Up to 1 billion monarchs winter in the mountainous pine-oak forests of central Mexico after completing one of the most spectacular migrations on Earth.

From November to March they hibernate here, waking only long enough each day to sip water and gather pollen. As the weather warms they fatten up and mate. The males die quickly, but the pregnant females head north to Texas in search of a milkweed plant on which to lay their eggs.

Four days later each egg hatches into a tiny caterpillar that feeds non-stop for two weeks before molting into a small cocoon-like chrysalis. Hormonal changes lead to the "birth" of the next generation of monarch, which continues the northward migration. Over the course of the spring and summer three successive generations of butterflies repeat this cycle on their northward trek. By the time the fourth generation emerges it is once again time to head south - a journey last undertaken by this generation's great-great-grandparents.

The El Rosario sanctuary is open to visits, but it's a long drive from Zitacuaro and you need to get here early in the day to catch the monarchs when they are active. It also requires some steep hikes at elevations great than 10,000ft (3,000m).
Up to 1 billion monarchs winter in the mountainous pine-oak forests of central Mexico after completing one of the most spectacular migrations on Earth.

From November to March they hibernate here, waking only long enough each day to sip water and gather pollen. As the weather warms they fatten up and mate. The males die quickly, but the pregnant females head north to Texas in search of a milkweed plant on which to lay their eggs.

Four days later each egg hatches into a tiny caterpillar that feeds non-stop for two weeks before molting into a small cocoon-like chrysalis. Hormonal changes lead to the "birth" of the next generation of monarch, which continues the northward migration. Over the course of the spring and summer three successive generations of butterflies repeat this cycle on their northward trek. By the time the fourth generation emerges it is once again time to head south - a journey last undertaken by this generation's great-great-grandparents.

The El Rosario sanctuary is open to visits, but it's a long drive from Zitacuaro and you need to get here early in the day to catch the monarchs when they are active. It also requires some steep hikes at elevations great than 10,000ft (3,000m).