Sunday, April 28, 2013

Follow the sun: how to navigate like a monarch

From now until early June, young monarch butterflies are emerging  in the southern U.S. after developing from eggs laid by their moms several weeks ago. They wait a full  4-5 days before mating (can you say precocious?) and then demonstrate for yet another year their species' miraculous ability to find their way north to summer homes, laying their own eggs along the way.

monarch butterfly en route to some thistle nectar
photo credit: Daniel Cook/Monarch Watch

These little orange-and-black tigers fly thousands of kilometers south each autumn to avoid winter cold and then north each spring to provide their offspring with the toxic milkweed (yummy!) they need to maintain their nasty bitter taste that birds avoid.

swallows on the wing
photo credit: Trendsupdates
Unlike larger migratory animals with longer lifespans -- such as birds, wildebeest, or whales -- individual monarchs don't complete the round trip. Their grandchildren and great-grandchildren are the butterflies that complete the journey, and their own descendants return south the following fall.

So, how do monarchs find their way from breeding to wintering site, or vice versa, if they’ve never been there? They have a built-in solar-driven compass, of course!

Yep. The butterflies have both a solar compass and an equally fascinating biological clock. Monarchs use the sun's position and movement to judge their latitude and direction, and they use major geographic landmarks, such as the Appalachian and Rocky Mountains, to help keep them on course.

Migration map from Queen's University Dept of Psychology
Monarchs use these mountains and coasts to stay on course

monarchs use various cues to find their way home
photo credit: Ann Ryan/Monarch Watch
Scientists have known that monarchs use the sun as a calendar to tell them when to begin the journey south to Mexico and as a compass to guide them on their way.

But a compass based on the sun's moving position in the sky would not work unless the insects also have an accurate clock to tell them the time of day.

A gene in the monarch butterfly acts as a biological clock that helps the butterfly determine where to fly. The gene produces a protein that is sensitive to light, which counts the hours of the day and sends this information to the monarch's solar compass.

So even though the monarch's brain is tiny, it somehow arranges information about time and space that allows it to fly in the right direction at the right time.

There's more -- they also steer with their antennae

monarchs' antennae are more important than we knew
photo credit: Thomas Bresson, Wikimedia
“Monarchs have a biological clock in their brain, but also one in their antennae that tells them when the sun rises and sets,” says Chip Taylor of Monarch Watch.

It's a sun-based compass that contains both a light sensor and a clock: by keeping track of the sun’s location in the sky, the monarchs born in early autumn sense that days are getting shorter, and nighttime temperatures are decreasing (brrrrr), meaning it's time to fly south!

The north-south migration leads the butterflies to spend the winter either in central Mexico (for eastern North American monarchs) or coastal California (for western monarchs). There they start revving up for mating and make the return trip north in late February/March, as days grow longer and warmer again.

monarchs in Mexico revving up for the spring flight north
photo credit: Jim Lovett/Monarch Watch

Map or no map? Monitoring monarch flight patterns

Recently, scientists in Canada tested whether monarchs kept a “map” of the Earth by examining their flight patterns from two locations. They first set the monarchs free within a flight simulator in Ontario in eastern Canada and watched as the butterflies flew southwest, in the direction of their wintering grounds in Mexico.  Right on track!

When the scientists moved these same monarchs to Calgary, in western Canada and set them flying again, the butterflies again flew in the same general southwestern direction, rather than adapt to their new starting point. The butterflies’ flight patterns indicated that the monarchs didn’t know that their starting point had moved 2,500 kilometers (1,500 miles).

monarchs enjoy a nectar drink together
photo credit: Montgomery Allen, US Fish & Wildlife Service

In nature, monarchs that migrate to Mexico do so from the eastern part of North America. Other scientists have thus questioned whether artificially moving the monarchs in this study well outside their normal range, into terrain they would never have entered naturally, was a true test of their possession of an internal map.

Monarchs from western Canada and the U.S. tend to overwinter on the California coast, not central Mexico. Perhaps Calgary was just off the map, so to speak, for these test monarchs from the East coast, and perhaps similar tests from sites within the normal monarch migration zone would help us understand the true nature of the monarchs' amazing navigational feats.

While the monarchs’ innate assumption that they are in the northeast of North America suggests that they may not judge longitude as well as latitude, it also indicates that new generations must be consistently returning to the same areas every summer. Because their ability to find their way seems to be tied to the trees and groves of their ancestors, the navigation ability of each traveler is key to not only their own well-being but that of future generations.

Learn more about monarchs pull this off every year!

  • Follow the Monarch's spring and fall migration via Journey North.

  • Experts discuss monarch migration mysteries.

  • The University of Minnesota's Monarch Lab provides essentials on monarch biology, migration, conservation.

  • A BBC discussion on how scientists showed the importance of antennae to monarch migration.

  • Learn how the University of Massachusetts Medical School team identified the key monarch gene that serves as their biological clock.

  • Some techniques used by Queens University Psych Department to study monarch navigation.

  • The original paperon moving monarchs across Canada:  Henrik Mouritsen, Rachael Derbyshire, Julia Stalleicken, Ole Ø. Mouritsen, Barrie J. Frost, and D. Ryan Norris. An experimental displacement and over 50 years of tag-recoveries show that monarch butterflies are not true navigators. Proceedings of the National Academy of Sciences, 2013; DOI: 10.1073/pnas.1221701110

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