Nepenthes rafflesiana, a large pitcher plant commonly found in swampy forests of Borneo. Photo by Rhett A. Butler
Pitcher plants (Nepenthes species) have long captivated our fascination. Typically growing in acidic and nutrient-poor soils, they have developed the ability to eat insects and other small prey to supplement their diets. Nepenthes grow modified leaf structures which form a saucer cup with a slippery rim lined with visual or edible bait. When an insect approaches to investigate the reward, they slip off the rim and into the cup which is usually filled with a viscoelastic fluid. Unable to escape, the prey is slowly digested and absorbed by the plant.
However, observers of the Nepenthes rafflesiana, found in Borneo, discovered that the effectiveness of some traps is dependent on the weather. The rim (known as the peristome) is only slippery when wet from rain or increased humidity, but stays dry during the day allowing ants—a staple prey—to freely come and go.
“For up to eight hours during dry days, these traps are ‘switched off’ and do not capture any of their insect visitors,” said Dr. Ulrike Baur from the University of Bristol whose study into the phenomenon was recently published in Proceedings of the Royal Society B: Biological Sciences. “At first sight, this is puzzling because natural selection should favor traps that catch as many insects as possible.”
And indeed, younger N. rafflesiana plants do tend to produce a slippery wax crystal coating that is wetness-independent allowing for a constant intake of prey—but the upper pitchers of more mature plants forgo that strategy in favor of the intermittently slippery peristome. To understand what is happening here, Baur and her colleagues manipulated several plants to study how effective they are at capturing ants.
After blocking off natural routes to the pitchers, they tied three strings between neighboring vegetation to each, providing ants with controlled access to the study traps. Using a drip system consisting of IV bags filled with a 3% glucose solution, they kept some plants consistently wet and slippery, while allowing others to maintain their natural wet-dry cycle. Then, they counted dead ants.
Nepenthes rafflesiana pitcher plants
Interestingly, the wet group and control group showed no significant difference in the number of single-ant capture events. On the other hand, batch-capture events (more than 5 ants at once) were significantly more frequent in the control group. Those pitchers captured batches of 10 or more ants more than twice as frequently as the wet group, with a maximum capture of 20 ants at once. The control group also posted the highest two total catches: 132 and 40 ants. All of these findings indicate that the wet-dry cycle has the potential to increase the plant’s nutrient intake.
Baur explains the findings with the ‘scout hypothesis:’ ants typically send scouts out to search for food. When they find a source—say, the nectar produced by a pitcher plant—they return to the colony to recruit others to help them harvest it. If N. rafflesiana captured every scout, the steady trickle of ants would be random and infrequent, but by allowing some scouts to escape and return with friends, the plant is able to lure greater numbers in at once.
“By ‘switching off’ their traps for part of the day, pitcher plants ensure that scout ants can return safely to the colony and recruit nest-mates to the trap,” Baur explains. “Later, when the pitcher becomes wet, these followers get caught in one sweep. What looks like a disadvantage at first sight, turns out to be a clever strategy to exploit the recruitment behavior of social insects.”
However, in the fight for survival, this is a high stakes lottery. Batch captures are still infrequent, which may explain why it is a strategy employed typically by more mature plants with an abundance of pitchers. Younger N. rafflesiana plants with fewer pitchers tend to rely on the “slow and steady” method of capture, maintaining a constantly slippery surface.
This study is only one in a long-line of interesting discoveries regarding pitcher plants, and their complex relationships with other tropical species. Including frogs and bats which use pitcher plants for shelter, and rodents which find them to be a convenient toilet.
