Food Fight

Hangry caterpillars fight, 'Hunger Games' style

When food is scarce, the monarch butterfly caterpillar gets ugly.

We all know what it's like to feel a little hangry. You haven't eaten in hours, your stomach begins grumbling, and you start to get a little on edge. Maybe you snap back at a coworker, or unexpectedly raise your voice in a conversation with loved ones.

It may be some consolation to know that humans aren't alone in experiencing this unique blend of hunger and anger.

In a study published Thursday in the journal iScience, scientists reveal that late-stage monarch butterfly caterpillars display aggressive tendencies when faced with limited food supply.

An example of aggressive feeding among monarch caterpillars.Credit: Alex Keene

Chow Time — One of the unique traits of the monarch butterfly (Danaus plexippus) caterpillars is that the caterpillars feed on the milkweed plant. It is the monarch caterpillar's primary source of food.

In the latest stages of their development, monarch caterpillars chow down like there's no tomorrow, rapidly going through their food supply and leading to competition over scarce resources.

"Monarchs primarily feed on milkweed, and we noticed that the late-stage caterpillars strip the leaves off the entire plant," Alex Keene, lead author on the study and professor of biological sciences at Florida Atlantic University, tells Inverse.

In the wild, the researchers noticed the caterpillars becoming aggressive in their final days before they became a pupa — the stage when they hang out in a cocoon before they transform into full-grown butterflies.

"We set out to systematically test the relationship between food availability and aggression in a controlled laboratory setting," Keene says.

This photo shows caterpillars fighting for space on the same milkweed leaf.Credit: Alex Keene

The link between aggression and food — or aggressive behavior at all — in caterpillars has never been scientifically documented before, according to the researchers.

Keene's team built a milkweed garden behind their laboratory in Boca Raton, Florida, letting the monarchs come to them. The scientists then placed the caterpillars near milkweed leaves in their laboratory.

The results were clear: Caterpillars initiated significantly more attacks on their fellow critters when the supply of milkweed was low and they were searching for food.

In most cases, the attacked caterpillar was feeding at the time of the violent encounter, suggesting that the attacking caterpillar was trying to clear out the competition so they could eat the milkweed leaf themselves.

This video shows caterpillars competing for the same food, including aggressive head butting behavior. Credit: Keene et al.

Future Implications — The study is a fascinating examination of how caterpillars initiate a Hunger Games-style brawl over scarce resources. It's also a timely piece of research.

In recent years, scientists have bemoaned the decline of monarch butterfly species, in part due to dwindling amounts of milkweed.

This could spell even more competition among the caterpillars for increasingly scarce food resources. The caterpillars that survive could be more aggressive as a rule.

"One could imagine that further competition for food sources increases aggression," Keene says.

"I imagine this means we are more likely to see this behavior in the wild, but also it may increase selection for aggressive caterpillars over time."

Abstract: Food represents a limiting resource for the growth and developmental progression of many animal species. As a consequence, competition over food, space, or other resources can trigger territoriality and aggressive behavior. In the monarch butterfly, Danaus plexippus, caterpillars feed predominantly on milkweed, raising the possibility that access to milkweed is critical for growth and survival. Here, we characterize the role of food availability on aggression in monarch caterpillars and find that monarch caterpillars display stereotyped aggressive lunges that increase during development, peaking during the 4th and 5th instar stages. The number of lunges toward a conspecific caterpillar was significantly increased under conditions of low food availability, suggesting resource defense may trigger aggression. These findings establish monarch caterpillars as a model for investigating interactions between resource availability and aggressive behavior under ecologically relevant conditions and set the stage for future investigations into the neuroethology of aggression in this system.

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