Carnivorous pitcher plants that trap and consume insects can change their trapping strategy to increase the feeding success, showing that being clever doesn't necessarily require possessing a brain, a study finds.

Instead, evolution can create strategies just as effective as those that come about through thinking through problems and challenges, researchers say.

Pitcher plants of the worlds' tropical region use surfaces around their "pitchers" to trap and capture insects, and can make those surfaces wet by secreting sugary nectar or let them dry out.

"The plant's key trapping surface is extremely slippery when wet, but not when dry," says study leader Ulrike Bauer of Bristol University in England.

For many hours during dry days, the traps are "switched off" and cannot capture any insect visitors, Bauer and her colleagues found.

"At first sight, this is puzzling because natural selection should favor traps that catch as many insects as possible," she says.

Why would the plants let their traps go dry, which would allow visiting ants in search of the nectar to escape?

The researchers discovered it was an evolutionary adaptation that improved the plant's chances of capturing large groups of ants from a single species.

"Ants are social insects," Bauer explains. "Individual 'scout' ants search the surroundings of the nest for profitable food sources."

When those scouts locate a pitcher plant with a trap full of sweet nectar, they will return to their colony and gather large numbers of ant workers expecting a nectar harvest.

However, if the pitcher plant kept its trap slippery at all times, those scout ants would be captured and the plant would only have them, instead of a large group of ants recruited by the scouts.

"By 'switching off' their traps for part of the day, pitcher plants ensure that scout ants can return safely to the colony and recruit nest-mates to the trap," Bauer says.

"What looks like a disadvantage at first sight, turns out to be a clever strategy to exploit the recruitment behavior of social insects," she says.

The researchers, writing in the journal Proceeding of the Royal Society B, described their studies of wild carnivorous pitcher plants in Borneo, where they found that when they artificially kept the plants' pitchers wet the plants were no long able to capture large batches of ants.

Around 600 species of carnivorous plants are known worldwide, usually growing in nutrient-poor habitats, which is why they've evolved to feed on captured insect or animal prey.

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