Source: Science Daily
When food is scarce, monarch butterfly caterpillars go from docile to domineering. The results look something like a combination of boxing and ‘bumper’ cars. The less food, the more likely caterpillars were to try to head-butt each other out of the way to get their fill, lunging and knocking aside other caterpillars to ensure their own survival. And, they are most aggressive right before the final stages of their metamorphosis.