Fledgling birds change rules for caterpillar color

During seasonal flush of naïve predators, larvae lose advantage of cautionary marks


IN BAD TASTE  A caterpillar called a lettuce shark sports bright coloring that warns predators of toxins or a bad taste. But the colors can be a liability for the caterpillar when young birds are learning what to eat, a study suggests.

Kimmo Silvonen

For a few weeks every summer in central Finland, orange may become the new black. Bright colors on caterpillars temporarily stop working as well as plain black bodies in reducing risks of getting eaten by birds, a new study says.

Flashy markings on caterpillars, as on many other animals, can serve as memorable warnings of toxins within. Predators, however, have to learn what the warnings mean. Every year, a new generation of fledgling birds pecks brightly colored caterpillars and gets an education in what not to eat.

Caterpillar-like blobs of Plasticine
BITE ME Caterpillar-like blobs of Plasticine stood in for real insects so that researchers could count peck marks and monitor attacks from birds. J. Mappes et al./Nature 2014

That surge of young birds can cause a short, seasonal reversal in evolutionary pressures, so less colorful caterpillars are temporarily favored, says Johanna Mappes, of the University of Jyväskylä in Finland. In an outdoor experiment with 1,243 handmade Plasticine caterpillars wired to wild plants, she and her colleagues concluded that early in birds’ breeding season, when adult birds do the hunting, bright warnings work better than camouflage. Fake caterpillars sporting an orange patch suffered fewer bird pecks than all-black fakes. Then, for much of June and July, those orange patches became riskier than usual.

During that spell, the orange-spotted fake caterpillars in the experiment got gouged and pecked more often than the harder-to-spot all-black ones. Records from bird banding and netting surveys in the region suggested that these were the weeks when many young birds started venturing away from the nest, Mappes and her colleagues report September 23 in Nature Communications.

“It’s costly to be conspicuous in the middle of the season when the naïve birds don’t know what they’re doing,” Mappes says.

This season of danger may be something that the caterpillars have adjusted to, she and her colleagues suggest. Combing an unusual trove of photographs of the last larvae of caterpillars of 688 species in the region, researchers took note of the ones most likely to be conspicuous to birds because of their color. The more eye-catching species represented a low proportion of the caterpillar population during the fledgling-bird period, Mappes and her colleague report.

Results from the birds’ pecking at orange-spotted fakes make a convincing case that warning colors wax and wane in effectiveness during a single year, says sensory and evolutionary ecologist Martin Stevens of the University of Exeter in Penryn, England. But because he questions whether the all-black fakes were as camouflaged as real caterpillars, he hesitates to compare the relative value to caterpillars of camouflage versus warning signals.

Mappes says she was not trying to get full realism but a continuum of coloration from easier to harder to spot on the same caterpillar stand-ins. What she wonders about is how this kind of experiment would play out in the tropics, which lack the big rush of migrants arriving at nearly the same time to breed.

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