As if it's not impressive enough for a slow-moving plant species to have learned to trap insects, Venus flytraps have learned another skill we normally associate with animals – the knowledge of how to count, in order to determine when to close their jaws on their prey.
"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," said Professor Rainer Hedrich of the Universität Würzburg, Germany, in a statement.
The inside of the trap has hair-like touch sensors. When these are stimulated mechanically an electrical signal is induced. However, it is well known that a single trigger is insufficient to cause the plant to close. Hedrich touched the hairs with a force that simulated 24 milligrams and found the first signal electrically primes the trap so that it is ready to close should a second signal occur within 15 to 20 seconds.
Ready for a meal. Marco Uliana/Shutterstock
Once the jaws have closed, insects naturally seek a way out but their movements just make things worse for them, repeatedly touching the trigger hairs. After a minimum of three further touches, the inner surface of the trap releases enzymes that dissolve the catch, with additional touches increasing the quantity of enzymes.
The capacity to count is an important evolutionary adaptation for the flytrap. Without it the plant would frequently snap shut on raindrops. Not only do closed traps catch no insects, there is an energy cost to frequent opening and closing. Even after closing the trap needs to make sure it has an insect inside before releasing precious digestive enzymes, so the capacity to count is important.
"The number of action potentials informs [the plant] about the size and nutrient content of the struggling prey," Hedrich said. "This allows the Venus flytrap to balance the cost and benefit of hunting."
The flytrap is far from unique in its capacity to sense movement, although it has taken things to an extreme few plants can match. "The model plant species Arabidopsis senses touch without any specialist cell types or organs," notes a paper in Current Biology, of which Heidrich is senior author. "Tendrils of climbing plants, however, have specialized buds that allow a conversion of already weak mechanical forces into a curvature of the climbing organ toward a given support."
Like other carnivorous plants, Venus flytraps use the insects they catch to compensate for growing in nutrient-poor soils. Heidrich found this includes swift absorption of sodium, which may substitute for potassium scarcity. However, there is a substantial price to be paid. It is thought flytraps allow small insects to escape through gaps between their "teeth" because the enzymes required to digest tiny prey are more valuable than the underwhelming meal.
Despite its popularity as a houseplant, the Venus flytrap is threatened in the wild because the fires it depends on to avoid being overgrown by competing species are being suppressed.