And when Ulmar Grafe from the University of Brunei Darussalam looked inside them, he saw seven times fewer insects than in other pitchers but instead, he found small bats. Credit: Schöner et al./Current Biolgoy 2015.
It was that discovery that led to the study on the ultrasound communication in the first place, as the researchers wondered how the carnivorous plants were so successful at grabbing the attention of bats flying by.
When the researchers have pinpointed the benefits of the plants and bats to each other, rich fertilizer for the plants and a safe roosting spot for bats, scientists were keen to find out how exactly did the two find each other in a lush environment filled with thick, tropical vegetation.
Schöner, senior author Gerald Kerth, and their colleagues determined that the bats poop in and around the plants, keeping the latter well fertilized with their droppings. The Nepenthes isn’t so much after the bats, but after their guano, which is a nitrogen-rich plant food.
Developing these leaves has cost the carnivorous plant some of its ability to attract insects to digest, but the extra bat poop makes up for the losses. It turns out that the mouth of N. hemsleyena is shaped like a dish, and capable of acting as an echoing chamber for the cries that bats give off. After all, both species are relatively rare, and they live in a rather crowded place.
In fact, some neotropical flowers have a similar problem: they depend on nectar-feeding bats to pollinate them. They found that bats preferred to approach enlarged or normal pitcher plants over plants without the reflective structure.
“With these structures, the plants are able to acoustically stand out from their environments so that bats can easily find them”, explains co-author Michael Schöner of Ernst-Moritz-Arndt-University of Greifswald in Germany. It now appears that the unrelated pitcher plant N. hemsleyana does the same thing, but for a different reason. In order to find out how these plants and bats manage to locate one another, the researchers decided to use an artificial biomimetic bat head that emits and records ultrasounds to test the pitchers’ acoustic reflectivity. Those studies uncovered a strong echo reflection from the pitchers’ back walls, where the plant form works perfectly as an effective reflector.
The study answers a longstanding question about these particular plants: Why don’t they feast on many insects versus what other pitcher plants do?
The mutualistic relationship between the pitcher plant and the bats is one more example of the way in which nature, through evolution, can solve problems, the researchers say.
Subsequent behavioral experiments showed that the bats respond to those sounds echoed back to them from the plants.