Monarch butterflies are unique — they’re the only butterfly to travel thousands of miles when the seasons change. They travel from as far north as Canada all the way down to a few very specific mountaintops in central Mexico.

They don’t have a car, or an airplane ticket. They just have their two little wings. So we’re asking: How do they do it? How do they migrate thousands of miles? And why?

We’ll also look at why other insects don’t have to travel south for the winter — hint: it has to do with something in their blood. Of course, we have a mystery sound for you to decipher, too.

Butterfly incubator
A peek inside a refrigerator at the University of Minnesota Monarch Lab. They keep live monarchs inside envelopes in the fridge. They try to make the climate in the fridge like the mountaintops in Mexico where monarchs spend the winter.
Molly Bloom
Right-click to download episode transcript.

Find out more about Flight of the Butterflies, the IMAX 3D film discussed in the show.

Be a citizen scientist!

By using your skills of observation you can help scientists figure out a lot about monarchs — like how many there are, when they’re migrating, and if they’re being attacked by parasites.

Here’s a list of places to get started:

• The nationwide The Monarch Larva Monitoring Project based at the University of Minnesota
• You can report your observations to Journey North about monarchs, and also hummingbirds, bald eagles and other wildlife
• Help the Natural History Museum of Los Angeles County track monarchs, as well as spiders, geckos, zombie flys and ladybugs
• Collect samples of monarch parasites for MonarchHealth

Sound effects in this episode provided by freeSFX

Audio Transcript

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HARRY: You're listening to Brains On, where we get curious about the science of everything.

MOLLY BLOOM: You know what? We're just going to do it.

HARRY: What?

MOLLY BLOOM: I know you don't want to wait.

ALLY: What?

MOLLY BLOOM: We're going to start off with the mystery sound.

ALLY: Yes.


SUBJECT 1: Mystery sound.

MOLLY BLOOM: I'm your host Molly Bloom, and here is the mystery sound.


I have two co-hosts here with me today who are going to give us their best guesses, 12-year-old Harry Hanzy-Francis and 9-year-old Ally Hanzy-Francis. They're brother and sister from Minneapolis.

ALLY Hello.


MOLLY BLOOM: Do you guys have any guesses about that mystery sound?

HARRY: It sounds like rain.

MOLLY BLOOM: What do you think, Ally?

ALLY: It sounds like birds that are flapping away really fast.

MOLLY BLOOM: Both excellent guesses. We're going to hear the sound again a little later in the show, so you can think about it a little bit more.

HARRY: But let's tell them what this show is all about first.

MOLLY BLOOM: Good call. Today is all about butterflies, and not just any butterfly. It's the--

ALLY: Monarch butterflies.

MOLLY BLOOM: How would you describe what a monarch looks like for people who might not know?

HARRY: Orange and then like either really dark brown or like black.

ALLY: We know who are boys and girls because the boys have black spots and the girls have fatter stripes of black.

MOLLY BLOOM: The pattern on their wings are different if they're a boy or a girl.

ALLY: Yeah.

MOLLY BLOOM: I didn't know that. That's a good fact. So how do you know so much about monarchs?

HARRY: We just started raising them.

MOLLY BLOOM: So you raise monarchs at your house?


MOLLY BLOOM: Do you put them? How do you raise them, outside, inside?

HARRY: We raise them inside, so that rain and stuff doesn't get to them. And we basically just put them in a fish tank with a bunch of leaves.

MOLLY BLOOM: I'm going to ask you some more questions about raising monarchs a little later.


MOLLY BLOOM: And the reason we're talking about monarchs today is that they're unique. They're the only butterfly to travel thousands of miles when the seasons change.

HARRY: They head South during fall.

ALLY: And in spring, they come back North.

MOLLY BLOOM: They don't have a car.

ALLY: Or an airplane ticket.

HARRY: And they don't hitch rides on the back of a motorcycle.

MOLLY BLOOM: They just have their two little wings.

HARRY: So how do they do it? How do they migrate thousands of miles?

ALLY: And why?

MOLLY BLOOM: Let's find out right now. Keep listening. Keep listening. Keep listening?

