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Masquerading animals aim to fool

Early thorn (Selenia dentaria) larva on hostplant (hawthorn-crataeous species).
[Image courtesy of Hannah Rowland]

Many birds enjoy snacking on caterpillars, but the caterpillars of Brimstone and Early Thorn moths have a handy defense. Instead of looking like juicy, green treats, they resemble brown, knobby twigs.

Scientists have now discovered that this disguise does more than just help the caterpillars blend in to the background – it actually tricks birds into thinking the caterpillars are boring, inedible twigs.

Brimstone moth larva (Opisthograptis luteolata).
[Image courtesy of Kimmo Silvonen]

Plants and animals use their visual appearances in all kinds of ways to avoid detection by predators or to capture prey. Scientists have paid a lot of attention to "crypsis," in which an organism blends in with its environment. But, they know much less about another strategy, called "masquerade," in which an organism resembles inedible and generally inanimate objects such as twigs, leaves, stones–even bird droppings.

John Skelhorn of the University of Glasgow and colleagues studied how domestic chicks reacted to Brimstone and Early Thorn caterpillars. They report their findings in the 1 January issue of the journal Science.

Some of the chicks were shown real twigs before being shown the caterpillars, and these chicks took longer to attack the caterpillars and handled them more carefully than chicks that were not shown real twigs first. There were no other sticks in the testing area, so the caterpillars didn't blend into their environments. Nonetheless, the chicks who knew what sticks were still seemed to avoid the caterpillars.

These results suggest that masquerade is in fact different from crypsis. They also suggest that predators' abilities to recognize and identify prey are important influences in evolution.