Survival of the Feces: Why Some Caterpillars Look Like Poop

Curling up to look like a pile of poop might not sound appealing, but it's a useful strategy that some species of caterpillars use to hide from hungry birds, a new study finds.

The moment the caterpillar uncurls, birds are more likely to realize that it's not a pile of excrement but rather a tasty snack, said study researcher Toshitaka Suzuki, a postdoctoral fellow of evolutionary studies at the Graduate University for Advanced Studies in Kanagawa, Japan.

The bird-poop disguise is a type of camouflage called "masquerade," a defense that helps animals look like inedible objects, such as twigs, stones or bird droppings. Suzuki noticed that certain caterpillars curled up their bodies to masquerade as poop, but it wasn't clear whether this disguise increased their rate of survival, he said. [In Images: Spiders Camouflage Themselves as Bird Poop]

To investigate, Suzuki and his co-author Reika Sakurai created artificial caterpillars out of dough. (The creepy-crawly recipe is detailed in the study, and includes flour, lard, water and food coloring.) They decided to test the importance of the caterpillar's color and posture, so they created two types of fake caterpillars — a black-and-white one and a green one — and left them in curled or straight positions on cherry trees around Tokyo.

Cherry trees were a fitting location because Apochima juglansiaria, a species of bird-poop masquerading moth caterpillar, eats cherry tree leaves and rests on the leaves and branches, Suzuki said. What's more, birds — such as grey starlings (Sturnus cineraceus) and tree sparrows (Passer montanus) — that eat caterpillars were seen at all nine of the cherry tree sites, he said.

In all, the researchers pinned 404 bogus caterpillars on about 200 trees. After waiting 7 hours, they found that birds had attacked about 20 percent of the fake caterpillars, either stealing them away or leaving visible peck marks on the dough, the researchers wrote in the study. 

"Birds seemed to misrecognize our pastry caterpillars as real caterpillars (or bird droppings for bent caterpillar models with bird-dropping coloration), since many caterpillar-eating birds attacked and ate our models," Suzuki told Live Science in an email.

Unsurprisingly, the fake green caterpillars didn't fare well regardless of their posture, as their color didn't camouflage them against the cherry tree branches, the researchers said. But posture made a big difference for the artificial black-and-white caterpillars.

"It was a surprise to us that the bent posture reduced bird attacks on bird-dropping caterpillars by almost one-third," Suzuki said. "Since birds have very good eyesight and many hunt visually, our results propose that this selective force led to two adaptations — the adoption of bent posture, as well as bird-dropping coloration — which can overcome even clever birds." [Gallery: Out-of-This-World Images of Insects]

Masquerading as bird poop is so advantageous that it isn't limited to A. juglansiaria. Other moth caterpillars, such as Macrauzata maxima and Acronicta alni,also bend themselves to look like bird droppings, the researchers said. Swallowtail butterfly caterpillars masquerade as bird excrement as well, but they're short and thick, so they likely don't have to bend to look like the real thing, the researchers said. Orb web spiders have also figured out the bird-poop camouflage trick.

Suzuki said he plans to continue his research on defensive caterpillar coloration.

"Bent posture is very common in moth caterpillars with bird-dropping coloration, but some butterfly caterpillars do not bend their bodies even if they have bird-dropping-like coloration," he said. "We are now investigating the ecological reason why some caterpillars adopt bent posture, whereas the others do not."

The study is published in the July issue of the journal Animal Behavior.

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