Why do some plants eat flesh?

Certain plants have gone from helpfully purifying the air to devouring the flesh of animals. But how? Find out why some plants went rogue and began capturing living prey. More importantly, can we stop them from one day eating us?

We think that carnivorous plants are cool. Why? Because we're sure they can't eat us unless we willingly feed them pieces of our flesh. They only eat bugs. Where's the threat? Then we find a group of pitcher plants in the Philippines that eat rats, and the gaping maws of venus flytraps don't look so cute anymore. How did the gentle greenness that makes hippies wax rhapsodic turn mean? It's tough to say, but it involved first fighting for survival against tough odds, and then indulging an ever-increasing bloodlust.

From Salt to Meat

Charles Darwin studied the venus flytrap, which he called "one of the most wonderful plants in the world." He followed the digestive processes of the plant and managed to make connections to the enzymatic digestion done by animals. He also noted how much the protein boosted the plants' growth. Neither Darwin nor modern botanists had much success with the fossil record. Plants don't leave behind the bones that make, under the right conditions, for long-lasting fossils. Generally, paleobotanists can identify the seeds of carnivorous plants, and occasionally their pollen, but can't find adult plant fossils. We know carnivorous plants are commonly found in nutrient-poor soils, but whether they migrated there after they turned predatory or whether they turned on animals for survival after they were stranded is unclear.

The entire business attained a little clarity after DNA analysis got involved. With clarity comes a sense of unease. Carnivory wasn't achieved due to the leap of one desperate, starving species of plant. It looks like plants made the jump about six different times. They couldn't wait to eat us.

Still, it seems that need played a part in it. Researchers discovered that one of the lines of carnivorous plants came from plants that were salt-tolerant. Most plants can't live in salty soil, or salt water. Some plants, however, have managed to outlast their more discriminating kin. They suck up the salt in the soil, and use special glands to excrete it along their leaves. Researchers think that this secreted salt might have been the first step towards carnivory. If the plants had started excreting other substances from their leaves, substances that killed insects, the remains of the insects would enrich the soil around the plant, or perhaps even be taken into the plant itself. A survival mechanism turned into a killing mechanism.

The Plants Hunt Bigger Prey

Why do some plants eat flesh?

The earliest carnivorous plants were most likely what's known as sticky traps. They put out long stem-like tentacles covered in tiny hairs. Those hairs have droplets of what look like water on them. The "water" is actually viscous fluid that traps the insect, at which point the tentacle very slowly curls around it. One such plant is the Drosera regia, also known as a sundew. It turns out to be the closest living relative of two hunting plants - the venus flytrap and the waterwheel, which snaps at underwater prey.

Again, scientists think the change came from a switch of secretions. While the curling reflex quickened to a snap, and the hairs stopped being inert and began to sense moving prey, the sticky solution turned into digestive enzymes. The plant evolved as a predator by using an already-effective mechanism and making it more deadly. And this time it did it not just to survive, but thrive. A snapping mechanism helps it trap larger prey, and any increase in length or width of prey, researchers argue, translates to a major increase in volume. The plants were getting hungry for more meat. Sound familiar?

Carnivorous plants already hang down through the air on vines, float in the water, and hunt underground. How do we survive them? By collaborating. Another mammal has already showed us the way. Bats are bedding down, with their young, in the hollows of pitcher plants.

Pitcher plants look like elongated cups. Any insects that stumble over the lip of the cup get trapped in the plant like a spider in a bathtub. Bats can fly out of the trap, but choose to stay for a night among the enzymes. The plant keeps away biting insects, giving the bat a safe room. In return, the bat provides nutrients, in the form of its nitrogen-rich poop. If we do the same, we might give the ever-hungrier plants reason to spare our lives. So please. Poop in your plants. Think of the future safety of the world.

Sundew Image: Brocken Inaglory

Via Journal of Experimental Botany, The BBC, Sarracenia.