Many birds, such as white storks and swallows, have been known to migrate, each year, from one region of the earth to another. Aside from birds, there are also mammals, reptiles, fish, and even butterflies that migrate. Among these journeys, the migration of monarch butterflies (Danaus plexippus) is perhaps the most fascinating.

After a caterpillar undergoes metamorphosis, a mature Monarch butterfly lives for approximately six weeks under normal conditions. But as we will see, there is one exception to this rule.

These butterflies, with a wing span of 8-10 centimeters, live in Canada and the Northern USA and they migrate to South America during their migratory season. They increase their body weight by consuming flower nectar before starting the journey, to prepare for the 5000 km long route ahead of them. During this journey, they change direction many times because of rain storms and extreme weather, but they are never lost.

Towards the end of August, Monarch butterflies begin their journey from North America to the Oyamel forests in Mexico. They fly around 80 km a day, reaching speeds up to 20 km/hour. Though most generations only live for six weeks, these butterflies live for months. After an arduous journey of 8 to 10 weeks, they arrive at the 3000 meter high mountains in November and December. The Mexican mountains make an ideal place for these animals. Butterflies hibernate in this region for four months, from December to March, and survive on their fat reserves.

While this north to south journey is completed by one generation, the return journey takes four generations to complete. With the onset of spring, Monarch butterflies start feasting on honey nectar to get the energy to head back north. Around mid-March, they start to migrate northward, headed back to their birthplace. Meanwhile, having reached maturity, they reproduce. Each member of this generation lays around 700 eggs in suitable places between March and April, and then dies near mid May.

Female butterflies lay their eggs near the leaves of poisonous plants. Hatching larvae feed on these leaves. These neurotoxic plant leaves serve as a protection against predators. This poison is found to remain in the tissues of the insect even after it becomes a mature butterfly and is not harmful to it. The protection is necessary, for monarch butterflies have charming colors that attract many other animals. Thus some birds try to prey on them, only to fail each time. Because of the neurotoxin, these birds become stunned as soon as they touch their beaks to the butterfly.

It takes about a month for an egg to mature to an adult butterfly. Thus, the first generation comes to existence on the journey home. Like a programmed machine, this generation continues their parents' journey north. They leave their eggs, for the second generation, during April and May, and then die in mid-June.

The second generation also continues the northward journey of their parents. They lay their eggs during May and June, and then die around mid-July.

Third generation Monarchs, however, arrive in Canada around June or July. The females of this generation lay their final eggs in mid-July and die during August. As a result, the fourth generation reaches maturity in a month, thus completing the annual tour. The fourth generation begins migrating south, towards the mountains of Mexico, at the end of the same month and hibernates there. This final generation lives six months more than the other generations, long enough to lay their eggs during spring. Without this character, the final generation Monarch butterflies would have died before spring, and would thus be unable to reach adulthood to lay eggs.

The migration to Mexico of a butterfly with such a delicate muscle structure confounded scientists for a long time. It is a fascinating, extraordinary event for millions of butterflies that, unlike migratory birds, lack a complicated nervous system. Yet despite this, they are able to pinpoint, and meet, in an area of around 1000 square kilometers.

An ultimate version of the global positioning (GPS) system as invented by the scientists is blessed as a birth-right to Monarch butterflies. Biologist Steven Reppert and his team from Massachusetts University have uncovered very interesting facts while studying butterfly antennae. When butterflies were placed in a flight simulator type of device with their antennae cut, they changed their direction after flying a while. Migrating butterflies with their antennae continued to move Southwest, however butterflies without their antennae flew in different directions. Not only did butterflies lose their navigational skills without their antennae but also had problems with their daily, time-dependent navigation.

Is it sufficient to explain this phenomenon by instinct only?

Pinar Celik has a PhD in medicine. She is a freelance writer from Turkey.


References

Zhan S, Merlin C, Boore JL, Reppert SM. "The Monarch Butterfly Genome Yields Insights into Long-Distance Migration". Cell 147 (5): 1171–85. 2011

Davis, A.K. "Wing color of monarch butterflies (Danaus plexippus) in eastern North America across life stages: migrants are 'redder' than breeding and overwintering stages". Psyche: 1–5. 2009

Merlin, C; Gegear, RJ; Reppert, SM. "Antennal circadian clocks coordinate sun compass orientation in migratory monarch butterflies". Science 325 (5948): 1700–1704. 2009

Kyriacou, CP. "Unraveling Traveling". Science 325 (5948): 1629–1630. 2009

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