The masked birch caterpillar creates its own home by weaving leaves together with silk. Once built, it vigorously defends its territory but, like many animals, it prefers to intimidate its rivals before resorting to blows. To display its strength and claim its territory, it drums and scrapes its jaws against the leaf. It also drags its anus across the surface to create a complex scratching noise. This “anal scraping” message seems utterly bizarre, but its origins lie in a far more familiar activity – walking.

Warding a rival off with your anus might seem unseemly to us, but caterpillars that do this turn out to be rather civilised species. The scraping is based on the same walking movements that their ancestors used to chase after rivals. The other parts of their signalling repertoire – drumming and scraping jaws – are ritualised versions of fighting moves like biting, butting and hitting. While their earlier cousins might resort to such fisticuffs, the anal-scrapers conduct their rivalries with all the restraint of Victorian gentlemen.

These signals and their evolution have been decoded by Jaclyn Scott from Carleton University. They a great examples of how ritualised animal communiqués evolve from much simpler actions that have little if anything to do with communication – walking, breathing, hunting and the like. Crickets, for example, sing by rubbing their wings together, which may originally have been done to release pheromones or to prep the wings for flight. The whistling of wind through the feathers of crested pigeons has turned into an alarm. The competitive knee-clicks of eland antelopes are made by tendons that slide as a natural part of their gait.

Often, these origins are hard to test and scientists need to be careful if they aren’t to rely on fanciful just-so stories. To avoid that, Scott analysed 36 species of caterpillars from two different families. Some of them had simple struts called “pro-legs” on their end segment, which they use to inch their way along. Other species lacked these structures and in their place, they had a pair of “anal oars” – thicker, harder, spatula-shaped versions of the caterpillar’s normal hairs. These are the instruments that the larvae use to scrape their leaves.

These two groups of caterpillars put their bums to different uses – walking and talking – but the movements they make are the same. They lift the anal segment forward, place it on the leaf and their push backwards against it. The big difference is that in the walkers, the end stays put and the front half launches forward, while in the talkers, the front stays attached and the bum moves backwards. When the masked birch caterpillar makes its anal scrapes, it is essentially talking by walking on the spot.

To confirm this analogy, Scott sequenced DNA from her three dozen species and built a family tree that charted their evolutionary relationships. She found that the species with the pro-legs – the walkers – came first. They solve their conflicts with violence, crawling towards intruders and physically attacking them.

Those with anal oars – the talkers – are an offshoot that descended from this larger group and lost their anal pro-legs. In this group, all the aggressive movements on their cousins have become ritualised into signals. Instead of a confrontational crawl, they do an anal scrape. Instead of biting, hitting and butting, they scratch and drum the leaves with their jaws instead.

Scott also found that the original movements already had the foundations of a good signal. The crawling and pushing movements of a defensive caterpillar produce vibrations that you can pick up over background noise. As these movements became ritualised into signals, they became simpler too, so that caterpillars that send messages by anal scraping produce much larger and more repetitive vibrations using fewer body movements. This is exactly what you’d expect to happen over time – as messy but informative behaviour evolves for use in communication, it becomes clearer and less ambiguous.

Reference: Nature Communications http://dx.doi.org/10.1038/ncomms1002

More on animal communication:

• Bee-ware – bees use warning buzz to refute the waggle dance
• Boom-boom-krak-oo – Campbell’s monkeys combine just six ‘words’ into rich vocabulary
• Orang-utans use leaves to lie about their size
• Cats manipulate their owners with a cry embedded in a purr
• Camouflaged communication – the secret signals of squid
• Singing fish reveal shared origins of vertebrate vocals