It's the first time such a feature has been discovered in plants, which mostly rely on flashy colors to attract pollinating insects. But hundreds of plant species use bats to pollinate, and researchers are still teasing out how the flying mammals home in on the plants.
In the lab, scientists noticed that Pallas's long-tongued bats excelled at finding hollow hemisphere shapes hidden among artificial leaves.
Study co-author Ralph Simon of Germany's University of Ulm then saw a picture of a Cuban plant called Marcgravia evenia. He "noticed the dish-shaped leaf above the flower, and thought, Wow, that's like a hemisphere ... that must be a signal for bats."
Study co-author Marc Holdereid of the U.K.'s University of Bristol added, "We didn't even know this plant was bat-pollinated at the time."
Bats Home in on Dish-Shaped Leaf
In experiments, bats found a hidden feeder about 50 percent faster when the feeder was accompanied by the specialized M. evenia leaf than without. Attaching a regular leaf reduced bat search times by just 6 percent.
The researchers also studied the intensity and direction of simulated bat sonar reflecting from the M. evenia leaves. These results showed the echoes would sound the same to a bat at almost any angle, which should be "conspicuously constant to a passing bat," the authors said in their paper, published July 29 in the journal Science.
M. evenia is rare and has a patchy distribution pattern in the wild, Simon said, so being able to attract bats from a distance is crucial for its survival.
"The bats have a large range and can bring pollen to specimens which are far apart," he said.
Cuban Plant Acts as True "Echo Beacon"
Another neotropical plant, M. holtonii, signals bats via a specialized acoustical "mirror" inside its flowers.
But "what we have found is going a step further," Holdereid explained. "In the first instance, it is a small reflector producing a local signal. The bat already needs to know where the flower is. In our case, we have found the true acoustic echo beacon."
Other than M. holtonii and M. evenia, the literature on plants that signal pollinators acoustically is slim, Holdereid said. (See pollinator pictures in National Geographic magazine.)
"I'd say we are only starting to uncover the full story," he said.
"We have looked at two [bat-pollinated plants] and found amazing things. We are expecting to find many more. I think the acoustic world out there is just waiting for us."