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Pitcher plant's tricky trap pulls in the ants

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Insect feast Why have a snack when you could have a feast? The pitcher plant has developed a clever way to help it dine on batches of ants at a time rather than individual ants.

The carnivorous plant adjusts the slipperiness of its pitfall traps according to changes in the weather to entice more visitors to their stickly downfall, a new study has found.

The research, published in the journal Proceedings of the Royal Society B was based on observations of the prey intake of Nepenthes rafflesian, a species of pitcher plant found in Borneo.

The plant's leaves form cup-shaped insect traps that look like a pitcher. When the rim of the plant becomes wet, it gets extremely slippery and ants walking on the surface fall victim to the voracious vegetation.

In hot, sunny weather, however, the surface dries and becomes safe for ants to visit. Individual ants serving as scouts for their colonies discover and collect sweet nectar from the trap and return to their nest to tell their fellow ants where to find a nice meal.

Numerous ants then march unwittingly into the trap in search of food and are captured because the plant has made its trap slippery and inescapable.

So by letting the individual scouts escape when the weather is dry, the plant eventually manages to capture much more prey.

To control when its trap is slippery, the pitcher plant secretes sugary nectar that primes the trapping surface to become wet through condensation at lower humidity levels than other plant surfaces. That activates the trap during afternoons when many day-active insects are still out and about.

"Of course a plant is not clever in the human sense - it cannot plot. However, natural selection is very relentless and will only reward the most successful strategies," says biologist Dr Ulrike Bauer of the University of Bristol.

There are about 600 species of carnivorous plants known worldwide. The pitcher plants generally grow in nutrient-poor habitats, which is why they capture animal prey to feed on. Most species trap insects. A few attract small mammals and collect their faeces for nourishment.

"What superficially looks like an arms race between nectar robbers and deadly predators could in fact be a sophisticated case of mutual benefit," says Bauer.

"As long as the energy gain (eating the nectar) outweighs the loss of worker ants, the ant colony benefits from the relationship just as much as the plant does."