In advertising, it's all about finding the right medium for your audience, and the natural world is no different. A new study shows that one plant species uses acoustic tricks to reach its customers—in this case bats that use echolocation to navigate. The plant, a vine found in the Cuban jungle, displays a special, dish-shaped leaf above its flowers to make itself more conspicuous to the sonar of bats, which the plant needs to spread its pollen.
The paper will change the way researchers and botanists look at forests, predicts Brock Fenton, a bat researcher at the University of Western Ontario in London, Canada, who wasn't involved in the study. "It has certainly changed how I look at it," he says. "That leaf is like a big neon sign over a restaurant."
Whereas bees are the best known pollinators, many plants rely on birds, beetles, or bats to transfer their pollen. "There are approximately 40 species of highly specialized nectar-feeding bats in South and Central America and several hundred plant species that have adapted to them," says biologist Ralph Simon of the University of Ulm in Germany, who carried out the new study with colleagues at the universities of Erlangen-Nürnberg, also in Germany, and Bristol in the United Kingdom. Among these adaptations are big, wide-open blossoms and enough nectar to cater to small mammals.
The challenge for the bats, many of which have poor vision and rely on echolocation, is to find the plants amid thousands of others. "Acoustics in a forest are hell," says Jürgen Tautz, a biologist at University of Würzburg in Germany. The echoes of plants are mostly random, and for a bat flying through dense vegetation, the patterns are constantly changing.
In earlier research, Simon and colleagues had trained bats in the laboratory to recognize objects of various shapes. They had discovered that the animals were very good at finding hollow hemispheres and distinguishing them from other shapes. So when, high up in the canopy of the Cuban rain forest, they found a vine called Marcgravia evenia that is pollinated by bats and has a leaf resembling a dish reflector above each flower cluster, they wondered whether the leaves might serve as a beacon. Because the flowers are very rare and difficult to access, the researchers tested their hypothesis in the lab.
They hid a nectar feeder within an artificial foliage background and measured the time that it took flower-visiting bats (Glossophaga soricina) that had been trained for the task to find it. Mounting a replica of an ordinary foliage leaf on top of the feeder did not significantly reduce search times. But adding a replica of the dish-shaped leaf cut them in half, the team reports online today in Science. "This leaf reflects a loud and constant echo in many different directions, making it easier to locate for a bat than a normal leaf," Simon says.
Even though the scientists have not shown that Cuban bats actually use Marcgravia's special leaf to find it, the evidence strongly suggests they do, says Ulrich Kutschera, an evolutionary biologist at the University of Kassel in Germany.
"It is an interesting example of the ongoing coevolution of flowering plants and their animal pollinators," he says. "Darwin would have been thrilled."
Fenton, meanwhile, raises another question that the scientists have yet to answer: Are the special leaves only displayed when the flowers are ready? "After all, when a restaurant closes, it also switches off the neon sign," he says.