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Monarch caterpillars head-butt each other to fight for scarce food

As milkweed supplies dwindle, the insects turn up the aggression, lab experiments show

Monarch caterpillar

Monarch caterpillars must bulk up before forming a chrysalis. Otherwise, they’ll never make it to their final life stage: a butterfly.

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By Curtis Segarra

- More than 2 years ago

When food and space get scarce, competition can bring out the worst in monarch caterpillars.

In the laboratory, researchers watched as roaming caterpillars looking for a hard-to-find meal started head-butting and lunging at fellow caterpillars munching on a milkweed leaf.