Plants Can Hear

 

As humans, we are experts at anthropomorphology. We easily identify aspects in other organisms that match our own. For example, we understand the eyes of animals, which appear largely similar to our peepers, provide the critters with sight. In art, a long tradition of attributing human traits to non-human entities has provided a rich metaphorical layer to our tales (think J.R.R. Tolkien’s ents, George Orwell’s Animal Farm, or the Muppets).

In many ways, it makes sense: we perceive and create with what we know. If alien life exists, it’s hard for us to imagine sentient beings without eyes, limbs, brains, or mouths. We tend to be less adept at thought experiments to ponder the reverse. Are there other ways to sense the universe than our own?

With just a little thought, the answer is certainly “yes.” Though one might quibble with the definition, scientists have recently discovered evidence that plants can hear!

The ents were J.R.R. Tolkien's anthropomorphized trees

Before we get to the plants, it’s important to recall the essence of sound. We tend to take for granted that we simply experience music, wind through leaves, or rushing water. These examples are not magical absolutes that we happen to hear. Sound is actually a mechanical phenomenon, propagated by vibrations through the air or other media. Most times, humans perceive sound because vibrations hit our eardrums and our brains translate these movements into our sense of hearing. Thinking in human terms, one might wonder if an “ear” is necessary for hearing. In a way, the answer is yes; an entity does require some sort of receiving organ to sense sound. However, that organ need not be an ear with a drum.

In fact, even those whose ears do not work properly can experience the waves of sound. Beethoven famously adapted to deafness by placing a rod in his mouth that was connected to his piano. He taught himself to discern the different frequencies of notes through vibration. One story about the debut of his masterwork Symphony No. 9 involves standing ovations the conductor needed to be informed were occurring since he could not hear clapping through the air.

Perhaps two more modern examples might help illustrate the notion of non-human ears. The parabolic shape of a classic satellite dish is optimized for radio waves. The waves hit the dish and focus on a point that contains a device, called a feedhorn, which begins the process of translating the waves into usable information. Another instance might hang on your wall, sit on your table, or in your pocket. Alexa, Siri, and Cortana can all “hear” you without an eardrum.

“Ears” can be anything that can parse vibrations.

A parabolic satellite antenna - photo by Richard Bartz

An evolutionary theoretician at Tel Aviv University named Lilach Hadany (great moniker for this story) pondered the “natural resource” of sound. Could an entire swath of life on Earth skip utilizing an information stream that constantly flows? If plants were to somehow “hear” the vibrations of the world, they could potentially add a substantial ability to transmit their genes. The traditional view is that plants do not use sound. How could you test the hypothesis?

Hadany and team developed a fantastic way: bees!

They studied a flower called evening primrose, a plant that relies on pollination from bees and produces measurable amounts of nectar. In a laboratory, the team subjected the primroses to five different levels of sound: silence, bees buzzing, and three levels of computer-generated frequencies. The team measured the increase in nectar sugar concentration with each of the five. The flowers exposed to silence, high frequencies, and medium frequencies showed no significant change in nectar production. But the flowers that “listened” to the bees and the low-frequency sounds, which mirror the frequencies of the buzzing, went into super-sugar mode, yielding 12-20% more nectar.

Astonishingly, the change transpired within three minutes!

The evening primrose - photo by Ximenex

Hadany believes that sweeter nectar will draw in more pollinators. If a bee happens to visit a flower, nectar production needs to happen in a hurry, before the insect flies off to another location. These experiments indicate that flowers can quickly up production. Further, the team noted in the field that pollinators were nine times more likely to visit a plant that had been visited by another pollinator within the previous six minutes. It seems as if the flowers and the bees have developed a fast, feedback-loop give and take. The sweeter the nectar, the more enticing the flower; usage breeds popularity.

To extend the ear metaphor, consider the shape of many flowers. Numerous feature concave bowls, which just happen to resemble parabolic satellite dishes! The research team determined that the vibrations of the pollinators actually resonate with the shape of the flowers. They tested this notion by duplicating the experiment on flowers missing petals. These flowers did not resonate with pollinators!

So, it seems, at least one species of flower can hear sounds, which it uses to its advantage. The implication of this discovery is rather seismic. Could plants have been listening all along and we never knew it? Hadany dubbed this new area of research “phytoacoustics.” 

Talking to National Geographic, she said, “We have to take into account that flowers have evolved with pollinators for a very long time. They are living entities, and they, too, need to survive in the world. It’s important for them to be able to sense their environment—especially if they cannot go anywhere.”

If plants can hear, what else can they do? With this discovery, researchers will likely discover a whole litany of talents in the future. Perhaps not to the level of talking trees, but, as they say, one never knows!

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