The stories about migrating animals are fascinating. However, today I only want to talk about one specific but extraordinary migration. I am referring to a journey over 4000 miles long, from Canada to Mexico. The traveler is a 4 inches wide and 0.01 ounces animal. Any guess? I’m talking about monarch butterflies.
The reason for the trip is quite clear: butterflies migrate to Mexico after the summer so they can survive. In Canada, when the heat is not as intense and the sun is not as vibrant, the monarch butterflies start their way to avoid the risk of frost. Neither the monarch butterflies nor its preliminary stages (egg, larva or chrysalis) would be able to survive freezing winters. The question is: how these fragile animals can travel thousands of miles without having previous experience? There are many open questions regarding this trip. There are several possibilities to explain such a surprising migration. Some theories suggest a migration triggered by the intuition. As I said, also low temperatures, less hours of sun and lower solar light intensity may be another cause, but also senescence of the plants which lay eggs, or chemical variations of the nectar in fall might cause the migration too.
Every year there are four generations of monarch butterflies. Each generation lives between two and six weeks. Well, not really. Only three of the four generations live a few weeks; the ones staying all their life in the same region. The individuals of the fourth generation, born in September, live up to eight months. These 4th-generation butterflies are also larger, have bigger wings, are fatter and have a mission: reach the south where temperatures and humidity are high. At the beginning of their migration, monarch butterflies come into a state of diapause to minimize biological activities so they are able to complete the great journey. Instead of remaining fairly immobile, monarchs remain active until they reach the winter rest location. The areas towards they migrate are always the same and very focused: Oyamel trees reserve near Angangueo (Michoacan, Mexico), Pacific Grove in California and southern Florida. To give an example of how localized is the winter rest, Brower described in 1977, 14 billion butterflies in 1.5 Ha. The very same reason explain the survival of the monarch butterflies is also nowadays the main danger. What happens when humans cut down the resting trees? As in the case of hummingbirds (I recommend the article about hummingbird threats, April 2016), habitat loss is a major threat monarch butterflies are facing. Fortunately in Monterey, California, there is a butterfly protection season in place, and fairly successful. Also, in Mexico, the numbers of individuals are recovering, although the present numbers are still 30% below what was considered normal a few decades ago.
