The carnivorous pitcher plant is nature's own honey trap, luring insects to their doom with drops of nectar and other enticements. But once an insect comes to call, pitcher plants have a problem: how to make sure their prey doesn't escape.
Now, researchers have discovered an ingenious insect-trapping mechanism, or rather two, in a South American pitcher species. The throat of the little-known Heliamphora nutans, new research shows, is covered with a pelt of tiny, precisely oriented hairs that prey can't easily ascend. The hairs also help to create a slippery, wet film on the pitcher's inner walls, making it nearly impossible for the plant's victims to get a grip. Instead, they plunge into the bottom of the pitcher, where they drown in a pool of water.
Other carnivorous plants also resort to strange techniques for seizing prey, says Jonathan Moran of Royal Roads University in Victoria, Canada, but the features described in the new study are "very, very sophisticated. … It's not perhaps the most outrageous method of doing it, but it certainly seems to be very effective."
It's not easy to study this kind of pitcher in the wild, says first author and plant biologist Ulrike Bauer of the University of Cambridge in the United Kingdom. H. nutans lives atop high plateaus in the wilds of southeastern Venezuela, an area dubbed "The Lost World." To access the area, scientists need a helicopter and a permit the Venezuelan government is reluctant to grant. As a result, no one knows H. nutans's precise prey, only that it mostly consumes ants. So Bauer and colleagues worked with specimens of the plant from the collections of London's Royal Botanic Gardens, Kew.
When the researchers examined H. nutans with a scanning electron microscope, they saw that the pitcher's throat is densely carpeted with more than 100,000 tiny hairs, all pointing downward.
When they allowed an Asian ant species, Camponotus rufifemur, to stroll into a dry pitcher, only 29% fell into the abyss, as shown in the first section of the accompanying video. The bolder ants that ventured onto the longer hairs toward the bottom were most likely to fall, for reasons that are unclear. But when the ants were released into a wet pitcher, 88% went down the slippery slope to oblivion, as seen in the second section of the video. The scientists call it "insect aquaplaning."
To find out why that happens, the researchers outfitted another Asian ant species, Oecophylla smaragdina, with tiny strain gauges. The team also removed the adhesive foot pads—which help ants grip smooth surfaces—from one group of ants but left another group intact. Then the ants were placed on sections of pitcher wall, which were pulled out from underneath the ants' feet.
In tests replicating an ant's journey down into a pitcher, ants without adhesive pads couldn't get a grip on a dry pitcher wall, whereas intact ants couldn't get a grip on a wet pitcher wall but did manage to get a purchase on a dry wall. The results show, the researchers report online today in the Proceedings of the Royal Society B, that the wet pitcher surface works to counteract the ant's adhesive foot pads. And the ant can't use its claws—its second gripping tool—to save itself because all of the hairs are pointing downward.
Some Asian pitcher plants also use the "aquaplaning" method—a textbook example of convergent evolution, Bauer says, because Asian and South American pitchers are as related to each other "as we are to flatworms." And the researchers note that hairs similar to H. nutans's are found inside the hood of North American pitcher plants, so perhaps they capture their prey in a similar way.
The findings are "fascinating," writes carnivorous plant specialist Laurence Gaume of France's National Center for Scientific Research in an e-mail, adding that they provide useful information on how insects attach to plants. Still, she thinks this method is probably only one of a wide variety of methods that carnivorous plants use to entrap their prey, given the diversity of their diets. Gaume would also like to see field experiments to confirm the greenhouse studies.
Bauer agrees. "We really know way too little about these plants," she says. If Venezuela loosens the rules, "I would be the first in line for a permit."