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In the absence of euphorbia – their favorite food – the caterpillars of monarch butterflies (Danaus plexippus) go from peaceful feeders to aggressive fighters. Researchers reporting in the journal iScience on November 19 he observed that caterpillars with less access to food were more likely to lunge at others to set them aside, and caterpillars were more aggressive during the final stages before metamorphosis.
“I saw that there were virtually no published papers on aggression in this or any other caterpillar species, but a lot of exciting work has been done on fruit flies where they found single pheromone receptors or single genes that trigger the aggression, “says lead author Alex Keene, a professor of biological sciences at Florida Atlantic University. “Now we may be able to take that powerful neurobiology and genetics and study it in a more ecologically relevant organism.”
While Keene usually studies neurobiology using fruit flies, he was inspired to investigate monarchs after observing their fighting behavior in his garden. Because they are large and recognizable compared to many other insects, monarchs are a useful indicator of the status of pollinator populations in an ecosystem. If monarchs are decreasing, other pollinators are likely to be decreasing as well.
Monarchs also influence the milkweed plants they consume: in their largest and most hungry phase, a single caterpillar can eat an entire milkweed leaf in less than 5 minutes. “If you compare it to a fruit fly where there are a lot of larvae on a rotting piece of fruit, there is less competition there,” says Keene. “But each of these caterpillars will encounter resource constraints at some point in their development cycle.”
To model this resource limitation, the researchers faced the challenge of maintaining a population of monarchs. Inspired by his butterfly garden at home, Keene and his team built an open euphorbia garden behind their Boca Raton-based laboratory and let nature take care of harvesting the caterpillars. Back in the lab, the researchers placed the caterpillars in groups with varying amounts of milkweed. The results were clear: the less food, the more likely the caterpillars would try to head each other to satiate themselves. But the process of getting there was anything but simple.
“We certainly had a lot of challenges. We had a hard time raising monarchs in the lab and found that almost all nurseries sell their milkweed with pesticides. So, we ended up having to grow our own,” says Keen. “But I like to say that resilience is one of the main characteristics that scientists need to have because most of what we do doesn’t work.”
Although this research has shown that caterpillars respond aggressively to limited food, the researchers still hope to learn more about what drives this response in their brains, which is important to learn more about how these responses work outside. of the laboratory.
“One of the fundamental problems with a job like this is that we’re testing animals in a very derivative environment. And that’s not what the brain evolved to do,” Keene says. “So now that we have this model of invertebrates in a relatively controlled environment, but it does ecologically relevant behavior, which becomes important in terms of observing the mechanism and function of this behavior in more complex organisms.”
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The authors were supported by the National Science Foundation.
iScience, Keene et al.: “Aggression is induced by resource limitation in the monarch caterpillar” https://www.cell.com/iscience/fulltext/S2589-0042(20)30988-3
iScience (@iScience_CP) is an open access journal from Cell Press that provides a platform for original research and interdisciplinary thinking in the life, physical and earth sciences. The main criterion for publishing in iScience it is a significant contribution to a relevant field combined with solid results and underlying methodology. Visit: http: // www.
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