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When milkweed leaves are scarce, hungry caterpillar get angry

Caterpillars fight over milkweed leaves by headbutting their rivals out of the way. Photo by Alex Keene
Caterpillars fight over milkweed leaves by headbutting their rivals out of the way. Photo by Alex Keene

Nov. 19 (UPI) -- New research suggests monarch butterfly caterpillars, Danaus plexippus, get hangry when there aren't enough milkweed leaves to share.

As many parents are acutely aware, caterpillars are driven by a voracious hunger. In the iconic Eric Carle book, The Very Hungry Caterpillar, a caterpillar happens upon a smorgasbord of goodies -- all for himself. In the real world, caterpillars are rarely so lucky.

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Unlike Carle's caterpillar, monarch caterpillars only eat one thing: milkweed. When the time for metamorphosis nears and the supply of milkweed leaves dwindles, new research suggests monarch caterpillars turn aggressive.

In lab experiments, researchers found hangry caterpillars headbutt their peers out of the way when jockeying for food. It's the first time scientists have documented aggression among groups of caterpillars.

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"The field of insect aggression has been understudied but there is growing appreciation for its importance," lead study author Alex Keene told UPI in an email.

"Studies in fruit flies have been able to identify single genes and neurons that contribute to aggression, so I used these studies as the basis for defining aggression in monarchs," said Keene, a professor of biological sciences at Florida Atlantic University.

Keene first noticed the behavior in his butterfly garden at home. To collect monarch caterpillars for the study, Keene and his research partners grew a butterfly garden outside their lab.

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Researchers collected caterpillars from the leaves of milkweed plants and brought them into the lab. After dividing the caterpillars into groups, researchers offered the insects varying amounts of milkweed.

Their observations revealed a simple pattern: caterpillars with access to less milkweed were more likely to headbutt their peers.

"It's very exciting that the behavior is triggered by limited food availability," Keene said. "We can ask questions now how the brain senses food scarcity to initiate aggression. Colloquially, we can look for 'hanger' genes."

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Amusing as it is, caterpillar aggression is no joke for those jockeyed out of the way by their bigger, bolder peers.

"The caterpillars on the receiving end typically leave the food source. In the wild, this is probably very costly to them," Keene said.

In followup studies, scientists plan to study the neural patterns that support aggressive behavior among monarch caterpillars.

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To do so, researchers must genetically engineer caterpillars to carry calcium ions in their neurons.

"This type of work is commonly performed in fruit flies, worms and other commonly used lab animals," Keene said.

"There are technical hurdles to developing new models, but we think its within our capabilities," he said. "This type of work can tell us which neurons are involved in aggression, and how there activity is regulated by food availability. We can effectively watch the behavior occur within the brain."

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