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Retrograde monarch butterfly populations are difficult to restore



  Declining monarch butterfly populations are difficult to restore.

Monarch butterflies participate in a spectacular migration spanning several generations. They surf on the green wave of spring as it spreads north, bringing forth new generations along the way. Then, as autumn sets in, this generation of butterflies will stop breeding and begin to fly south, eventually reaching their overwintering areas in unimaginable numbers. However, these numbers have grown much less in recent years. The loss of some of these wintering areas, the destruction of habitats across North America, and other threats have steadily reduced the migrant population to the point where the expulsion of endangered species is being considered.

The decline in population has inspired people across North America to try to give butterflies a hand. Their efforts include planting more of the most popular insect food plants, protecting the butterflies in their juvenile stages, and even ordering Monarch dolls from commercial suppliers. The rearing of monarchs is also a common school project.

However, a new study raises the question of whether buying dolls really helps butterflies. A group of researchers at the University of Chicago found that the monarchs they buy from commercial vendors may not be able to migrate effectively, potentially adding only a temporary boost to the monarch population.

Homebodies

Migrant monarch populations are subject to both behaviors and physiological changes in the fall. Their reproductive system is limited and the females will carry far fewer unfertilized eggs in the fall. In addition, the animals usually orientate themselves in the open to the south, the direction of their current migration. So the question of the researchers was whether the descendants of monarchs raised by captive suppliers would behave the same way.

To find the answer, the researchers took monarchs from a commercial breeder and caught others in the wild. Both were reared in an outdoor environment so they could get seasonal indications of whether they should migrate. In the summer, the females carried many eggs, and the total population showed no preference orientation when they were put into a "Monarchs Flight Simulator" (I do not think so). In the autumn, however, the butterflies caught in the wild showed reduced fertility and a strong tendency to southward orientation. Both did not apply to the butterflies sourced from commercial suppliers.

This may indicate that long-term captive breeding had opted against the ability to migrate in the commercial population. But there is an alternative. From their winter base in Mexico, the monarchs have spread to the Caribbean, South America and the Pacific to Australia. None of these additional populations are subject to such migratory behavior as the North American population. So there is a possibility that the commercial population originally came from a non-migrant population.

To find out, the researchers turned to genetics. These have shown in the past that the three non-migrant populations descended independently of the North American monarchs. When the researchers conducted a similar analysis, the results indicated that the indigenous population was a separate branch ̵

1; indeed a fourth non-migratory line.

Turning South

While it is tempting to attribute the differences between populations solely to genetics, the researchers found an additional complication. When they brought a couple of dolls up for a few days, they found that even this short indoor time could affect the butterflies' ability to orient themselves to the south, even if they were reared in containers holding the temperature and temperature Light of imitate the autumn. In this way, however, only a few monarchs have been tested, so researchers are reluctant to draw clear conclusions.

There are many possible implications for recovery efforts as this is not the case Every commercial butterfly is a problem. For one thing, in the wintering areas of Mexico butterflies were found that were obtained from another commercial provider, suggesting that at least some of them can migrate. In addition, the researchers found that migration behavior already returned after two generations. Thus, butterflies obtained early enough in the year can help to increase the migratory population.

Even then, the authors argue that monarchs sold at schools do much to raise awareness of the problems of the species, and they have support for others, possibly for more effective conservation measures.

Separately, it's about raising monarchs indoors. "Summer hobbyists raise monarchs in their homes all summer and fall and then release them," the authors say, "in the hope that they or their descendants will fly south to Mexico." If the limited indoor rearing results are maintained, the autumn releases may be counterproductive.

In addition to conservational aspects, the work adds another level of complexity to the migration process. The behaviors associated with migration are complex enough. Now we know that the decision to migrate or not includes a mix of environmental and genetic influences.

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


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