Deep in the forests of Michoacán, several hours drive north of Mexico City, tourists watch millions of Monarch butterflies as they cling to fir trees, gather on bushes, or take off in flight.
The striking black and orange insects winter in these mountains every year, traveling up to 4,800 kilometers to get here, from as far away as Canada. They stay through the winter, then, in the spring, the females leave these fir tree forests and head as far north as Kansas, to lay their eggs on milkweed plants.
It's an odyssey that inspires Chip Taylor, director of the Monarch Watch program in Lawrence, Kansas, over 2,000 kilometers north of the butterflies' winter habitat.
"We see them usually between the 14th and 21st of April if they get this far north," he says, adding that many of them can barely fly after their journey. "A lot of those butterflies are so tattered and broken, that they're crawling from milkweed and milkweed to lay the eggs."
A journey guided by the senses
Just as remarkable as the distance they fly is how the butterflies navigate. After all, they don't have guides, or maps, or GPS equipment. Instead, they use sight, smell, touch and other senses to find their way. To study that, butterfly scientists track the insects' normal flight paths, then they confuse them and track their path again.
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Taylor says his research teams used this method to discover that butterflies need accurate information about the earth's magnetic fields. "Ten seconds of exposure to the strong magnetic field was enough to totally disorient the butterflies. They could not maintain a course," he reports.
Sunlight matters, too, and by manipulating artificial sunlight, scientists can fool butterflies into traveling in the wrong direction.
At the University of Massachusetts, neuroscientist Steven Reppert studies the Monarch butterfly's migration with a sophisticated apparatus. It's a kind of flight simulator; essentially a large barrel. His research team connects a butterfly to a thin wire tether to track its normal flight path and then to track how that path changes once they've confused it.
Based on flight simulation studies, Reppert says that Monarchs use visual cues about the daylight sky to set their internal compass, as well as their brain's circadian clock, which regulates hormones and behavior. Reppert says researchers thought they had this control system mostly figured out. But then they took a closer look at the insects' antenna.
A multipurpose organ for navigation
"The antenna are really remarkable organs," Reppert says. "They're odor sensors. But they do a lot more than that. They can sense vibrations, they can act as ears, they can sense changes in barometric pressure, so there's a number of things that the antennae can do."
To study these abilities, Reppert's team decided to confuse the butterflies, at least a little, by removing their antenna. But this did more than eliminate their sense of smell and vibration. It totally befuddled their navigation.
Reppert says, "This finding was actually quite heretical to us. We did not know quite what to do with it at the time." So, they devised another experiment.
On the chance that smell mattered more than they had realized, the researchers painted butterflies' antenna to block their odor receptors. And just in case sunlight played a role, Reppert's team used clear paint on some butterflies' antenna and black paint on others. The clear paint allowed in daylight, and those butterflies navigated normally. The ones painted black could not.
Antenna data leads the way
This meant that the Monarch's antenna can detect light. But an even bigger surprise was up ahead. Scientists knew that the circadian clock in the insect's brain responds to light signals from the eyes. They had assumed that that system provided all the daylight data that the butterfly's navigation system needed.
But it turns out that the circadian clock signals produced in the butterfly's antenna are at least as crucial as the clock in its brain. The butterfly's bug-sized GPS needs that antenna data, so when winds and other surprises hit, the Monarch still flies in the right direction.
As for how all these pieces work together, Reppert says, the clues are tantalizing. "The antenna are talking to the brain and controlling the behavior in a very specific way. And we need to understand that."
Reppert's lab plans more experiments. He predicts they'll find that the ability of the Monarch's antenna to sense odors and magnetic waves may also help them set the course for the day. And he says the lessons learned will help us understand the role of circadian clocks in other animals, including people.