American monarchs in Mexico

This is what it looks like when millions of Americans head south for the winter. Monarch butterflies (Danaus plexippus) cover trees for as far as the eye can see.

“It’s a mind-blowing sight,” says Venkat Talla, an evolutionary biologist from Emory University in the US. “It makes you wonder how they all know how to get there.”

Exploring that question – along with where they came from and what they did on the way – is one part of an ongoing research program that saw Talla and colleagues make a field trip to an overwintering site inside and adjacent to the Monarch Butterfly Biosphere Reserve northwest of Mexico City.

Research leader Jaap de Roode took these photos.

Most of his subjects are likely eastern monarchs from the east of the US (some fly from as far away as the Canadian border to get there – a 4600-kilometre journey). Western monarchs tend to prefer a shorter trip down the Pacific Coast to California.

However, the team’s recent full-genome analysis suggests that more than a few western monarchs may be choosing Mexico, where the two groups mingle. And they don’t all go back where they came from.

The research – published in the journal Molecular Ecology – also suggests the long-held view that they are genetically different populations may be wrong. Talla analysed more than 20 million DNA mutations in 43 monarch genomes and found no evidence for genomic differentiation.

From wherever they came, all the butterflies in these photos are first-time visitors, says co-author Amanda Pierce.

After monarchs leave their overwintering sites, they fly north and lay eggs. The caterpillars turn into butterflies and then fly further, mating and laying another generation of eggs.

The process repeats for several generations until finally, as the days grow shorter and the temperatures cooler, monarchs emerge from their chrysalises and start to fly south.