Plants don't have brains, so the Venus flytrap doesn't do anything that we'd recognize as "counting," in a cognitive sense. But according to this new study, the plant somehow keeps track of the number of times it's touched, which allows it to react appropriately to its prey.
When the scientists probed at their plants with mechano-electric pulses, they found that one touch set the flytraps into high-alert mode – but didn't actually result in any action on the plant's part. If a second touch happened within a few seconds, the trap snaps partially shut. It only shuts all the way after more touches, and the fifth touch triggered the release of digestive enzymes. After that, more touches mean more digestive enzymes. This allows the plant to expend just enough energy to successfully subdue and consume its prey: A bigger, livelier insect will get more attention than a weak bug.
"The number of action potentials informs [the plant] about the size and nutrient content of the struggling prey," Hedrich said in a statement. "This allows the Venus flytrap to balance the cost and benefit of hunting."
Or, in the words of The Atlantic's Ed Yong: "The plant apportions its digestive efforts according to the struggles of its prey. And the fly, by fighting for its life, tells the plant to start killing it, and how vigorously to do so."
So if you ever find yourself in a nightmarishly huge Venus flytrap (you won't, that's silly) try not to struggle. Or, alternatively, if the trap has already shut fully on you, fight valiantly to ensure that your death is as swift as possible.
The researchers hope to learn more about the mechanism behind this counting by sequencing the plant's genome.