Monarch butterflies engage in a spectacular migration that encompasses multiple generations. They surf the green wave of spring as it spreads north, producing new generations on the way. Then, as autumn sets in, that generation of butterflies shuts down reproduction and starts heading south, eventually reaching their wintering grounds in staggering numbers. But in recent years, those numbers have grown far less staggering. The loss of some of those wintering grounds, habitat destruction across North America, and other threats have steadily reduced the migrating population to the point where it's under consideration for endangered species designation.

The declining population has inspired people throughout North America to try to give the butterflies a hand. Their efforts include planting more of the insects' favorite food plants, protecting the butterflies in their pupal stage, and even ordering monarch pupae from commercial suppliers. Raising monarchs is also a common school project.

But a new study raises questions about whether buying pupae is really helping the butterflies. A group of researchers at the University of Chicago have found that the monarchs purchased from commercial suppliers may not be able to migrate effectively and so might only give the monarch population a transient boost.

Homebodies

Migrating monarch populations undergo both behavioral and physiological changes once the autumn rolls around. Their reproductive system is limited, and females will be carrying far fewer unfertilized eggs in the autumn. In addition, when placed outside, the animals will generally orient toward the south, the direction of their ongoing migration. So the question on the researchers' minds was whether the offspring of monarchs raised in captivity by suppliers would display the same behaviors.

To find the answer, the researchers obtained monarchs from a commercial breeder and caught others in the wild. Both were raised in an outdoor environment, so they could pick up on any seasonal cues as to whether they should be migrating. In the summer, the females carried lots of eggs, and the population overall showed no preferential orientation when put in a (I am not making this up) "monarch flight simulator." In the autumn, however, the butterflies caught in the wild showed reduced reproductive capacity as well as a strong tendency to orient to the south. Neither of those were true for the butterflies obtained from commercial suppliers.

This could suggest that long-term captive breeding had selected against the ability to migrate in the commercial population. But there's an alternative. From their winter base in Mexico, monarchs have spread to the Caribbean, South America, and out into the Pacific, reaching as far as Australia. None of these additional populations undergoes the sort of migratory behavior seen in the North American population. So, there's a chance that the commercial populations were simply originally source from a nonmigratory population.

To find out, the researchers turned to genetics. In the past, these have shown that the three non-migratory populations were all independently derived from the North American monarchs. When the researchers performed a similar analysis, the results suggested that the domestic population was a distinct branch—effectively a fourth non-migratory lineage.

Turn south

While it's tempting to ascribe the differences between the populations entirely to genetics, the researchers found an additional complication. When they brought in a few pupae to raise for a few days, they found that even this short time spent indoors may interfere with the butterflies' ability to orient to the south, even when they were raised in containers that mimicked the temperature and lighting of the autumn. Only a few monarchs were tested this way, however, so the researchers are hesitant to draw any strong conclusions about it.

There are a lot of potential implications here for efforts to help restore the species, as this is not a case where any commercial butterfly is a problem. For one, butterflies obtained from a different commercial supplier have been found in the wintering grounds in Mexico, suggesting at least some of them can migrate. In addition, the researchers found that migratory behavior returned after just two generations. So, butterflies obtained early enough in the year may help expand the migratory population.

Even then, the authors argue that monarchs sold to schools do plenty to build awareness of the species' problems, and they built support for other, possibly more effective conservation measures.

Separately, there's the issue of raising monarchs indoors. "Summertime hobbyists raise monarchs in their homes throughout the summer and autumn and then release them," the authors note, "hoping that they or their offspring will fly south to Mexico." If the limited results regarding indoor raising hold up, then the autumn releases may be counterproductive.

In addition to the conservation aspects, the work adds another layer of complexity to the process of migration. The behaviors involved with migration are complex enough; now we know that the decision of whether to migrate or not also involves a mix of environmental cues and genetic influences.

PNAS, 2019. DOI: 10.1073/pnas.1904690116 (About DOIs).