Making Sense Of The Predatory Pitcher Plant

Design flaw in a carnivorous plant could be its greatest asset.

Many types of pitcher plants are only occasionally slippery, which may help them trap more insects overall. Pictured above is Sarracenia purpurea.

Rachel Kaufman, Contributor

(Inside Science) -- Carnivorous plants hold a place of special fascination in elementary science classrooms and botany labs alike. Many of these plants have an obviously predatory look about them (think: Venus flytrap). But pitcher plants, as successful as they are at capturing insects for their nutrients, don't make a lot of sense at first glance. A new study aims to make sense of the fact that the plants' traps aren't always slippery along the edges, even though a slippery edge would, in theory, catch more insects not sure of their footing.

A pitcher plant's trap contains a deep cavity filled with liquid, which has a slippery coating along the insides. Along its rim is a bug lure: bright colors or sweet nectar. But the rim itself is only slippery when wet.

In the case of Nepenthes rafflesiana, a pitcher plant native to Borneo, "wet" means after a rainstorm or at night, when humidity increases. During a dry day, N. rafflesiana might not catch a single insect.

But, says a new study, published this month in Proceedings of the Royal Society B, that design quirk may be intentional.

Lead author Ulrike Bauer, a botanist at the University of Bristol in the United Kingdom explained that about two thirds of N. rafflesiana's diet is ants, and with ants, the longer you wait, the more you get.

"It's a self-reinforcing mechanism," she said. "You have one scout going back to the colony, bringing a nectar sample back. Then you get maybe four or five ants following that scout. They all leave chemical trails."

It's only a matter of time before 20 or more ants are all traipsing back to the pitcher plant. In the late afternoon, the rim becomes slippery, and they march to their doom.

Bauer and her colleagues studied this model in N. rafflesiana in the field, by hooking plants up to IV drips that kept their rims constantly moist, and counting the number and types of insect the plants caught versus control plants. The moist traps caught more individual insects but fewer "batches" of ants. Bauer hypothesizes that the batch capture strategy, while less reliable, is more favorable to pitcher plants once they reach a certain number of traps -- some Nepenthes plants can have hundreds of pitchers.

However, pitcher plant species across the world have a similar slippery-when-wet rim, including in Australian or American pitcher plants, which "are as unrelated [to Asian pitcher plants] as you and I are to a flatworm or marine sponge," Bauer said. Yet even these unrelated plants have wettable surfaces. "This mechanism may be a far more general adaptation than we initially thought."

Other scientists are waiting for more data.

"I love the idea of IV drips leading to pitchers -- this is a scene right out of the 1951 movie, 'The Thing from Another World,'" said Barry Rice, a botanist and astronomer at Sierra College in Rocklin, California, in an email to Inside Science. But, he said, the paper shows that pitcher plants can capture ants in bulk, not necessarily that the plants have evolved to encourage bulk capture. To prove that to his satisfaction, Rice said, he would have to see a plant with features that encourage drying or increase nectar production during dry times. That would imply that the pitcher plant is actively increasing the time the trap is inactive to attract more scout ants, rather than simply, passively, waiting out the dry periods in the day.

Either way, don't feel too badly for the ants tricked into leading their sisters to certain doom. Bauer said, with the usual caveats about further research being required, that the ants and pitcher plants may be in much more of a mutualistic relationship, "an exchange rather than exploitation," she said. Losing even 20 or 100 ants doesn't mean much for a colony. "Especially if the trap is such a good sugar resource [that helps] the colony grow faster."

Pitcher plants produce pitchers, and therefore nectar, year-round, and live many years. That's "crucial" for an ant colony in need of carbohydrates. The ants as a group may in fact benefit overall from this arrangement.

N. rafflesiana isn't endangered, but a number of pitcher plants are lost due to poaching, climate change and habitat loss.

"I've lost two major field sites to development in the past three years," Bauer said; one became a highway and the other is now a chicken farm. "Habitats are lost quickly, especially in the tropics."

Yet pitcher plants and other carnivorous plants are "fascinating to people," in no small part due to their myriad adaptations. There's a pitcher plant native to only two mountains in Borneo that has evolved to serve as a rodent's toilet. There's another that uses ants as its attack squad to protect it from weevils. And another that has a lid like a springboard. When it closes, it catapults insects into the murky depths below. "There’s an amazing lot of strategies, specializations," Bauer said. "We’re only just starting to understand how these things work."

Rachel Kaufman is a freelance writer based in Washington, D.C. She tweets at @rkaufman.