How monarch butterflies navigate to a forest they have never seen

HostIt's always a little bit sad when the weather turns cold and you see that last butterfly of the summer fluttering around the garden. You know it probably won't be there in a few days because its life is so short. But as it turns out, that one butterfly might be about to do something that seems physically impossible. I was reading that the ones we see in late autumn aren't like the ones we see in June.

GuestThat's exactly right. Most monarchs live a very fast, very frantic life. They basically have about two to five weeks to find a mate, lay their eggs, and that's it. But the very last generation of the summer is a complete biological outlier. When the earth tilts and the air starts to cool down, it triggers a massive change in their bodies. They enter a state called reproductive diapause. It sounds complex, but it basically means they put their aging process on hold. They stop growing their reproductive systems and divert all that energy into building up fat and preparing for a long flight. Scientists call them the Methuselah generation because they can live up to eight times longer than their own parents or grandparents. Instead of a few weeks, these butterflies live for eight months, which is the only reason they can survive a three-thousand-mile journey.

HostSo they're essentially super-butterflies born just for this one trip. But they have never been to Mexico before. Their parents never went, and their great-grandparents never went. How do they even know which way to fly?

GuestIt's a huge mystery that we're only just starting to piece together. We know they use the sun as a compass to keep a steady path south. But the sun is a moving target. It crawls across the sky all day, so if you just flew toward it, you would end up flying in a giant circle. To fix this, they have an internal clock that lets them adjust their angle to the sun as the hours go by. For a long time, everyone just assumed this clock was inside the brain. But we found out it's actually located in their antennae.

HostWait, so the feelers on top of their head are actually the ones doing the navigating?

GuestYeah, it's wild. Researchers actually tested this by painting the antennae of some monarchs with black ink. When they did that, the butterflies could still see the sun perfectly fine with their eyes, but they completely lost their way. Because the ink blocked the light from hitting the antennae, the internal clock couldn't receive the signals it needed to calibrate the flight path. Without those feelers telling them what time it was, they couldn't use the sun to find south.

HostThat's incredible. But what happens if the sun isn't out? I mean, a trip that long is going to have plenty of gray, cloudy days where you can't see the sun at all.

GuestThey have a fail-safe for exactly that scenario. It turns out they have a backup magnetic compass built right into their eyes. They use these special light-sensitive proteins called cryptochromes to feel the magnetic field of the planet. But it doesn't work like the compass a hiker uses, which just points toward a pole. The monarch uses what we call an inclination compass. They actually detect the specific angle at which the magnetic field lines hit the ground. That angle changes depending on where you're on the earth. It gets steeper as you move away from the poles and toward the equator. So even if the sky is totally covered in clouds, they can feel their way toward Mexico by tracking those magnetic lines.

HostSo they have a sun clock in their feelers and a magnetic sensor in their eyes. But even with all those tools, they aren't just going to a general area. They end up in the exact same groves of trees every single year. How do they pick out a few specific acres in a whole mountain range?

GuestWell, you have to remember they aren't looking for a home they remember. They're looking for a very specific kind of refrigerator. They're heading for the Oyamel fir trees high up in the mountains of central Mexico. These trees create a very specific microclimate. It has to be just cool enough to keep their metabolism low, which means they don't burn through their fat stores too quickly. But it also has to stay just warm enough so they don't freeze to death. To find these tiny spots, they follow geographic funnels created by the shape of the mountains. They also pick up on the distinct moisture levels of the high-altitude cloud forest. That dampness tells them they have finally found the perfect spot for their winter dormancy.

HostIt's amazing to think that the same little insect that was just sitting on a flower in a backyard a few months ago can sense the tilt of the planet and the magnetic pull of the earth.

GuestIt really is, and they do all of it just to reach a forest they have never seen, where they'll wait for months until the cycle starts all over again.

HostThat tiny butterfly in the garden isn't just a visitor passing through, it's a long-distance traveler following a map written into its own body.