How plants sound as well as how they look helps them to attract pollinators, a new study by scientists at the University of Bristol, UK, and the Universities of Erlangen and Ulm, Germany has found.
The researchers discovered that a rainforest vine, pollinated by bats, has evolved dish-shaped leaves with such conspicuous echoes that nectar-feeding bats can find its flowers twice as fast by echolocation. The study is published in Science.
While it is well known that the bright colours of flowers serve to attract visually-guided pollinators such as bees and birds, little research has been done to see whether plants which rely on echolocating bats for pollination and seed dispersal have evolved analogous echo-acoustic signals.
The Cuban rainforest vine Marcgravia evenia has developed a distinctively shaped concave leaf next to its flowers which, the researchers noticed, is reminiscent of a dish reflector. By analyzing the leaf's acoustic reflection properties, they found that it acts as an ideal echo beacon, sending back strong, multidirectional echoes with an easily recognizable, and unvarying acoustic signature -- perfect for making the flower obvious to echolocating bats.
They then trained nectar-feeding bats (Glossophaga soricina) to search for a single small feeder hidden within an artificial foliage background, varying the feeder's position and measuring the time the bats took to find it. The feeder was presented on its own or with a replica of either a foliage leaf or the distinctive dish-shaped leaf. Each feeder type was randomly tested once at each of the 64 positions within the artificial foliage background.
Search times were longest for all bats when the feeder was presented on its own and were slightly, but not significantly, shorter when a replica of a foliage leaf was added. However, a dish-shaped leaf replica above the feeder always reduced search times -- by around 50 per cent.
Although the leaf's unusual shape and orientation reduce its photosynthetic yield compared to a similarly sized foliage leaf, the researchers argue that these costs are outweighed by the benefits of more efficient pollinator attraction.
Dr Marc Holderied of Bristol's School of Biological Sciences, co-author of the paper, said: "This echo beacon has benefits for both the plant and the bats. On one hand, it increases the foraging efficiency of nectar-feeding bats, which is of particular importance as they have to pay hundreds of visits to flowers each night to fulfill their energy needs. On the other hand, the M. evenia vine occurs in such low abundance that it requires highly mobile pollinators."
Bats, with their wide home range and excellent spatial memory, are exceptionally efficient pollinators and many other neotropical plants depend on them for pollination. As the acoustic and perceptual principles shaping the echo beacon leaf of Marcgravia evenia should work for all echolocating pollinators, the researchers expect to find other instances of plant species that use acoustic signalling to attract their bat pollinators.