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Ass-Dragging Caterpillars Evolved from Bullies

April 22, 2010 Suppose you’re a caterpillar. You’ve just built yourself a nice home by sewing leaves together with silk and then some jackass invades your turf. How do you defend your home? You could walk right over to that intruder and push him, maybe smack him around a bit or even bite him. Ha! That’d teach him not to trespass.

That’s exactly what you’d do if you were a poplar lutestring (Tethea or). But violence isn’t always the answer.

Some caterpillars, like the masked birch caterpillar (Drepana arcuata), are quite civilized in their approach to property disputes. Instead of walking over to kick the s**t out of the trespasser, they stay put and use their words (kind of).

When an intruder encroaches on its little leaf shelter, the masked birch caterpillar plants its front legs and moves its rear end, scraping its anal oars along the leaf surface. Then the caterpillar thumps its jaw against the leaf, scrapes its jaw along the leaf and performs another anal scrape. These movements produce vibrations which roughly translate to “get the f*** off of my property” in caterpillar-ese.

What makes one species so brutish in its approach to intruders while another seems so, umm, mature? A recent study (published in Nature’s new and mostly open access journal “Nature Communications”) examined 36 caterpillar species to find out. They found one major difference. The species that approach their intruders (and proceed to head butt and bite the offending caterpillar) have a pair of “anal prolegs” that they use for walking. The caterpillars that scare their intruders away by scraping their rear ends on a leaf lack anal prolegs, but have a set of hair-like anal oars in their place.

To dig deeper, the scientists sequenced the DNA of all 36 species. They found that the caterpillars with prolegs—those that walk towards, hit and bite their intruders—came first. At some point (evolutionarily-speaking), the more peaceful anal-oared species branched off from their prolegged ancestors. Instead of using the old school bullying techniques of their ancestors, the newer species turned those aggressive tactics into ritualized signals.

When faced with a trespasser, both types of caterpillar produce similar movements that produce similar vibratory cues. While anal prolegged caterpillars walk towards the intruder, anal-oared species walk in place, producing an anal scrape. While prolegged species head-butt and bite the interloper, anal oared species scrape and bash their jaws against the leaf.

The old school and new school methods are both successful, but the anal scrapers are a whole lot safer. Perhaps the lesson here is: When you feel like fighting, drag your ass instead.

Scott, J., Kawahara, A., Skevington, J., Yen, S., Sami, A., Smith, M., & Yack, J. (2010). The evolutionary origins of ritualized acoustic signals in caterpillars Nature Communications, 1 (1), 1-9 DOI: 10.1038/ncomms1002

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