Why do monarch butterflies migrate

Monarch butterfly

Four generations hatch within a year

With its orange-colored wings, which are covered by a black-colored net, and the white spots of color on the wing tips, it is easy to distinguish from other butterflies native to the USA. There the monarchs mate so diligently that they can produce four generations in one summer.

The caterpillars are conspicuously drawn and signal their predators: "I am inedible!" This is also true because they absorb toxins from their forage plants, certain swallowweed plants.

Strong flight muscles bring the moths south

The last generation of each year, which hatches before winter sets in, is different from the others. Their performance made the monarch butterfly a real star among butterflies: This generation has extremely well-developed flight muscles. And it is precisely these that make her a true masterpiece of the animal kingdom.

Although the moths don't even weigh a gram, they embark on a journey of many thousands of kilometers. The animals migrate in groups to winter in the south, as far as southern California or Mexico. An average of 70 kilometers per day and up to 330 kilometers on peak days. After two months the first reach their destination, after around three months the last.

6000 kilometers - to Mexico and back

For many decades it was a great mystery where exactly the monarch butterflies spend the winter. It was not until 1975 that a large winter quarter was found: It is located in the Sierra Madre Mountains northwest of Mexico City at an altitude of 2750 meters above sea level.

Millions of butterflies here cover an area of ​​several hectares, densely packed with trees and soil, and hibernate in a kind of rigid cold.

But in the coming year, even before spring really moves in in the winter quarters, they will start their return flight. And on their way home they are getting married, because meanwhile the individual individuals have developed their sexual organs.

Eggs are laid on the way and the first travelers die on the way, that is, they no longer reach the place of departure. Those who come to the point where they first lived as an egg, then as a caterpillar and finally as a butterfly last year have flown a total of around 6,000 kilometers. An absolute record for a butterfly.

Sun, geomagnetic field, polarized light - the question of orientation

But how is it possible with such huge distances that the animals find their way to their destination and back again? When it comes to orientation - i.e. where the route is going - the sun helps the butterfly, researchers found.

Henrik Mouritsen from the University of Oldenburg and Barrie Frost from Queen's University in Kingston, Canada wanted to find out how the monarchs orient themselves. To do this, the researchers had several animals fly in a kind of flight simulator on a stream of air coming from below.

The result: the moths tried to set off in a south-westerly direction, i.e. in the direction where their southern winter quarters are located. So the sun is a kind of directional giver for the butterfly.

However, even when the sky is overcast, the monarchs are single-mindedly flying south. So there has to be another navigational aid that gets the butterflies safely to their destination year after year.

Another possibility for coping with the long haul, in addition to the sun, would be to orientate yourself towards the earth's magnetic field. Some kind of built-in biological compass could help the animals stay on course.

So much for the theory, but a change in the magnetic field in the experiments of the two scientists had no effect on the chosen flight route. But how does the targeted long-haul flight work? Polarized light from the sky could be the solution to the riddle, because aligning the light with the magnetic field helps many insects, such as bees, to achieve their goal.

The question of how long-haul pilots will find their destination has not yet been fully clarified, but one thing is certain: They will fly, at least as long as monarch butterflies still exist.

In recent years, biologists have repeatedly reported a threat from the destruction of the wintering areas in Mexico, where deforestation and deforestation have been destroying the typical biotope of this butterfly for decades.

And in their summer quarters in the USA, the butterflies suffer from the fact that their forage plants are considered weeds and are therefore increasingly being combated.