National Geographic By Michelle Donahue January 15, 2019
“I’d like people to understand that hearing is not only for ears.”
EVEN ON THE quietest days, the world is full of sounds: birds chirping, wind rustling through trees, and insects humming about their business. The ears of both predator and prey are attuned to one another’s presence.
Sound is so elemental to life and survival that it prompted Tel Aviv University researcher Lilach Hadany to ask: What if it wasn’t just animals that could sense sound—what if plants could, too? The first experiments to test this hypothesis, published recently on the pre-print server bioRxiv, suggest that in at least one case, plants can hear, and it confers a real evolutionary advantage.
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Hadany’s team looked at evening primroses (Oenothera drummondii) and found that within minutes of sensing vibrations from pollinators’ wings, the plants temporarily increased the concentration of sugar in their flowers’ nectar. In effect, the flowers themselves served as ears, picking up the specific frequencies of bees’ wings while tuning out irrelevant sounds like wind.
The sweetest sound
As an evolutionary theoretician, Hadany says her question was prompted by the realization that sounds are a ubiquitous natural resource—one that plants would be wasting if they didn’t take advantage of it as animals do. If plants had a way of hearing and responding to sound, she figured, it could help them survive and pass on their genetic legacy.
Since pollination is key to plant reproduction, her team started by investigating flowers. Evening primrose, which grows wild on the beaches and in parks around Tel Aviv, emerged as a good candidate, since it has a long bloom time and produces measurable quantities of nectar.
To test the primroses in the lab, Hadany’s team exposed plants to five sound treatments: silence, recordings of a honeybee from four inches away, and computer-generated sounds in low, intermediate, and high frequencies. Plants given the silent treatment—placed under vibration-blocking glass jars—had no significant increase in nectar sugar concentration. The same went for plants exposed to high-frequency (158 to 160 kilohertz) and intermediate-frequency (34 to 35 kilohertz) sounds.
But for plants exposed to playbacks of bee sounds (0.2 to 0.5 kilohertz) and similarly low-frequency sounds (0.05 to 1 kilohertz), the final analysis revealed an unmistakable response. Within three minutes of exposure to these recordings, sugar concentration in the plants increased from between 12 and 17 percent to 20 percent.
A sweeter treat for pollinators, their theory goes, may draw in more insects, potentially increasing the chances of successful cross-pollination. Indeed, in field observations, researchers found that pollinators were more than nine times more common around plants another pollinator had visited within the previous six minutes.
“We were quite surprised when we found out that it actually worked,” Hadany says. “But after repeating it in other situations, in different seasons, and with plants grown both indoors and outdoors, we feel very confident in the result.”
Flowers for ears
As the team thought about how sound works, via the transmission and interpretation of vibrations, the role of the flowers became even more intriguing. Though blossoms vary widely in shape and size, a good many are concave or bowl-shaped. This makes them perfect for receiving and amplifying sound waves, much like a satellite dish.
To test the vibrational effects of each sound frequency test group, Hadany and her co-author Marine Veits, then a graduate student in Hadany’s lab, put the evening primrose flowers under a machine called a laser vibrometer, which measures minute movements. The team then compared the flowers’ vibrations with those from each of the sound treatments.
“This specific flower is bowl- shaped, so acoustically speaking, it makes sense that this kind of structure would vibrate and increase the vibration within itself,” Veits says.
And indeed it did, at least for the pollinators’ frequencies. Hadany says it was exciting to see the vibrations of the flower match up with the wavelengths of the bee recording.
“You immediately see that it works,” she says.
To confirm that the flower was the responsible structure, the team also ran tests on flowers that had one or more petals removed. Those flowers failed to resonate with either of the low-frequency sounds.
What else plants can hear
Hadany acknowledges that there are many, many questions remaining about this newfound ability of plants to respond to sound. Are some “ears” better for certain frequencies than others? And why does the evening primrose make its nectar so much sweeter when bees are known to be able to detect changes in sugar concentration as small as 1 to 3 percent?
Also, could this ability confer other advantages beyond nectar production and pollination? Hadany posits that perhaps plants alert one another to the sound of herbivores mowing down their neighbors. Or maybe they can generate sounds that attract the animals involved in dispersing that plant’s seeds.
“We have to take into account that flowers have evolved with pollinators for a very long time,” Hadany says. “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.”
This single study has cracked open an entirely new field of scientific research, which Hadany calls phytoacoustics.
Veits wants to know more about the underlying mechanisms behind the phenomenon the research team observed. For instance, what molecular or mechanical processes are driving the vibration and nectar response? She also hopes the work will affirm the idea that it doesn’t always take a traditional sense organ to perceive the world.
“Some people may think, How can [plants] hear or smell?” Veits says. “I’d like people to understand that hearing is not only for ears.”
Richard Karban, an expert in interactions between plants and their pests at the University of California Davis, has questions of his own, in particular, about the evolutionary advantages of plants’ responses to sound.
“It may be possible that plants are able to chemically sense their neighbors, and to evaluate whether or not other plants around them are fertilized,” he says. “There’s no evidence that things like that are going on, but [this study] has done the first step.”
Editor's Note: This story has been updated to correct the percent increase in nectar's sugar concentration.