Monarch Caterpillars Butt Heads Over Milkweed

In the animal kingdom, even the creatures that might strike humans as cute or cuddly will get downright nasty when the chips are down. A new study finds that the cartoonish, candy-striped caterpillars of the monarch butterfly will aggressively headbutt their brethren when milkweed, their favorite food, is scarce, reports Katherine J. Wu for the New York Times.

“We think about monarchs as being these beautiful, dazzling creatures that fly around and pollinate flowers and lay eggs,” Adriana Briscoe, a butterfly researcher at the University of California, Irvine, who wasn’t involved in the study, tells the Times. “We don’t usually think of them as having this sort of darker underbelly.”

These caterpillars’ feisty lunges are designed to interrupt their targets mid-munch in hopes of gaining access to the food source before it’s all gone, the researchers reported last week in the journal iScience.

Research Footage of Monarch Caterpillars

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Alex Keene, a biologist at Florida Atlantic University and one of the paper’s co-authors, tells Curtis Segarra of Science News that the study came about by chance.

“My wife pointed out in the backyard that these two monarch caterpillars were fighting with each other,” Keene tells Science News. “I went on YouTube, and there were videos of this behavior [but for monarchs] it wasn’t documented anywhere in the scientific literature.” According to Science News, prior research has reported similarly aggressive behavior among the caterpillars of other species.

To take a scientific look at the behavior, the researchers placed groups of four monarch caterpillars in lab dishes with varying quantities of milkweed leaves, according to the Times. The team also found that the amount of food impacted the ferocity of the bouts.

“The less food that is present, the higher their level of aggression,” Elizabeth Brown, a biologist at Florida Atlantic University and co-author of the new research, tells Karina Shah of New Scientist.

“Some would just roam off and eat,” Brown tells the Times, but if one caterpillar spied another with a particularly tasty morsel it would “rear up and, with their head, make a lunge onto the body of the other.”

According to Science News, this bruising competition for food is owed to the fact that each budding butterfly needs to store up calories to power its metamorphosis. For most monarch caterpillars, which can only eat milkweed, the bush they’re born on is the only food source they can feasibly reach. This restriction means if they happen to be sharing it with other caterpillars there may not be enough to go around, and anything they can do to secure a larger share of the greenery gives them an advantage.

Brown tells New Scientist the most aggressive caterpillars were generally the ones closest to their big transformation, which she says probably meant they needed the most food.

Brown adds that in the world of baby butterflies, being big helps. “There’s a clear winning caterpillar and losing caterpillar,” she tells New Scientist. “This often scales with their size.”

Keene tells Science News he may explore whether more aggressive caterpillars grow up to be more aggressive adult monarchs, and, speaking with New Scientist, suggests that these caterpillars could be an ideal way to study the genetic roots of aggressive behavior.

Alex Fox | | READ MORE

Alex Fox is a freelance science journalist based in California. He has written for the New York Times, National Geographic, Science, Nature and other outlets. You can find him at Alexfoxscience.com.