Mother Nature has devised all sorts of ways for plants to attract pollinators: colorful flowers, enticing odors, and yes, even echo acoustics.
A team of scientists has found the loudest example yet of a plant – Marcgravia evenia – that attracts nectar-feeding bats to its flower by manipulating sound. It has a large, cup-shaped leaf above its red-and-pink buds – a veritable dinner bell for passing bats. Its deep shape reflects bat sonar with a strong echo that says “Come and get it!” The findings are published today in Science.
Biologist Ralph Simon at the University of Ulm in Germany and his colleagues had done training experiments with bats before, testing to see which shapes they could detect best with their sonar. They found that bats preferred a hollow hemisphere. The team was “totally amazed,” says Simon, to find that very shape on a rainforest vine in Cuba that depends on bats for pollination
Back in the lab, the team made a replica of the leaf and tested its acoustic properties. Whereas sound bouncing off any other leaf changes with the angle of sound incidence, the deep, round leaf reflected a strong echo from many angles, meaning the signal doesn’t change as the bat moves around. That would help the leaf stand out against a background of varying vegetation, say the researchers.
“It’s a very reliable signal,” says Simon.
Simon and his colleagues then created an artificial vegetation backdrop on a wall in their lab and trained captive nectar-feeding bats to search for a feeder hidden in the mix. The bats found the feeder almost 50% faster – in about 12 seconds – when it was under one of the cupped-leaf shapes, versus under a regular leaf shape or on its own.
Although a cup shape reduces the light the leaf can trap for photosynthesis, it more than makes up for the loss by attracting an important pollinator to the rare flower, say the researchers. M. evenia is one of many plants that depends on bats for spreading its pollen, so the authors say they expect to find even more acoustically unique varieties.
“This study shows convincingly that the odd-shaped leaf found in M. evenia has unique echolocation characteristics that should make them highly ‘visible’ to nectar bats,” says bat biologist Theodore H. Fleming, emeritus professor of biology at the University of Miami. “However, to really wrap this case up, I would have liked to see field evidence,” he says, noting that video data from the field that shows native Cuban bats visit the plants more often than those without the oddly-shaped leaves would have completed the story.
Image courtesy of Ralph Simon