The plight of monarch butterflies—the extraordinary transcontinental journeys made on fluttering wings and across multiple generations, the sudden collapse of their populations—has inspired public concern unprecedented for an insect.
Across much of North America, citizen scientists track monarch sightings; gardeners and schoolchildren plant the milkweeds they need to reproduce. Sometimes they nurture eggs and even adult butterflies purchased from companies that breed monarchs in captivity.
New research suggests, however, that these captive-raised butterflies and their progeny may have lost the ability to migrate. They may even disrupt wild migrations. More research is necessary to know for sure, but the results, published in the Proceedings of the National Academy of Sciences, offer a potentially important note of caution.
“Our results provide a window into the complexity—and remarkable fragility—of migration,” write the study’s authors, who were led by biologists Ayse Tenger-Trolander and Marcus R. Kronforst of the University of Chicago.
To investigate their navigational habits, the researchers raised dozens of monarchs from eggs laid by wild-caught and commercially-purchased butterflies. These butterflies were kept outdoors, in net-enclosed field nurseries. Then, in late summer and autumn, the researchers put the butterflies into an apparatus that tracked the directions of their flights.
Descendants of wild monarchs quickly oriented southwards, as expected from creatures whose autumn migration ends in Mexico. Descendants of captive-bred butterflies, however, showed no such directional preference. Subsequent genetic analyses found them to be distinct from any wild migratory monarch population.
Non-migratory monarchs do exist, but those studied by the researchers didn’t trace their ancestry to them. Rather, they were descended from migrating populations—but just a relative few generations spent in captivity had profoundly altered their evolutionary trajectory.
As the butterflies studied came from just one company, it’s unknown to what extent the same patterns hold for monarchs from other companies. Indeed Tenger-Trolander and colleagues noted reports of captive-bred monarchs from another commercial breeder being released in Texas and spotted in Mexico, so the loss of migration doesn’t seem to be universal.
Still, “our results indicate that at least one group of commercially bred monarchs are much less likely to migrate than wild” monarchs, wrote the researchers. “Nonorienting monarchs released in the autumn are unlikely to migrate successfully and will not contribute to monarch population recovery or to the gene pool.”
Another part of the study showed just how quickly migration may be lost in captivity. The researchers hatched monarch eggs gathered in the wild and raised them in an incubator that simulated autumnal conditions, a common practice among hobbyist breeders; those butterflies likewise failed to fly south.
“We do not know what specifically about the indoor environment prevents the development of migration behavior,” the researchers write, but just a short indoor spell had short-circuited it. Even as the biology of migration is powerful enough to guide a butterfly thousands of miles, it’s also quite fragile.
All this is not reason to stop raising monarch butterflies, say Tenger-Trolander’s team. That goes extra for kids, whose experience raising monarchs is likely a key reason why so many people care about them—but these butterflies, they suggest, “should be locally sourced and subsequently reared outdoors where they will be exposed to the full spectrum of natural environmental conditions.”
Only then will reared monarchs have the best chance of completing their migration. And these butterflies will also be a reminder that, once lost, processes shaped by millions of years of wild interactions may not be easily restored.
Source: Tenger-Trolander et al. “Contemporary loss of migration in monarch butterflies.” Proceedings of the National Academy of Sciences, 2019.
Image: US Fish and Wildlife Service
About the author: Brandon Keim is a freelance journalist specializing in animals, nature and science, and the author of The Eye of the Sandpiper: Stories From the Living World. Connect with him on Twitter, Instagram and Facebook.