The Negative Nancys and Debbie Downers of the world might say that all the major scientific discoveries have been made already, that no new surprises remain in the natural world waiting to be stumbled upon. Obviously, people who say that are wrong. Today I finally got around to reading a new paper that recently came out from Veits et al. that absolutely blew my mind (in a good way). While not officially published yet, the article is available as a pre-print on biorxiv. In it, the authors report that the evening primrose (Oenothera drummondii) can not only detect sounds made by the beating wings of the bees and moths that pollinate it, but also responds to these sounds by producing sweeter nectar.
In other words, the plant hears its pollinators coming and responds accordingly.
The group of researchers, all based at Tel Aviv University in Tel Aviv, Israel, used a series of elegant experiments to demonstrate that the evening primrose can detect sounds produced by pollinators. They used speakers to play artificial sounds at low, high, and intermediate frequencies, as well as the recorded sound of a bee flying. When they measured the sugar concentration of nectar produced after exposure to these sounds, it was 20% sweeter after exposure to the sound of a flying bee or to the low frequency sounds (which includes the frequency produced by flying bees). In addition, the group showed that it is the flowers that function to detect sound: plants that had their flowers covered did not produce sweeter nectar when exposed to any sound, and vibration of the flower petals was also directly observed.
Nobody has ever shown a plant responding so rapidly to an airborne sound before, let alone one that is ecologically relevant. It’s one of those things that makes perfect sense when you read about it, but still evokes awe at the wonderful facts the natural world is hiding all around us. There are a couple key takeaways from Veits et al. that are particularly cool for me.
Unwrapping the Umwelt of Plants
In 1934, an Estonian-born German scientist named Jakob von Uexküll introduced the concept of “umwelt” to the study of animal behavior. Literally meaning “environment” or “surroundings” in German, von Uexküll posited that those studying animal behavior must take care to view the animals as subject in their own unique sensory worlds. In other words, he acknowledged that animals have very different sensory capabilities than our own, and so viewing animal behavior through the lens of human abilities is misleading and unhelpful for generating true understanding.
This paper now adds to our understanding of the sensory worlds of plants. We already knew that plants can detect and respond to many stimuli, such as light, gravity, moisture, touch, and chemicals, but now we can airborne sound to the list. These results open the door for a brand new area of scientific research focusing on how plants hear and how and why flowers evolve to detect sounds in the air. A new piece of the plant umwelt has been unlocked!
A New Piece to the Co-evolution Puzzle
I have a high opinion of bees, and part of that opinion is a massive appreciation for just how smart they are. Bees are incredible at learning, and when you think about how they go about finding food, it makes sense–a bee that can learn and remember flowers that provide lots of nectar and pollen will do better than a bee that can’t. We thus would expect bees to evolve, via natural selection, to have good cognitive abilities…and alas, they do.
What this paper demonstrates is that plants have also evolved to exploit the cognitive abilities of bees and other pollinators. By increasing nectar sweetness only when pollinators are near, the flower is essentially increasing the motivation for pollinators to visit other flowers of the same type. Pollinators search for food in complex floral marketplaces, so they only bother learning and remembering the best food sources. Sensing sounds produced by approaching pollinators enables the plant to only put resources into sweetening the nectar when it is most beneficial; being super sweet all the time would be energetically wasteful!
We have long known that bees and other pollinators can pick up on signals from the flowers they visit, such as shape and color. Now we know that at least one plant’s flowers are fine-tuned to eavesdrop on the auditory world buzzing through the skies, increasing the closeness of what was already an evolutionarily marvelous relationship.
As Always, Now We Must Know MORE!
As cool and exciting as these results are, the authors still only tested one plant species. Given that 87.5% of flowering plants depend on some kind of animal pollinator, it is more than likely that other plants have evolved similar sensory abilities as well. Still, this will need to be tested before any final conclusions can be reached. It is also possible that other plants may detect sound through something other than their flowers, such as the stem or the leaves (though the flowers make the most sense, as that is where the nectar is found). As is always the case in science, more research is definitely needed!
I for one am excited to see what the future of this research will hold, and what other fascinating facets of biology still await right in our backyards. And everyone, be sure to watch what you say…you never know who or what might be listening.