Monarch caterpillars are beating each other up over food

Monarch caterpillar snacking on leaf
Photo: Sarah8000 (Getty Images)

Dark stuff happens when you’re hangry. Best-case scenario, you snap at a loved one before you can get your hands on a Snickers. Worst-case scenario, you head-butt your peers into oblivion over a milkweed leaf. According to Science News, the latter is a relatively common smackdown strategy among monarch caterpillars as they grapple with a dwindling food supply. It’s rough out there on the larval scene.


According to Science News, researchers have witnessed roaming caterpillars head-butting and lunging at their caterpillar compatriots in an attempt to commandeer milkweed leaves. It’s apparently a tactic to score a beefier leaf in a time when climate change is killing off milkweed, the monarch caterpillars’ sole food source. Crawling from plant to plant takes energy, so if a caterpillar lands on a leaf that’s already, er, occupied, they might not get enough food to grow. Enter the larval smackdown. Science News also cites Elizabeth Brown, a biologist involved in the study. Brown explains that the victimized caterpillar often exhibits a “loser response,” which involves leaving the contested area entirely whether or not a stream of smack talk emerges from the caterpillars’ tiny mouths.

Turns out, other types of caterpillars have shown similar aggressive behavior. The big question: do aggressive caterpillars grow into aggressive butterflies? Is caterpillar aggression a troubling evolutionary trend? Can humankind expect a Hitchcockian butterfly onslaught in the next 10 to 20 years? The answers remain unclear—but I, for one, will be stocking up on butterfly nets. And milkweed.

Staff writer @ The Takeout, joke writer elsewhere. Wrangling dogs and pork shoulder in Chicago.


For the past several years the wife and I have truly enjoyed witnessing the mass migration of Monarch butterflies in late August as they make their way south. Being on the north shore of Lake Ontario, urban setting or not, literally thousands of them will fly by over the course of a day, either resting in parks before crossing the Great Lake(s) or otherwise high enough up to make a go of it. The whole spectacle lasts less than maybe 5 days at most before the numbers trickle off again. I’m genuinely amused at the notion that some of them might be assholes.