their bodies.

By the time they pupate and become butterflies, the toxin is still inside them.

So the monarch's bright orange and black is actually a warning: I'm poisonous, don't eat me.

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

That's why the American writer Diane Ackerman called the monarch a genius chemist.

Next comes their journey.

In winter, millions of monarchs gather in the mountain forests of central Mexico, packed in layers on the trunks of firs and pines.

When spring warms things up, they wake and start flying north.

They can fly over a hundred kilometers a day — with good weather and rising air, even three or four hundred.

Along the way they sip nectar to refuel, rest in the woods at night, and mate during the trip.

After mating, the males soon die, while the females must find milkweed, lay their eggs on the undersides of the leaves, and then they too reach the end of their lives.

About a month later, those eggs have become a new generation of butterflies.

The new generation keeps flying north — mating, laying eggs and dying again, handing the journey on to the next.

After a relay of three or four generations like this, the monarchs finally reach the northern US and southern Canada, nearly five thousand kilometers from Mexico, where they set out.

In autumn, as it turns cold, 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.

Earlier generations usually live only a month or so, but this one can live seven to nine months.

Science writers have nicknamed this generation Methuselah, after the man in the Bible who lived to 969.

These long-lived butterflies start flying south.

They navigate by the sun and can also sense Earth's magnetic field, flying four thousand kilometers over two months back to Mexico.

Remember, they themselves have never been to Mexico — it's only where their great-grandmothers and great-great-grandmothers set out from.

Once there, they pack onto the tree trunks again and huddle for warmth through a winter of more than four months.

The next spring, they wake up and become the first leg of a new journey.

Many animals have a homing instinct.

Chinese sturgeon, for example, must return to the upper Yangtze to breed, and green sea turtles drift at sea for twenty-plus years and still return to the beach 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.

This story comes from a nature essay published by Caixin, written by an author who saw monarchs about to head south by a roadside in Montreal, Canada.

She says she doesn't know why these butterflies set out when they know they can't finish.

All she knows is that, relay by relay, generation after generation, this migration route has been flown for tens of thousands of years.

So, back to the opening question: if a road were too long to finish in your lifetime, would you still set out?