Monarch butterflies look delicate, but they need to be super-tough to survive their annual migrations. The monarchs of eastern North America may travel thousands of miles to their winter home in Mexico’s Sierra Madre mountains. And, increasingly, they’re not making it, a problem that has been blamed on habitat loss, climate change and pesticides.
In an effort to boost the struggling insect’s numbers, some butterfly enthusiasts buy monarchs raised in captivity or breed their own, then set them free. But research published Wednesday in Biology Letters shows that captive-born monarchs are weaker than wild ones — adding evidence to the arguments of those who warn that releasing them does more harm than good.
Earlier research has shown that monarchs raised in captivity are less likely to reach Mexico. To find out why this might be, Andy Davis, an ecologist at the University of Georgia, and his co-authors raised 83 monarchs in two different indoor settings, using eggs from wild butterflies. They also caught 41 wild monarchs and brought them into the lab. Then — like a miniature NFL combine — they put the insects through a series of tests.
The scientists measured the monarchs’ wings, because larger, more elongated wings are known to help with migration. They also assessed the orange color of the butterflies, which can range from pale yellow to nearly brick-red. Monarchs with darker orange wings are more successful migrators, though this probably has to do with the butterflies’ overall fitness and not the color itself.
Finally, the scientists tested the monarchs’ strength. During migration, monarchs may have to hold on tightly to trees during high winds or storms. The researchers attached a wooden rod to an electronic force gauge and wrapped the rod in plastic mesh so the butterflies could grip it. Then they had each butterfly grasp the perch with its feet, and gently tugged the butterfly upward until it let go.
Wild butterflies dramatically outperformed those born in captivity. On average, captive-born monarchs were less than half as strong as wild ones. Though their wing size wasn’t significantly different, the captive-born monarchs had less elongated wings. They were also paler in color.
Something about rearing monarchs in captivity seemed to make them less fit for migration. Dr. Davis thinks the most likely explanation is that hand-raising caterpillars is too safe.
In the wild, monarch caterpillars often become food for other animals. Only about five percent reach adulthood, Dr. Davis said. That’s before migration, which itself takes a huge toll on the population. “Only the strongest, fittest individuals ever make it to Mexico,” he said. But in captivity, all the insects survive. “You’re basically bypassing natural selection.”
Dr. Davis thinks releasing these wimpier bugs en masse could harm the whole monarch population. Even if most captive-born bugs die during migration, the few that survive might spread their weaker genes. (Dr. Davis, citing annual summer surveys, questions whether the population as a whole is declining. Either way, though, he says the failure of many butterflies to reach Mexico shows their migrations are being disrupted.)
Facebook groups are full of hobbyists who raise and release monarchs. Commercial businesses sell the butterflies for mass releases at weddings and other events. In all, Dr. Davis estimated that people free “easily hundreds of thousands” of captive-born monarchs each year. Although these people have good intentions, Dr. Davis said, adding huge numbers of monarchs that may be poor migrators to the population is “the last thing we should be doing.”
Marcus Kronforst, an evolutionary biologist at the University of Chicago who has also studied captive-born monarchs’ migratory problems but was not involved in this study, said that Dr. Davis’s study shows “pretty striking differences” in the physical traits of captive-born monarchs. However, he noted that more might be learned by studying captives that were raised outdoors, too. And releasing butterflies that weren’t culled by natural selection may not harm the gene pool of the whole population, said Ayse Tenger-Trolander, Dr. Kronforst’s co-author and a graduate student in his lab.
The researchers stress that raising a few monarchs as a fun family project, or for education or citizen science, is fine. Dr. Davis said he doesn’t want to stop people from rearing a few monarchs at home.
But either way, the evidence shows that raising and releasing monarchs isn’t a good conservation strategy. “Our resources may be better spent on habitat conservation and fighting climate change, rather than rearing armies of monarchs,” Ms. Tenger-Trolander said.