The monarch butterfly Danaus plexippus is renowned for its two-way annual migration in North America, though it’s also found in New Zealand and Australia, where it’s called the wanderer.

In autumn, all of the monarchs in Canada and northern USA migrate south to Baja California and, most famously, the fir groves of Mexico’s Michoacán Province, where they take shelter through winter.

In the volcanic mountains of central Mexico, the cool, sheltered conditions provide the monarchs with a perfect winter refuge: not cold enough to kill them; not warm enough to wake them too early.

They arrive in October after their 3,000km journey from the USA, and cluster together in a near-hibernation-like state known as torpor.

In February, awakening males begin to pursue females in fluttering courtship ballets, before heading north in March, completing the annual two-way migration.

Their journey will end when they come across a belt of milkweed, the plant on which the females lay their eggs. But the next generations will press on – some as far as Canada. The full migration spans the life of three to four generations of the butterfly.

Mike Unwin

Main image" Monarch butterfly © Getty Images