The very angry caterpillar: Before they become butterflies, the insects can be ‘aggressive fighters’

There’s something missing from Eric Carle’s heart-warming children’s book The Very Hungry <i>Caterpillar</i> - violence - in particular, headbutting.

Though it might take the classic tale in a rather different direction, surprising new research reveals some types of caterpillar can be “aggressive fighters” as they seek to dominate their preferred food sources.

Milkweed - also known as “butterfly flower” in the US - is the monarch butterfly caterpillars’ favourite food, but when there are shortages, these voracious green and black stripy larvae have no qualms about resorting to violence - mostly headbutting - in order to keep eating.

Researchers found that caterpillars with less access to food were more likely to lunge at others to knock them aside, and the caterpillars became increasingly aggressive until the final stages before metamorphosis.

“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,” said lead author Alex Keene, a professor of biological sciences at Florida Atlantic University.

“Now we might be able to take that powerful neurobiology and genetics and study it in a more ecologically relevant organism.”

Professor Keene said he was inspired to pause his studies into fruit flies and investigate monarch butterfly caterpillars after witnessing the combat between the larvae in his own garden.

The researchers said that because the caterpillars are large and recognisable compared to many other insects, monarchs are a useful indicator of the status of pollinator populations in an ecosystem.

If monarchs are declining, other pollinators are likely to be declining as well.

Monarchs have a significant impact on the plants they consume - at their largest and hungriest phase, a single caterpillar may eat an entire milkweed leaf in under five minutes.

"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," Professor Keene said.

“But each of these caterpillars will at some point in their developmental cycle encounter resource limitation.”

To model this resource limitation, the researchers faced the challenge of maintaining a population of monarchs.

Inspired by his own butterfly garden at home, Professor Keene and his team built an open milkweed garden behind their Boca Raton-based lab and let nature do the work of collecting caterpillars.

Back in the lab, the researchers placed caterpillars into groups with different amounts of milkweed. The results were clear - the less food, the more likely caterpillars were to try to headbutt each other out of the way to get their fill.

The research team said the process of getting their results was anything but simple.

“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,” Professor Keen said.

“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.”

While this research showed that the caterpillars respond aggressively to limited food, the researchers still hope to learn more about what drives this response in their brains, which is important for learning more about how these responses work outside the lab.

“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," Professor Keene said.

“So now that we have this invertebrate model in a relatively controlled setting, but doing an ecologically relevant behaviour, that becomes important in terms of looking at the mechanism and function of this behaviour in more complex organisms.”

The study is published in the journal iScience.