Monarch caterpillars resort to headbutting when food is limited

Monarch butterfly caterpillars can flip the switch from harmonious to hostile when their favorite food may become unavailable. In a new study published by Cell Press, scientists observed caterpillars with limited access to milkweed lashing out and lunging at others, attempting to headbutt them out of the way. The caterpillars were found to the most aggressive during the final stages before metamorphosis.

Study lead author Alex Keene is a professor of Biological Sciences at Florida Atlantic University.

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

Professor Keene normally explores the neurobiology of fruit flies, but he turned his attention to monarchs after observing their aggressive behavior in his garden. Monarchs are a good indicator of the pollinators present in any given ecosystem – if they are declining, other pollinator numbers are likely dwindling as well. 

At their largest body size, a single monarch caterpillar can eat an entire milkweed leaf in under 5 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,” said Professor Keene. “But each of these caterpillars will at some point in their developmental cycle encounter resource limitation.”

To investigate, the researchers built an open milkweed garden behind their lab in Boca Raton and waited for the caterpillars to arrive on their own. In the lab, the team grouped caterpillars together with different amounts of milkweed. The results were consistent. When there was less food available, the caterpillars were likely to headbutt each other out of the way to get more milkweed. 

Professor Kenne explained that the experiment itself turned out to be a bit complicated. “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. 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.”

Going forward, the researchers want to learn more about what drives the aggressive response to limited food in the brains of the caterpillars. This will provide insight into how these responses work in nature.

“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,” explained Professor Keene. “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.”

The study is published in the journal iScience.

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By Chrissy Sexton, Earth.com Staff Writer