Pollination is the process whereby plants turn animals into sex toys. With nutritious nectar, striking flowers (and the odd bit of deceit), they lure in animal carriers that can transport their pollen to another flower. These partnerships have painted the world in a resplendent palette of flowery hues. But pollination can create other feasts for the senses that are oblivious to us visually focused humans.
The Cuban rainforest vine Marcgravia evenia is pollinated by bats, which find their way around with sonar rather than sight. They make high-pitched clicks and time the returning echoes to “see” the world in rebounding sound. And M.evenia exploits that super-sense with a leaf that doubles as a sonar dish. It reflects the bats’ calls into strong, distinctive echoes, creating a sonic beacon that stands out among the general clatter of the forest.
The vine also has a ring of red and white flowers, sitting over a cup full of nectar. But above the flowers, it has one or two dish-shaped leaves that are twisted upwards, so the concave side faces out towards approaching pollinators. In cross-section, they could form the cap of a sphere.
These leaves are very different to those found elsewhere on the vine, and Ralph Simon from the University of Ulm in Germany thought that they might act as bat-attracting beacons. To test his idea, he hit the leaves with bursts of sound at the same frequency as the calls of Pallas’ long-tongued bats.
Normal leaves reflected stronger echoes straight ahead of them, but these reflections became drastically less intense at any sort of angle. However, Simon found that the dish-shaped leaves reflected strong echoes at angles of up to 60 degrees. A bat could detect these sounds over a larger volume of space than those of the normal leaves.
The sounds are also very distinctive. For a normal leaf, echoes change depending on where the listener is standing or flying. Even a small shift in direction can produce a very different sound. But the dish-shaped leaves are different – their echoes sound much the same in a wide variety of directions. The curve of the dish sends back two very strong echoes – one from its centre, and another that rebounds off two sides – that merge in a mostly constant way, over a wide range of angles. To a flying bat, it must be the aural equivalent of the Mona Lisa’s eyes, a burst of echoes that follow it wherever it goes.
To prove that these loud, broad and distinctive sounds are attractive, Simon trained bats to find a nectar feeder hidden among an artificial clump of foliage. When he moved the feeder, the bats took around 23 seconds to find it again. If he added a normal leaf on top, they took 22 seconds. If he added one of the vine’s dish-shaped leaves, the search time halved – the bats found the feeder in just 12 seconds.
Simon thinks that the leafy beacons benefit both the vine and the bat. The vine obviously gets sex-on-wings. Meanwhile, the bats find nectar more quickly. That’s useful because they have to make hundreds of such visits every night to fuel their gas-guzzling metabolisms.
Other plants that rely on bats for pollination may use similar tricks. Otto van Helversen, who was involved in Simon’s study, found another example back in 1999 – another tropical vine called Mucuna holtonii that also lures bats with concave leaves. It too reflects strong echoes in many directions and if you cut it off, the vine’s flowers are less often visited. Perhaps there is an entire world of similar echo beacons out there, with a acoustic richness that rivals the visual splendour of flowers.
Reference: Simon, Holderied, Koch & von Helversen. 2011. Floral Acoustics: Conspicuous Echoes of a Dish-Shaped Leaf Attract Bat Pollinators. Science http://dx.doi.org/10.1126/science.1204210
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