SUBJECT 2: Brains On.

MOLLY BLOOM: The answer of why monarchs travel South for the winter at the very heart of it is pretty simple.

ALLY: It's just too cold up North.

MOLLY BLOOM: It's the same reason some birds fly South for the winter.

HARRY: But the bigger question is, if monarchs need to travel South, why don't other insects like beetles or ants need to find a warmer home in the winter?

MOLLY BLOOM: To find out, we sent a Brains On reporter on a special assignment.

JACK: Brrr. It's cold here.

SUBJECT 3: Hey, kid, your mic's on. We'll live here.

JACK: Oh, right. My name's Jack. And I'm here on a special assignment somewhere very cold to interview-- what does that say?

BEETLE: A beetle.

JACK: A Beatle. Cool. I love The Beatles. Is it Ringo, or is it Paul? Where is he?

BEETLE: No, no, no, a beetle like a bug. Me, I'm a beetle. See.

JACK: Oh, that kind of beetle. Hey, there little bug. What are you doing out here in the cold? And why aren't you wearing a jacket? It's freezing.

BEETLE: Well, that's what I'm here to tell you about. I have a special way of keeping myself from freezing.

JACK: What? Do you have lots of body fat or something?

BEETLE: Are you saying I look fat?

JACK: No, I was just--

BEETLE: Kidding. Of course, I don't have body fat. I'm ectothermic.

JACK: What um, thermic?

BEETLE: Ectothermic. It means I don't really produce my own body heat. So insulating with fat wouldn't help me stay warm. I'm pretty much always the temperature of the environment around me. If it's hot out, I'm hot. If it's cold out, I'm cold.

JACK: So then why aren't you a bug icicle right now? It's so cold, even my fingers are icy. Look, I can see my breath. Ha.

BEETLE: Hey, not in my face.

JACK: Oh, sorry.

BEETLE: It's OK. I appreciate the attention. People don't think I'm that interesting. They don't even take the time to ask. But guess what? The reason I don't need a jacket or hot cocoa is that I have a type of antifreeze inside my body.

JACK: Huh? Antifreeze?

BEETLE: Yeah, imagine if I didn't have it. Ice crystals could form in my cells. Those cells would get destroyed. And that would be the end for me. So my body produces a protein that actually prevents water from turning to ice, even when it's below freezing outside. Antifreeze.

JACK: Whoa, that's a really cool trick.

BEETLE: It helps the fluid in my body stay liquid, even though outside my body water is freezing. Lots of bugs do this to survive the winter. Other bugs can even control the way the liquid around their cells freezes to minimize damage. And then some bugs just burrow into the ground where it's warm and wait for the summer. Us bugs have all kinds of ways of surviving the cold winter.

JACK: Ah, I guess that makes bugs pretty cool, right?


Well, I thought it was funny. Besides, aren't you a beetle, not a cricket?

BEETLE: I do impressions.

JACK: Since I don't have antifreeze in my blood, I better get out of here and go someplace warm. This is special correspondent Jack signing off.

MOLLY BLOOM: Was there anything in that skit that you found particularly interesting?

HARRY: I thought that it was interesting that they have antifreeze in their bodies.

MOLLY BLOOM: Do you wish you had antifreeze in your bodies?

ALLY: Yes.

HARRY: Well, considering that it's poisonous, no.

MOLLY BLOOM: Yeah. It's probably-- our bodies probably don't have it for a reason. But it's cool that bugs can use it to survive the winter.

HARRY: Yeah.

MOLLY BLOOM: But monarchs don't have it. And that is why they need to go South.

SUBJECT 4: (SINGING) Ba, ba, ba, ba, ba, ba, ba, ba, ba, ba, ba, Brains On.

MOLLY BLOOM: So we've got the why taken care of. Now for the how. Not all monarchs migrate to the same place. Monarchs from the West Coast go to California and some from the East Coast go to Florida. But many monarchs from Canada and the Midwest and even the Eastern part of the country travel to Mexico.

HARRY: To the tops of some very specific mountains in Central Mexico.

