Ping this flower
This one’s an old bit of research, but a favourite of mine. It’s not groundbreaking science, but when I first heard about it, I just went ‘oh, wow’, in amazement at what natural selection can come up with! In short, it’s a flower shaped to reflect sonar so that bats can find it.
Flowers can be pollinated by all sorts of animals. Bees and butterflies are familiar pollinators, but there are also flowers that stink of rotting meat, to attract flies. And, in the tropics, where nectar is available year round, larger animals (which live longer than a year) can become specialist nectar feeders. Bat pollinated flowers tend to be pale (colours are wasted at night), strongly scented, and produce very large amounts of nectar (larger creatures need more energy to fly). One species, however, has a further trick up it’s sleeve.
Mucuna holtonii doesn’t have an English name. It grows in Central America as a vine, and it’s a legume (a relative of beans). When its flowers are ready to release pollen, they raise the top petal (the ‘vexillum’), which you can see in the photo above. A bat turns up to drink the nectar, and when it pushes into the flower, the anthers explode, dousing it in pollen. Because the flower releases its pollen in one go, it doesn’t need to attract a second bat, so it doesn’t refill with nectar.
Like insect-hunting bats in Britain, Mucuna‘s pollinators use echolocation: they make a brief squeak, too high for us to hear, and use the echoes to work out where things are. The scientists had noticed that, using a loudspeaker to do the same sort of thing, the top petal reflected much more sound than something its size ought to. How was it doing that, and was it advertising to the bat?
The shape of the petal suggested the how. Bike reflectors and cats eyes (the ones on the road) have mirrors at right angles. As the diagram on the right shows, these work to bounce back light in the direction it came from—so when your headlights shine on it, you see the light. Sound and light are both waves, and Mucuna‘s top petal is working in the same way for the bats’ sonar pulses. The German scientists tested this using a loudspeaker and a microphone: sure enough, the echo was loudest when the two were in line, both when the loudspeaker was in front of the flower, and when it was 30° off.
To show that this really was a signal to the bats, they got two more lines of evidence:
- When some of the flowers had the top petal cut off, or filled with cotton wool, they were far less likely to be visited by a bat. 88% of unmodified flowers were visited, but only 21% of those with the top petal cut off, and 17% of those where it was filled with cotton wool.
- Some closely related species are pollinated by bats which don’t use echolocation. The top petal in their flowers is not shaped in the same way to reflect sound.
Case closed, if you ask me. In 2003, the same scientists claimed that the bats could, by echolocation, distinguish newly opened flowers from those that a bat had already visited (so would have no nectar). That’s not implausible, but I’m not convinced: the bats could just be remembering which flowers opened when.
von Helversen, D., & von Helversen, O. (1999). Acoustic guide in bat-pollinated flower Nature, 398 (6730), 759-760 DOI: 10.1038/19648
von Helversen, D., & von Helversen, O. (2003). Object recognition by echolocation: a nectar-feeding bat exploiting the flowers of a rain forest vine Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology, 189 (5), 327-336 : 10.1007/s00359-003-0405-3