Each fall, monarch butterflies east of the Rockies make the long migration to their overwintering site in Mexico. Scientists have exposed the internal rhythms that tell the butterflies where to go, despite the fact that the site was last visited by their great-grandparents. But how do the butterflies know when it’s time to leave Mexico and head north again?

Researchers at the University of Massachusetts suspected that temperature or daylight had something to do with it, so they collected wild monarchs at the start of their southward fall migration, brought them into the lab, and divided them into three groups. Two of the groups were exposed to the same cooler temperatures they would experience in their overwintering ground in Mexico. In addition, one of these two was also exposed to the same changing light levels they would experience south of the border. For the third group, the temperature remained warm and light levels never changed.

When placed into a flight simulator 24 days later, the first two groups of monarchs began flying northward. The third group, not exposed to cooler temperatures, continued flying southward.

If temperature alone determines when the butterflies start their northward migration, scientists raise concerns about the effects climate change will have on these beauties. “Without this thermal stimulus, the annual migration cycle would be broken, and we could lose one of the most intriguing biological phenomena in the world,” says Steven Reppert. His study, coauthored with Patrick Guerra, appeared last month in Current Biology.

Sadly, on the heels of that study comes a report issued last week on the decline of monarch numbers in Mexico this past winter. The study blames drought in the American southwest and an increase in soy and corn farming. Both are responsible for a loss in the milkweed plants the butterflies rely on for food during their long migrations.

“If people want to help,” says Craig Wilson of Texas A&M University, “they can pick up some milkweed plants right now at local farmer’s cooperative stores and this would no doubt be a big boost to help in their migration journey. It is important to have a national priority of planting milkweed to assure there will be monarchs in the future. If we could get several states to collaborate, we might be able to provide a ‘feeding’ corridor right up to Canada for the monarchs.”

Image: Kenneth Dwain Harrelson/Wikipedia

These particular butterflies have never visited the site in Michoacan before—in fact, their grandparents were likely the last generation there—but somehow the orange and black beauties know exactly where to go.

Circadian clocks in the monarchs’ antennae and brain direct the butterflies in their migration, but researchers at UMass Medical Center wanted to dive deeper. “There must be a genetic program underlying the butterflies’ migratory behavior. We want to know what that program is, and how it works,” explains Steven M. Reppert, MD, chair of neurobiology.

So he and his colleagues sequenced the monarch’s genome. Nature’s newsblog reports:

The 273-million basepair genome is the first of any butterfly and is considerably smaller than—and quite different from—that of the commercial silk moth (Bombyx mori), which has 432 million basepairs, suggesting rapid evolution in the Lepidoptera group, which includes both butterflies and moths.

The entire study is published in a recent edition of the journal Cell.

Within those 273 million basepairs, an estimated set of 16,866 protein-coding genes, comprising several gene families, are likely involved in major aspects of the monarch’s seasonal migration, according to the UMass researchers. These genes influence all of the monarchs’ senses in order to navigate: visual input gathers clues from the sun; monarch-specific expansions of odorant receptors exist for long-distance migration; a full repertoire of molecular components exist solely to support the monarch circadian clock; additional molecular signatures orient flight behavior; and a variant of the sodium/potassium pump underlies a valuable chemical defense mechanism to fend off predators during the migration.

“Dissecting the genetic basis of long-distance migration in the monarch may help us understand these mechanisms not only in monarchs but more generally in other migrants, including migratory birds and sea turtles,” Reppert says.

Image: Sonia Carolina Madrigal Loyola/Wikipedia

In the animal kingdom, females have very high expectations. Across thousands of species, they choose their mates based on the brightness of plumes or the boldness of facial markings. By contrast, males pull out all the stops as suitors in the hope that the female will like what she sees. But new research into the sexual behavioral patterns of butterflies has shown that, given the right environment, males can be just as choosy.

The process by which males evolve beautiful (and often impractical) attributes to impress the ladies is called sexual selection. The term, coined by Charles Darwin, describes the struggle between males for the attention of a female.  Males evolve ever bigger and more elaborate ornaments over many generations, competing against other males for the attention of a female.

But one member of the animal kingdom does things a bit differently. Yale University scientists, led by Kathleen Prudic, were intrigued by the squinting bush brown butterflies (Bicyclys anynana) because courtship behavior seemed to be reversed.

The majority of the time, the butterflies’ behavior wasn’t that unusual. Males had black eye-shaped wing spots and displayed them to a female who chose her favorite. But sometimes, the roles are reversed. In these cases, it’s the females who develop the wing spots and display them for the males. The males then choose their favorite female. Prudic and her team set out to find the answers behind this puzzling behavior. Their results are described in the January 7^th issue of Science.

They thought this role reversal could be linked to changes in weather, so they raised two groups of butterfly larvae in two very different temperatures: one group was raised in warm and moist conditions, the other in a cool and dry place.

When the butterflies emerged from their cocoons, Prudic and her team noticed the differences immediately. The female butterflies that were raised in the warm environment didn’t develop the black wing spots. Moreover, they were quite choosy and more likely to mate with males who had the biggest wing spots. Females raised in the cool and dry environment were very different. They developed their own comparable black wing spots and played the role of suitors, trying to impress their male counterparts.

Prudic believes this role reversal may have something to do with food. When males mate with females, they not only deliver sperm, but much needed nutrients as well. In cooler, dryer climates, food is harder to find. Females must think outside the box to survive. In the case of the squinting bush brown butterflies, this means displaying to and mating with as many males as possible.  Males, on the other hand, need to conserve their resources, and tend to be choosier.

Prudic and her team are excited what this research could mean about the developmental and evolutionary mechanisms behind all that sexual selection has produced—plumes, feathers, and all.

Anne Holden, a docent at the California Academy of Sciences, is a PhD trained genetic anthropologist and science writer living in San Francisco.

Image credit: William H. Piel and Antonia Monteiro/Courtesy of Yale University