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Mark Freeth via Flickr // CC BY 2.0
Mark Freeth via Flickr // CC BY 2.0

Scientists Say Venus Flytraps Can Count

Mark Freeth via Flickr // CC BY 2.0
Mark Freeth via Flickr // CC BY 2.0

Grownups are well aware of the power of the slow count and its ability to shut down rambunctious behavior. (Think a teacher yelling “I’m going to count to five. . . ” to a rowdy classroom of students.) As it turns out, the venus flytrap employs a similar technique, counting to five before beginning the punishing process of digesting its prey, according to findings published last week in the journal Current Biology.

"The carnivorous plant Dionaea muscipula, also known as Venus flytrap, can count how often it has been touched by an insect visiting its capture organ in order to trap and consume the animal prey," study co-author Rainer Hedrich said in a press release.

Each D. muscipula trap is lined with sensitive, hairlike fibers that detect movement through changes in electricity. To find out how the flytraps were using that information, the researchers hooked D. muscipula plants up to electrical sensors, then fed them crickets. Each time a bug came into contact with these electrosensing fibers, the researchers found, the plant took note. The first time each insect made contact, the trap stayed open, but the plant went into “ready-to-go” mode, similar to a runner crouched at the starting line.

At the second touch, the plant’s jaws snapped shut—not tightly enough to kill the cricket, but enough to keep it from getting away. Here, the plant reset its tally and started counting from the beginning. 

“If you don’t panic, the trap will open,” Hedrich told New Scientist. “But like everybody, once you are trapped you panic, so you will try to escape – and once you are struggling, you touch the other hairs.”

After five hits, glands inside the trap started pumping out the digestive juices that would dissolve the cricket alive. The cricket’s bid for freedom was the very thing that spelled its downfall.

Why would the plants wait at all? First, to ensure that the first contact wasn’t a false alarm. Snapping shut and digesting food both require a lot of energy, and the plant doesn’t have room to make mistakes. Second, each instance of contact gives the plant information about the size of its prisoner, which allows it to decide if the prey is even worth it. A single trap can only reach up to about three inches, which means that a fat cricket could make a fine meal.

“It measures the touches to see what type of visitor it is,” Hedrich said in New Scientist. A bigger bug would be more work to contain and would require the production of more bug-eating juice, but the payoff would also be greater. The researchers say the math and cost-benefit analysis suggest a real process of calculation. Unconscious calculation, but calculation nonetheless.

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Bernard Dupont, Flickr // CC BY SA-2.0
Carnivorous Plant Uses Rain to Spring Its Trap
Bernard Dupont, Flickr // CC BY SA-2.0
Bernard Dupont, Flickr // CC BY SA-2.0

“Rain, rain, go away, come again another day” is not a sentiment that many would disagree with. But then there’s Nepenthes gracilis, a carnivorous plant found in Southeast Asia that relies on rain to help it eat. 

Carnivorous plants thrive in poor soils where most other plants would fail, because they get nutrients from consuming insects instead of taking them from the ground. They can’t stalk, chase, and attack their prey like animal predators, though, so they have to rely on lures and traps. Many of these traps, like the snapping “mouths” of Venus flytraps, take lots of time and energy to reset after use, while others, like the catapulting tentacles of sundews, can only be used once. In a new study, biologist Ulrike Bauer shows that N. gracilis works around these constraints by using rain drops to drive its traps and set them off with no energy costs. 

N. gracilis is a pitcher plant, and captures and digests insects in a modified leaf that looks like a champagne flute. Bugs are lured in by the plant’s smell and the nectar that coats the underside of the pitcher’s lid. While crawling around on the lid and collecting nectar, the bugs get knocked off and into the pitcher, where they’re devoured. 

Instead of flicking its prey into the trap by moving the lid itself, Bauer discovered, the plant lets the impact of rain drops do the work for it. Her study began when she noticed that N. gracilis’s lid is more rigid than those of its cousins. After taking high-speed video of the traps in action and looking at the mechanics of the lids in their lab, Bauer and her collaborators found that N. gracilis’s stiff lid doesn’t bend under the impact of rain drops like other pitcher plant lids do, but pivots on a flexible hinge at its base. That means there’s no safe spot for bugs to avoid getting shaken, like there would be if only part of the lid bent downward. Because the lid is small and light, it moves very quickly, and its rain-driven shaking is faster than both the Venus flytrap’s snapping trap and the sundew’s tentacles. 

All of this happens without any work from the plant. It doesn’t need to set the trap, activate it, and then wait to reset it, but can use it, Bauer writes, “instantly and indefinitely as long as the external driver, rain drop fall, persists.”

Useful as the effortless trap is, it does seem to have one big drawback in that it only works when it's raining. This isn’t too much of a problem for the plant, though, the researchers say. Water can keep dripping from other vegetation above the plants and keep the trap swinging long after a rain shower stops. When there’s no rain, social insects like ants that survive their encounter with the plant can recruit their nest mates to the food source, and they might not be so lucky to come check it out on a dry day. 

