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Hangry: Study Reveals Caterpillars Will Fight Each Other for Food

Think you get cranky when you're hungry? Research shows monarch butterfly caterpillars will headbutt each other when there's a shortage of their favorite food.

(CN) — Research shows that you're not the only one who gets grouchy when you’re hungry. According to a study released Thursday, when monarch butterfly caterpillars are faced with a shortage of milkweed, they become easily agitated and aggressive, and will physically headbutt competing caterpillars for leaves.

Details of the study are published in the journal iScience where the authors discuss how these normally calm, peaceful critters become little fighters once their hunger gets the best of them.

The monarch butterfly is one of the most recognizable butterflies in the world, with their distinctive orange, black, and white wings. They are native to North and South America but will thrive anywhere they can get their favorite food: milkweed. This food source also provides them with an added defense, as it contains toxins that remain in their bodies after consuming it, making them poisonous to predators. 

When a female monarch lays her eggs, it is on a milkweed plant, and when those hundreds of eggs hatch, the caterpillars get to work eating and growing. After a couple of weeks, they spin themselves into their cocoons and enter the chrysalis stage and soon after emerge as elegant butterflies.

Lead author and professor of biological sciences at Florida Atlantic University Alex Keene has conducted important research in the past analyzing aspects of neurobiology in fruit flies, specifically taste memory, but he became interested in the monarch butterflies after observing their behavior in his own backyard. 

"I saw that there were basically no published papers on aggression in this or any other caterpillar species, but there's a lot of exciting work that's been done on fruit flies where they've found single-pheromone receptors or single genes that trigger aggression," Keene said. "Now we might be able to take that powerful neurobiology and genetics and study it in a more ecologically relevant organism."

The monarchs face a series of environmental challenges, one of the most daunting being competition for their single food resource. When a caterpillar reaches its largest size before metamorphosis, it can consume a whole milkweed leaf in under five minutes. 

This led Keene and his team of researchers to wonder how the caterpillars handle a lack of resources amongst themselves.

"If you compare that to a fruit fly where there are a lot of larvae on one piece of rotting fruit, there is less competition there," Keene said. "But each of these caterpillars will at some point in their developmental cycle encounter resource limitation."

To investigate this, they constructed a milkweed garden behind their lab in Boca Raton, Florida, and after a matter of time they were able to collect the caterpillars left by the monarch population. 

They then split up the caterpillars into groups with varying amounts of milkweed and observed how they reacted. The team found that the groups with less food were more competitive and combative, often headbutting each other in a race to get to the milkweed.

"We definitely had a lot of challenges. We had a hard time breeding the monarchs in the lab, and we found that almost every nursery sells their milkweed with pesticides. So, we ended up having to grow our own," Keen said, commenting on the difficulty of sustaining the monarchs. "But I like to say that resilience is one of the main characteristics scientists have to have because most of what we do doesn't work."

The results of the study clearly showed that when faced with a food shortage, the monarchs did not hesitate to fight others for that last milkweed leaf. In the future, the researchers hope to study what happens within the brains of these insects to prompt this response, and how it presents itself in their natural habitats.

Furthermore, conservation experts have urged people to plant more milkweed in their gardens and other plants that promote butterfly population growth. The monarch butterfly is a species not yet declared as endangered but is in a clear state of decline. 

Since the 1980s, the monarch butterfly has declined anywhere from 80% to 99% due to pesticides, lack of available food and habitats, and climate change.

"One of the fundamental problems with work like this is that we're testing animals in a very derived setting. And that's not what brains evolved to do," Keene said. "So now that we have this invertebrate model in a relatively controlled setting, but doing an ecologically relevant behavior, that becomes important in terms of looking at the mechanism and function of this behavior in more complex organisms."

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