Carnivorous Plants Are More Conniving Than You Think

Pitcher plants are the sarlacc pits of the natural world. Unsatisfied with the energy yields of photosynthesis alone, these plants have evolved into carnivores capable of imprisoning everything from insects to rodents in their pitfall traps. Once prey has slipped into the eponymous pitcher, it is next to impossible for it to climb out, and the plant goes about slowly digesting its kill. Who says botany can't be metal?

But as it turns out, the pitcher plant's deliciously gruesome bait-and-switch is even more ingenious than it looks. Biologist Ulrike Bauer of the University of Bristol has studied pitcher plants closely for over a decade. Early on in her career, she noticed that some plants alternate between having a wet, slippery trapping surface, resulting in several ant kills, and a dried up surface, which allowed ants to safely investigate without falling into the trap.

"During my PhD research, I ran an experiment where I observed ants visiting a pitcher trap for a whole day and night, and counted visitors and capture events at different times," Bauer told me. "During the hot and dry hours, not a single ant got trapped but when the air humidity went up, the trapping surfaces got wet and suddenly up to 80 percent of the visitors slipped into the trap. I started wondering how such an 'imperfect' trapping device could possibly evolve—shouldn't a trap be optimized to capture at all times?"

In a study published today in Proceedings of the Royal Society B, Bauer and her colleagues show how this seemingly detrimental strategy actually nets the plant far more grub in the long run. The key is instilling a false sense of safety in scout ants that discover the plant while it's dry. These solo scouts will go back to the colony to recruit a large group of workers to share the free nectar. By the time the group returns, the plant has moistened again, and that's how it bags itself a feast.

"We did exactly that experiment," said Bauer, "and found that naturally alternating traps caught in total almost 2.5 times as many ants as the constantly wet ones." She pointed out that this wasn't an evenly distributed rate, and that most plants had very little luck in catching ants at all. But on the other side of the spectrum, there were also some major successes that threw off the bell curve.

"[U]nder natural, alternating wetness conditions, a few pitchers made spectacular catches, sometimes more than 100 ants within just two days," she told me. "This never happened when the traps were kept wet all the time."

The entire process seems to be disconcertingly Machiavellian for a plant, though to be fair, it is just one of many creative ways that organisms take advantage of the gullibility of ants (the Claviger beetle is a particular favorite of mine). But there are downsides to the alternating wetness strategy too. Plants that opt for the always-wet strategy catch more non-recruiting insects, like flies, bees, and beetles, while the alternating plants let plenty of prey escape during their dry periods.

"[T]iming is probably a matter of chance," said Bauer, when I asked if the plants could anticipate large catches based on the daily habits of ants. "We often observe pitchers with dozens of ants harvesting nectar, and safely leaving, and even though we don't have data to prove it yet, it is highly likely that scout ants also arrive at wet times and get caught."

"However, the fact that the traps are 'inactive' for up to eight hours in a dry day means that, statistically, one third of all scouts, assuming equal activity around the clock, will get away safely and start recruiting," she continued. "For a large plant with many—sometimes hundreds of—pitchers, that is enough to ensure a good catch."

Indeed, even when Bauer and her colleagues accounted for all the meals the alternating plants missed out on, they still came out ahead. "[O]verall, naturally alternating traps caught 36 percent more prey than wetted traps," she told me. "Such a stark difference explains why a mechanism that, at first sight, seems disadvantageous can persist in evolution."

The pitcher plant has long intrigued academics and the public alike, especially after David Attenborough narrated a spectacular segment on them in Planet Earth. So, it's exciting that biologists like Bauer are still able to unlock major secrets about their behavior, challenging our notions of what plants can accomplish.

"I have studied pitcher plants, and specifically their manifold and fascinating trapping mechanisms, over the past 10 years," Bauer told me. "This fascination is unlikely to wear off soon."