By Karina Shah

Very hungry monarch caterpillars get hangry, resulting in them headbutting and lunging at other caterpillars in an attempt to secure food.

“The less food that is present, the higher their level of aggression,” says Elizabeth Brown at Florida Atlantic University.

Monarch caterpillars, found across North and Central America, only eat milkweed leaves. Brown and her team gave the caterpillars three different amounts of food and found that they attacked each other significantly more when the leaves were scarce.

Larger monarch caterpillars – those in the final stages before starting to transform into butterflies – often showed the highest levels of aggression, probably because they need more food, says Brown.

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“There’s a clear winning caterpillar and losing caterpillar,” she says. “This often scales with their size.”

The hungry caterpillars only attack when their target is actively feeding, and this never occurred while a caterpillar was resting. The attacking caterpillar seeks to disrupt feeding and claim a food source for itself.

“You can often see a single caterpillar strip down an entire plant of its leaves,” says team member Alex Keene, also at Florida Atlantic University. “So, there is a big cost to these caterpillars if there are three of them on a plant with you.”

Many animals become aggressive when competing for food. The researchers hope to learn more about the genetic basis for aggression by studying the caterpillars. “There’s a lot we could learn about more complex animals from this ecologically relevant insect model,” says Keene.

Journal reference: iScience, DOI: 10.1016/j.isci.2020.101791

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