MOLLY BLOOM: For a monarch traveling from just outside our studios here in St Paul, Minnesota, that's over 2000 miles.

HARRY: We wanted to know how do they do that. So we talked to a biologist.

ALLY: Karen Oberhauser.

HARRY: She's been studying monarchs for 30 years. And she's a professor at the University of Minnesota. How do they go to Mexico? Like how do they find their way?

KAREN OBERHAUSER: Well, that is an excellent question. And I always like questions that we don't 100% know the answer to. So we know that the monarchs use the sun in the sky to know how to fly South. So if you imagine a map and it's morning, the butterflies need to keep the sun on their left in order to fly South, because the sun comes up in the Middle East. And if they keep the sun on their left, they'll fly South.

And if it's noontime, they want to kind of head toward the sun. And if it's afternoon, they want to keep the sun on their right. Then they know how to fly South. And if it's not sunny, the sun is under clouds like it is today, they can still migrate. And what they do is they use the Earth's magnetic field. So they kind of have a compass in their body.

But that doesn't completely answer your question, because if you were here in Minnesota and you wanted to get to a certain place in Mexico, and all you knew was which way is South, you could not get to that certain place. You'd have to know that you were in Minnesota. And what if you were in Maine, and you wanted to fly to that place in Mexico? Then you would have to fly a little bit West in order to get there.

So that's what we don't know is how the monarchs find that exact spot in Mexico. All we understand is how they know which way is South. It would kind of be like trying to get somewhere without a map. And all you have is a compass.

So you really need the map and the compass. And we don't know about the map yet. So maybe somebody like you who grows up and becomes a scientist can figure that one out for us.

HARRY: What I think about what it could be is it's like it's programmed into their like genetic code to know which way you need to go.

KAREN OBERHAUSER: That's probably the right answer. So we need to understand exactly what's programmed into their genetic code, how they actually sense the things in the outside world because they can't learn, because they're-- the butterflies that migrated the last time have been dead a long time, so there aren't any butterflies around to show them the way.

Like a lot of birds learn to migrate from their parents, like geese and things. They migrate with other birds who have already figured out the way. But butterflies have to just know how to do it. So it's got to be, just like you said, programmed into their genetic code.

One idea that people have is they use land farms. They kind of have these genetic rules built into their brain and it tells them go South as far as you can until you hit the ocean and then turn left or turn West. That's one possibility. Or they could actually be able to sense where they are. It's possible they have the ability to sense that they're at this place in the United States. So they need to fly in this direction to get to Mexico. We don't really know what the rules are that they use.

MOLLY BLOOM: How long does it take for them to get down to Mexico?

KAREN OBERHAUSER: That is a good question. Nobody has ever tracked one monarch all the way to Mexico. We know that they start leaving here in kind of the middle of August. And right around the end of October is when they start arriving at the overwintering sites in Mexico. So somewhere on the order of 2 and 1/2 to 3 months is probably how long it takes them.

MOLLY BLOOM: And then-- so they must stop a lot along the way, not just a straight shot to Mexico.

KAREN OBERHAUSER: Yeah, in fact, we know that it's really important for them to have nectar along the way. So from a conservation perspective, the monarchs, up here, they need a lot of milkweed. The caterpillars need milkweed. And the adults need nectar. But when they're migrating, they need energy to fuel that flight. They need to store up a lot of energy, because they have to stay alive all winter.

If we imagine-- let's just think of a monarch right now. So the monarchs that we saw out in the garden this morning, those monarchs are on their way to Mexico. We're not sure where they started, probably a little bit North of here. And they're flying South. So those butterflies are going to fly all the way through Iowa and South through Texas and into the middle of Mexico.

And they'll find mountaintops. So they go to certain mountaintops in Mexico. And they stay there all winter long. And in the spring, they sense what's changing in the springtime, the opposite of what's changing now. The days are getting longer. Yep, so they sense that the days are getting longer. So in the springtime, they know that they should fly back North.

So they fly North. And the very butterflies that we saw in our garden won't come all the way back to Minnesota. They'll get somewhere into Texas or maybe up into Oklahoma, and they'll lay eggs there. And then their eggs will hatch and become caterpillars and eat and eat and eat a lot of milkweed. And then they'll come up here.

