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The monarch caterpillars give their heads to fight for scarce food


When food and space are scarce, competition can bring out the worst in monarch caterpillars.

In the lab, researchers observed how itinerant caterpillars looking for hard-to-find food began to pounce on their heads and pounce on caterpillars eating an algae leaf. That aggressive behavior is apparently meant to interrupt the insects they feed on and help instigators score dinner. Biologist and neuroscientist Alex Keene and colleagues report online Nov. 19 on iScience.

Keene usually studies fruit flies and cave fish, but decided to adapt her lab to study monarchs after a casual observation. “My wife pointed out in the garden that these two monarch caterpillars were fighting each other,” says Keene, of Florida Atlantic University, on Jupiter. “I went on YouTube and there were videos of this behavior,” he says, but for the monarchs “it wasn’t documented anywhere in the scientific literature”. Other types of caterpillars showed similar aggressive behavior in other scenarios.

However, moving from a self-proclaimed “simple fly biologist” to a monarch researcher was a challenge. Hurricane Dorian in 2019 not only blew up the plants in the lab’s monarch garden, but also found pesticide-free algae plants that would eat the caterpillars was harder than expected. Once researchers overcame these challenges, they were able to film caterpillars competing with each other when researchers limited the amount of food available.

In a lab, researchers saw monarch caterpillars trapping and banging their heads over access to a leaf of dairy algae, their only source of food in nature.

“With declining food availability, we find higher levels of aggression,” as well as a “losing response,” with the caterpillar being attacked often leaving the area, says Elizabeth Brown, a biologist working in Keene’s lab.

This type of behavior occurs outside the lab, says Jaap de Roode, a biologist at Emory University in Atlanta who did not participate in the research. Competition can be tough, he adds, because monarch caterpillars are limited in their food choices. Insects only eat milkweed and are more or less attached to the plant in which they are born until they accumulate, because crawling from plant to plant takes energy (SN: 7/10/18). If there isn’t enough food from one plant to feed several caterpillars, “they won’t,” he says.

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Making things harder, the caterpillars need a huge amount of food to turn into a butterfly. “From egg to chrysalis, they multiply their weight 3,000 times and do it in about two weeks,” Roode says. That’s like a person gaining the weight of two blue whales in two weeks.

“Monarchs are like eating machines,” Keene says. But "some were much more aggressive than others." So the next step could be to explore whether or not the more aggressive caterpillars become aggressive butterflies, he says.


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