Slippery When Wet

Carnivorous pitcher plants gobble more ants by turning off their traps in dry weather.

Jan 14, 2015
Jenny Rood

WIKIMEDIA, ROBERT JONGPitcher plants survive in a nutrient-poor environment by luring tasty insects into their digestive juices with the sweet smell of nectar, but their intended prey can escape if conditions are dry. The ineffectiveness of the pitchers in low humidity benefits the carnivorous flora in the end, however, as the sticky trails left by the escapee insects later lead their colony mates to their doom when the weather turns damp, an international team reported this week (January 14) in Proceedings of the Royal Society B.

While the young Nepenthes rafflesiana pitcher plants of Borneo have slippery surfaces on their specialized pitcher-shaped leaves that “cause visitors to fall into the pitcher and drown in the digestive fluid” regardless of humidity, as Ulrike Bauer of the University of Bristol and her colleagues wrote in their study, it has long been a mystery why mature plants have seemingly ineffective and dry traps up to eight hours a day. To examine the apparent disadvantage of this adaptation, which is nevertheless common among many species of pitcher plant, Bauer and her colleagues compared the amount of prey caught by continuously wetted pitchers and leaves left alone on the same plant.

Constantly wet pitchers caught more flying insects, but the untouched pitchers overall nabbed 2.5 times as many prey over the study period, mainly due to a 10 percent increase in batches of five or more ants—the primary food source of N. rafflesiana—trapped at a time. The scientists thus reasoned that alternating between ineffective and highly effective trap modes allows the plant to exploit the behavior of “scout” ants—those insects responsible for sourcing food and alerting their colonies to its location—by letting one ant escape in order to capture many more. “What looks like a disadvantage at first sight turns out to be a clever strategy to exploit the recruitment behavior of social insects,” Bauer said in a statement.