So they migrate South. The same butterflies come North. They lay eggs in the South. So her children will come back here and lay eggs. So the caterpillars that you see next spring will be the grandchildren of the ones that left here.

As adults, those butterflies only live for about one month. And then they die. Their offspring continue on. But the ones that migrate to Mexico live up to eight or even nine months. And the reason they can do that is when they go to Mexico, it's quite cool in those mountains. So they're going to a place, it's kind of like a refrigerator.

Up here it's like a freezer in the winter. But where they are in Mexico, it's like a refrigerator. It's cool. And they can stay alive longer because their whole metabolism, everything about their body just slows down.

HARRY: It's like in the cooler down in the lab.

KAREN OBERHAUSER: Exactly. In fact, we often keep them all winter in our cooler and we kind of make it like the conditions in Mexico.

HARRY: Cool.

KAREN OBERHAUSER: Yeah, it's very cool. And the other thing that lets them live longer is they're not mating and laying eggs. So when they start producing eggs, that kind of takes a lot out of their body. So it makes them get old faster and die. So they-- the butterflies that migrates Mexico wait until the next spring before they lay eggs, so they can live all through the winter.

HARRY: That's really cool.

KAREN OBERHAUSER: It is very cool.


MOLLY BLOOM: Today, we're answering the question, how do monarchs travel thousands of miles. In past episodes we've answered questions like--

ALLY: Is there life on other planets?


HARRY: Can dinosaurs be brought back from extinction?

MOLLY BLOOM: We'd love to find answers to your questions. Send us your questions, big or small, and they may be answered on an upcoming episode. You can email them to us at That's brainson-- all one word-- @m-- as in Minnesota--

SUBJECT 2: Brains On.

MOLLY BLOOM: I'm Molly Bloom.

HARRY: And I'm Harry Hanzy-Francis. And I'm Alisandra Hanzy-Francis.

MOLLY BLOOM: So Ally and Harry, as we were talking about earlier, you know a lot about monarchs first hand because you've been raising them this year. What do caterpillars need to grow?

HARRY: They need food, and they need someplace where they can be sheltered from the elements and predators.

MOLLY BLOOM: What kind of food do they eat?

HARRY: Monarchs eat milkweed. And they basically just eat whatever they were born on.

MOLLY BLOOM: And what's it like to watch them turn from a caterpillar into a butterfly?

HARRY: It's very cool because you can see them in their different stages, like every instar you can see them growing.

MOLLY BLOOM: So explain what that is for people who don't know.

HARRY: The instar is basically their stages of growth. And it's like they'll be moving and moving and eating and eating and then they'll stop. And that's them shedding their exoskeleton.

MOLLY BLOOM: At what point do they go into the chrysalis?

HARRY: After about their fifth instar about.

MOLLY BLOOM: And then inside the chrysalis can you see through it to see what they look like?

HARRY: Not until the very last day in which they get all their colors. And then it turns black and orange. And it looks really cool.

MOLLY BLOOM: So that's how you can tell they're about to hatch out and become a butterfly when it gets darker?

HARRY: Hmm-mm.

MOLLY BLOOM: That's really cool. So when your butterflies came out, Ally, what did you do with them?

ALLY: We set them free.

MOLLY BLOOM: And where did you set them free?

ALLY: We set them free on our front porch.

MOLLY BLOOM: You said, goodbye, little butterfly, good luck. Have fun in Mexico.

HARRY: Yeah, basically.

MOLLY BLOOM: Your family is doing a lot to help butterflies. And that's very important because Karen Oberhauser explained to us that the population of monarchs is actually declining. The number of monarchs migrating has dropped by a lot in recent years.

HARRY: Yeah. Professor Oberhauser told us that one of the reasons for the drop is there's just not enough milkweed anymore.

KAREN OBERHAUSER: People are using the land in different ways. So for example there used to be quite a lot of milkweed growing and fields, but now the milkweed doesn't grow there because the farmers are keeping the milkweed out of their fields. They need to do that. They need to keep weeds out of their fields.