N. gracilis’s unique trap shakes up our picture of carnivorous plants, the researchers say. Traditionally, they’ve been divided into two groups: those with “active” moving traps like the Venus flytrap and those with “passive” motionless traps like pitcher plants. N. gracilis has a trap that the researchers call “passive-dynamic,” and uses movement but requires no activity from the plant. It suggests, they write, that the dividing line isn’t so clear and there’s a continuum between active and passive trapping mechanisms, with more traps like this potentially out there waiting to be discovered. 

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Wikimedia Commons
Pitcher Plants Catch More Bugs When Their Traps Take Time Off
Wikimedia Commons
Wikimedia Commons

To get by in soil that is low in nutrients like nitrogen and phosphorous, some plants do something almost unthinkable—something that seems to go against the natural order of things: They become predators and carnivores, turning the tables on animals and adapting to eat them instead of being eaten.

There are some 600 known species of carnivorous plant in the world. The Venus fly trap is the most well known, but the pitcher plants probably come in a close second. These plants have modified leaves, shaped like pitchers or champagne flutes, that serve as traps. When an insect lands on one of these leaves, a slippery film of water and waxy secretions on the inside surface sends the bug sliding deeper into the pitcher, where it lands in a pool of digestive fluids that drown and dissolve it.

For some plants, though, these traps don’t work all the time. Some pitcher plant species don’t produce the waxy coating, and rely only on water to slick their leaves. When it’s raining, or when it’s humid and there’s some condensation on them, they’re deadly slip-and-slides. But when it’s sunny out or the humidity is low, the leaves stay dry and a bug can trot over them with no problems. Raffles' pitcher plant (Nepenthes rafflesiana), for example, grows in open sunny areas with low humidity, and its trap is inactive for as much as eight hours each day.

It shouldn’t work like that, says biologist Ulrike Bauer, because natural selection should favor adaptations that maximize prey capture and food intake. A trap that switches on and off according to the weather and doesn’t work for a third of the day goes against expectations as much as a plant that eats animals. Yet, traps that aren’t “always-on” are widespread in pitcher plants, so they can’t be too much of a disadvantage. Now, Bauer has found that they’re actually a neat adaptation, and that a temporarily ineffective trap can help plants capture more insects.

Raffles' pitcher plants feed on a wide array of insects, but ants make up about two-thirds of their diet. Ants are highly social creatures and share information with each other for the good of the colony, and when a lone scout ant sniffs out the pitcher’s bait, it soon recruits its nest-mates to share in the food. Therein lies the advantage of a sometimes-dry trap: the first ant can scope the pitcher out, but lives to tell its friends about it and inadvertently lure them to their doom.

Bauer and her colleagues showed this in Raffles' pitcher plants in Borneo by testing the capture rates of different traps on several plants. Half of the leaves were allowed to fluctuate between wet and dry naturally, while the other half were kept continuously wet by a water drip that the scientists rigged up.

After several days of monitoring the plants, the researchers removed all the captured insects and sorted and counted them (the pitchers had been plugged at the bottom with a foam ear plug to keep the prey from being lost). They found that flying insects were a reliable, steady, but not very plentiful source of food for both the always-on and the on-and-off plants. While the always-on pitchers trapped more of these flying bugs and had a higher baseline capture rate, the on-and-off pitchers caught more prey overall—about a third more than the always-wet ones.

The difference was because of ants. The pitchers that switched on and off naturally attracted an increasing number of ants over the course of the day, while the always wet ones didn’t. They also captured more ants in total than the always-on ones, capturing them twice as often in batches (10 or more ants) and were the only pitchers that captured them in large batches (sometimes trapping as many as 20 bugs at the same time). While their base capture rate was lower, the plants that dried out and became wet again over the course of the day made up for it with these occasional bigger meals.

The bouts of binge eating are possible because of the ants’ social nature. An ant scout that finds a food source will let other ants know about it so they can use it, too. A pitcher trap that’s always slippery will capture these scouts before they can spread the word, cutting off the plant’s prey supply. But a pitcher that’s off for part of the day and then becomes slippery later gives scout ants safe access to food and some time to recruit other ants, who arrive later and get captured in one sweep when the pitcher is wet. The plant does more with less, and exploits the ants’ collective behavior similar to the way some animal predators attack schools of fish or herd their prey together to maximize their catch.

Other pitcher plants have a more cooperative relationship with ants. Nepenthes bicalcarata forms a partnership with Camponotus schmitzi ants and provides living space for them in a swollen tendril at the base of its pitchers. In return for housing, nectar and some leftover bits of the plant’s catch, the ants protect the plants from weevils that eat its developing buds, and clean the sides of the pitcher traps to keep them nice and slick.

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