Fairly bad pitcher traps triumph in the end

Fumbles pay off in pitcher plants’ lax ant hunting strategy

Three pitcher plant traps

DRY MOUTH  When the lips of pitcher plants’ traps are dry, ants can swarm over them without slipping in.

 

U. Bauer

Incompetent, says who? Carnivorous pitcher plant traps rarely catch much, but their lackadaisical hunting turns out not to be so lame after all. Ask the ecologist who set up hospital IV drips to test Nepenthes rafflesiana traps in the shrubbery of Brunei.

Biologists have observed that pitcher plants “have pretty lousy traps,” says Ulrike Bauer of the University of Bristol in England. Captured nutrients give them the edge in difficult environments, but “most of the time when you look into a trap, there’s hardly anything there.”

Interest in poor trap performance intensified in 2004, when researchers reported being at the right pitcher at the right time. As it began to rain, ants that had been safely scurrying over the fat, nectar-rich collars along the rim of a pitcher started slipping into the digestive lake within. The colorful collar, long considered just a lure, actually creates an efficient death trap — when wet.

JUST ADD WATER If it rains, pitcher rims turn into slick death traps for insects. U. Bauer
Young plants with only a few pitchers have an additional slippery surface just below the collars of their traps. But in many Nepenthes species, adult plants with abundant pitchers lose the waxy backup and rely entirely on their iffy collars.

Biologists had mused that inefficient traps might exploit the social habits of ants, allowing scouts to taste the nectar, escape and bring back nest mates for a massacre, rain permitting. To test the idea, Bauer set up slow-release tubing to drip water on 46 selected traps of N. rafflesiana. “You can believe it looked quite funny to have all these hospital drips out in the field,” she says.

When she added up the ants captured during her study, pitchers allowed to dry out naturally caught almost 2½ times as many ants as tumbled into artificially wet traps, Bauer and her colleagues report in the Feb. 22 Proceedings of the Royal Society B. A few rainy-day mass captures in natural traps made the difference.

Trap surfaces that work only when wet also evolved in North American and Australian pitcher plants. Yet “they’re about as closely related to the Asian species as you and I are to a flatworm,” Bauer says. “Whatever works, nature has discovered at least twice.”

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