If you have a garden, you have to weed your garden. So we've lost a lot of habitat that used to be available for monarchs. So what that means is we need to do everything we can to make sure there's other places for monarchs to find milkweed and nectar plants. So we need the plant gardens in our yards. We need to talk to people who own parks, like if there's a city park or a state park, they should plant milkweed and nectar plants in the land that they have. We could plant milkweed and nectar plants along roadsides. So we just need to be creative and think of other places that we can plant milkweed.

ALLY: We have a front garden that with milkweed--

HARRY: it's basically like a prairie garden.

ALLY: --and flowers.

HARRY: So we've got a bunch of milkweed.

ALLY: And we see monarchs fly over there and lay eggs and get nectar and stuff.

KAREN OBERHAUSER: That's great. So your family is really doing a lot to help monarchs. Enough people do that, we'll have enough space for monarchs. And the population can get back to what it used to be we hope.

MOLLY BLOOM: And not only can you help by making sure there's enough milkweed for monarchs.

HARRY: You can also help by becoming a citizen scientist.

KAREN OBERHAUSER: In the United States, there are 17 different monarchs citizen science programs. We have a program out of the University of Minnesota that's called the Monarch Larva Monitoring project. And in that one, people go out into their gardens or parks or somewhere there's milkweed, like you could do it in your yard. And they just look at as many plants as they can, and they write down how many plants they look at and how many eggs and caterpillars they see.

And there's another one where people-- it's called Journey North-- where people write down and enter on a website the first monarch they see in the spring. And if everybody writes down when they see their first monarch, we actually get this picture of the monarchs moving North. So there are a lot of things we can answer with citizen science data.

MOLLY BLOOM: By using your skills of observation you can help scientists figure out a lot about monarchs. For more about how you can become a citizen scientist, visit our website.



MOLLY BLOOM: Your eyes are needed to spot monarchs as they come through your town. But right now I want you to close them and use your ears. It's time again for the mystery sound. Let's hear it one more time.

Any new guesses since last time?

HARRY: Well, I'm pretty sure that it's rain. But it also might be the sound of a bunch of monarchs migrating.

MOLLY BLOOM: Here to reveal the answer is Peter Tilley.

PETER TILEY: So that's the sound of butterflies flapping their wings in the sun.

MOLLY BLOOM: So you were right here.


PETER TILEY: My name is Peter Tilley. I'm the supervising sound editor for Flight of the Butterflies, the IMAX 3D film.

MOLLY BLOOM: Since Peter was recording the sound of the monarchs for a movie, he and his team did a little bit extra to enhance the sound.

PETER TILEY: We actually had the butterflies landing on the microphones and very close to them. So that was one of the elements that we used. And then we just used textures of other plants, other pieces of cloth, and so on.

MOLLY BLOOM: Peter and his team made the very cool IMAX movie Flight of the Butterflies, a 3D movie tracking the journey of monarchs from Canada all the way to Mexico. It's playing all around the country. You can find out where at

In a couple of weeks, we're going to have a Minnesota with a bonus mystery sound. It's another one from this movie. So tune in to find out what it is. We've got our answer.

HARRY: Butterflies need a fridge and not a freezer to survive the winter.

ALLY: So they head South.

HARRY: It takes the monarchs months to travel over 2000 miles to Mexico.

ALLY: And to get there--

HARRY: They use their inner clocks.

ALLY: And compasses.

MOLLY BLOOM: But scientists still don't know exactly how they find their way to the mountains.

HARRY: We do know that we can help monarchs by planting milkweed and nectar plants to keep them going.

ALLY: And we can be citizen scientists.

MOLLY BLOOM: That's it for this episode of Brains On.

HARRY: If you like what you hear, you can subscribe to the podcast using your favorite app or review it on iTunes.

MOLLY BLOOM: You can also follow us on Twitter @Brains_On.

ALLY: And you can like us on Facebook.

HARRY: And you can always send questions and virtual high fives by email to a

MOLLY BLOOM: Many thanks to--


HARRY: This episode was produced--


ALLY: Thanks for listening.

SUBJECT 2: Brains On. Brains On.


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