Every fall, America’s eastern monarch butterflies migrate up to 3,000 miles from their northeastern homes to the mountains of Mexico for winter. To make their epic migration, the iconic orange insects run—or, rather, fly—a gauntlet. Only the toughest bugs arrive at the overwintering sites, and this year saw a 53 percent drop in butterflies that made it all the way.
Andy Davis, an ecologist at the University of Georgia, estimates that people release between 200,000 and 500,000 hand-reared monarch butterflies into the wild each year, he tells Discover magazine’s Leslie Nemo. But new research by Davis and others, published on Wednesday in the journal Biology Letters, suggests that butterflies raised indoors might not make it very far.
“Only the strongest, fittest individuals ever make it to Mexico,” Davis tells Elizabeth Preston at the New York Times. Butterflies raised in captivity, on average, have paler, shorter wings and weaker grip strength than their wild counterparts, the research found. As Davis puts it, “You’re basically bypassing natural selection.”
The new evidence builds on research published last year that found that butterflies raised indoors struggle to migrate normally, though captive butterflies raised outdoors could find their way. Per Discover, that difference inspired Davis to quantify other ways that the indoor environment affects monarch butterflies.
The research team raised just over 80 monarch butterflies indoors, and captured 41 wild monarchs to compare. While the wild butterflies had elongated wings, which are good for migration, the hand-reared cohort’s wings were more rounded. The two groups were also different colors, with the wild wings tending toward the darker, brick red and orange associated with strong migrators, while the other group had paler, yellow wings.
Lastly, the researchers measured the butterflies’ strength. It’s a delicate procedure—the researchers wrapped a short wooden rod with plastic mesh, so the butterflies could hold on to it. The rod was affixed to an electric force gauge, so that if you pulled up on the mesh, the gauge would measure the strength of the tug. Then, they brought in the butterflies.
Holding the critters gently by their wings, a researcher lowered the insect to the rod. Once the butterfly grabbed on, the researcher would lift the insect up again. The gauge measured the force when the butterfly released its grip, and instead began a six-legged doggy paddle.
Grip strength is important because during their migration, butterflies may need to hunker down on branches and wait out a gusty storm. Unfortunately, the hand-reared butterflies were less than half as strong as the wild insects.
University of Chicago evolutionary biologist Marcus Kronforst, who co-authored the 2019 study but was not involved in the new research, tells the New York Times that Davis’ study shows “pretty striking differences” between captive-born monarchs and wild ones.
Because they’re near the bottom of the food chain, only about five percent of caterpillars reach adulthood, Davis tells the Times. Without predation to worry about, caterpillars raised by humans all reach adulthood, including weaker individuals. The migration to Mexico whittles down eastern monarchs’ population even further.
“Mother Nature has a way of making sure every monarch that reaches the overwintering site is the cream of the crop,” Davis tells Discover magazine. Davis raises the point that if people are releasing weak butterflies into the environment, they may be introducing poor butterfly genes into the gene pool, possibly damaging the population down the line.
Speaking to the New York Times, Kronforst and his co-author Ayse Tenger-Trolander say that there is more to be learned from studying butterflies that are hand-reared outdoors, and that that any butterflies that make it to Mexico have proved their fitness.
The research adds evidence to the argument that raising monarchs at home is probably not a strong conservation strategy.
“I understand that desire to see something physical out of what you do. It feels way better to raise a bunch of monarchs and watch them leave.” Tenger-Trolander tells Discover. To the Times, she says, “Our resources may be better spent on habitat conservation and fighting climate change, rather than rearing armies of monarchs.