Zoologger: The chilly secret to monarch migration

Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world

Species: Danaus plexippus
Habitat: The Americas, western Europe, Australia, New Zealand and various islands ? although the North American populations are the really famous ones

In a few weeks, one of the most spectacular events in the natural world will begin. Having spent the winter in Mexico, millions of monarch butterflies will set off north to return to their breeding grounds in the US.

It's the final leg in a yearly cycle. The monarchs come south in the autumn to shelter from the freezing North American winters, and then fly north again in early March.

No one has ever been able to figure out what triggers the butterflies to leave the relative warmth of Mexico and head back to the US ? until now. Surprisingly, cold weather could be the decisive factor. That means the monarch migration could change profoundly over the coming decades, as the global climate warms.

Crammed on trees

Monarch butterflies breed through the summer in the US, where the females lay their eggs on milkweed plants. But as winter approaches and the weather turns colder, they are forced south. "They're originally neo-tropical butterflies, and will freeze to death if it's below freezing," says Steven Reppert of the University of Massachusetts Medical School in Worcester.

The monarchs spend the winter in the mountains of Mexico, crammed together on trees in a few isolated spots where conditions are suitable. "The temperatures hover just above freezing," says Reppert, but they are stable enough that the monarchs can wait out the winter in relative safety. The US's freezing winters, by contrast, would kill them.

Then in the first or second week of March, they set off north in search of warmer weather. The first wave of migrants reaches the southern US and breeds, the second generation gets a bit further, and it is only the third generation that returns to the northern parts of their range in Canada.

"Anecdotally, there are reports of butterflies that have gone all the way, but that's extremely rare," says Reppert. "Three is the magic number to complete the cycle."

Solar navigation

Reppert's team had previously shown that monarchs heading south navigate using the sun. To do that, they must also know roughly what time of day it is, so they can judge whether the sun's position corresponds to the eastern or western part of the sky. The butterflies achieve this with circadian clocks in their antennae.

Reppert has now shown that the monarchs use the same method when heading north in the spring. When he removed the antennae of 15 butterflies in Mexico, they became confused and flew in a wide range of directions ? rather than north-east as other butterflies did.

That suggests something in Mexico must flip the monarchs' orientation, given that the insects have relatively simple brains and should be incapable of adapting their flight orientation strategy without some kind of external forcing factor.

Duped in captivity

To find out what turns the butterflies around, Reppert captured 12 of the insects in the autumn, when they were heading south. He housed them under conditions that mimicked the short days and low temperatures of a Mexican winter. When he released them 24 days later, they flew ? even though under normal circumstances the insects would overwinter in Mexico for several months before returning north to the US

"We were very surprised," Reppert says. He repeated the experiment, this time without changing the day length. The butterflies again flew north. "It really was the cold temperature that was flipping them round," he says.

The fact that the northerly leg of the migration is triggered by cold temperatures means that it is vulnerable to climate change. "It has to be cold enough to cause the switch," Reppert says. If conditions in Mexico are too warm, the monarchs might set off in the wrong direction.

This doesn't mean monarchs are in danger. "The species is not threatened at all," says Ernest Williams of Hamilton College in Clinton, New York. There are populations on many continents, and most aren't migratory. "What is endangered is the migration."

It's hard to say what will happen if climate alters. One possibility is that the entire migration will shift northwards: in summer the butterflies might head further into Canada, and they might stop going as far south as Mexico in the winter.

"There will be more monarchs able to survive on the Gulf Coast," says Williams. But to expand their range northwards, they must have milkweed on which to lay their eggs. Williams says we don't know if milkweed is moving north, or even if it can survive and flourish at northerly latitudes.

Williams says there is another, less appealing possibility. The entire migration might reduce, with fewer butterflies migrating and only travelling short distances.

It's too soon to say, but if you want to see the monarch migration in its glory, maybe go sooner rather than later.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2013.01.052

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