Thomas' Plant-Related Blog

On plant science. Mostly.

Ping this flower

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This post was chosen as an Editor's Selection for ResearchBlogging.org

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.

Mucuna holtonii flowers (image credit: jmlynn, webshots.com)

Mucuna holtonii flowers (image credit: jmlynn, webshots.com)

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?

Rays being reflected towards their source by mirrors at a right angle

Rays being reflected towards their source by mirrors at a right angle

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:

  1. 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.
  2. 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.

References:

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

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Written by Thomas Kluyver

31 August, 2009 at 6:22 pm

Posted in Papers

6 Responses

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  1. Congrats on joining the blogger world! (this is a certain Kingsperson name of S. Gould in case you were wondering…) I’ve joined research blogger lately as well…I jut wondered, how do you get the nice little green tick “Research blogger” at the top of the relavent posts? I’ve imported posts in, and ticked the relevant option, but I don’t knoww how to get the symbol…

    Really interesting post by the way…I shall try to keep up with your blog!

    Lab Rat

    1 September, 2009 at 6:12 am

    • Thanks! When you generate a citation code on researchblogging, it will give you two codes, one for the citation, and one for the tick icon–if you just copy and paste the code for the icon at the top of the post, it should appear.

      I shall, in turn, go and have a look at your blog. 🙂

      Thomas Kluyver

      1 September, 2009 at 8:27 am

  2. […] that pollinate them. Thomas Kluyver points out that apparently, bat pollinated flowers evolve to be better sonar reflectors. […]

  3. btw…thought you might be interested in this: http://scienceblogs.com/grrlscientist/2009/09/scientia_pro_publica_needs_you_10.php they are really desperate for hosts right now…you you’dbe in with a great chance of getting a go at hosting it here. It’s a really good way to get your blog noticed.. quite a lot of people visit it and link to it.

    Lab Rat

    6 September, 2009 at 6:46 am

  4. Yes, well you should be convinced of something by the 2003 paper.

    Certainly bats can remember what flowers they have been too, but how do they know what flowers have already been visited by other bats. Certainly they can tell if you look at the visitation rate diference between unvisited and visited flowers. So they know what flowers have nectar.

    What you should consider is that after a flower has been visited by a bat, the flower has been tripped: i.e. the androecium is “burst” out of the other petals, and stands in the way of a clean echolocation signal from the banner (or top petal). Therefore the flowers that have been visited have a different sonar “look/feel” to them.

    Of course the nectar production is extremely low after the first visit too, and the bat could smell nectar concentration (bees do). Either way the bat has to get very close to the individual flower to tell if it has been tripped (and probably to identify which one smells stronger of nectar if that is what it does).

    Either way, it maps out the best flowers to visit using more than memory!

    Flower

    9 October, 2009 at 6:47 pm

    • Hi Flower.

      It’s probable that they do use echolocation to tell the difference, but it’s not quite certain. If they visit regularly, and know when different flowers open, they could simply ‘guess’ that recently opened ones haven’t been visited yet. That would still let them “selectively visit unexploded, ‘virgin’ flowers” as the paper says.

      If you really wanted to make sure that they were using echolocation to tell the difference, you’d have to do a controlled experiment involving manually ‘springing’ some flowers, and seeing whether bats visited them less than unsprung ones that had opened at the same time.

      Thomas Kluyver

      9 October, 2009 at 7:51 pm


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