Why do monarchs overwinter in Mexico?

Most people familiar with monarch butterflies know that they migrate to central Mexico to overwinter.

You can read in-depth how monarchs find their way here, but in general, they use their circadian rhythm to orient themselves with the direction of the sun and also use the earth's magnetic field to fly toward the equator.

However, many people don't know why monarchs overwinter in Mexico. Even scientists are still finding out more about what makes the oyamel forests in central Mexico the perfect winter habitat --- western U.S. monarchs overwinter in southern California and the Baja Peninsula.

Monarch migration has not been well understood for long. It is was only in the 1970s that scientists finally tracked where monarchs were overwintering, following them to an area approximately 70 miles wide outside of Mexico City.

In the volcanic mountains, forests of fir trees, now protected by the Mexican government, were found to host the monarch butterflies in the winter. Oyamel firs are rare --- only 2 percent of the original forest remains --- and they only grow at high altitudes between 2,400 and 3,600 meters above sea level. Only 12 mountaintops have been found to host the rare habitat for monarchs.

This area seems to be perfect for monarch butterflies because it is both cool and moist, in a region that is otherwise arid during the winter. The oyamels create a microclimate where cloud cover provides consistent moisture. Plus, the cool temperatures allow the monarchs' metabolism to slow to conserve lipid reserves during the winter without causing the monarchs to freeze unless there is a stretch of bad weather.

Protective trees and shrubs also protect the butterflies from occasional snow, rain and hail storms.

Monarchs also cluster together on the oyamel trees to stay warm. Tens of thousands of monarchs can cluster on a single tree. The branches are generally tough enough to withstand the weight, but they do occasionally break.