When people pose the old question about whether a tree falling in an empty forest makes a sound, they presuppose that none of the other plants in the forest are listening in. Plants, supposedly, are silent and unhearing. They don’t make noises, unless rustled or bitten. When Rachel Carson described a spring bereft of birds, she called it silent.
But these stereotypes may not be true. According to a blossoming batch of studies, it’s not that plants have no acoustic lives. It’s more that, until now, we’ve been blissfully unaware of them.
The latest experiments in this niche but increasingly vocal field come from Lilach Hadany and Yossi Yovel at Tel Aviv University. In one set, they showed that some plants can hear the sounds of animal pollinators and react by rapidly sweetening their nectar. In a second set, they found that other plants make high-pitched noises that lie beyond the scope of human hearing but can nonetheless be detected some distance away.
After the team released early copies of two papers describing their work, not yet published in a scientific journal, I ran them past several independent researchers. Some of these researchers have argued that plants are surprisingly communicative; others have doubted the idea. Their views on the new studies, however, didn’t fall along obvious partisan lines. Almost unanimously, they loved the paper asserting that plants can hear and were skeptical about the one reporting that plants make noise. Those opposite responses to work done by the same team underscore how controversial this line of research still is, and how hard it is to study the sensory worlds of organisms that are so different from us.
The concept of floral communication has long been controversial, especially after decades of pseudoscientific (but very popular) claims about plants growing well to classical music or being attuned to human emotions. Those hokey claims “have never been substantiated by rigorous experiments,” says Richard Karban from the University of California at Davis, and they tainted the entire field of study, making scientists skeptical about the very notion of plants exchanging signals.
But after many careful studies, it’s clear that plants can send airborne, chemical messages, warning faraway relatives about marauding plant-eaters, and that animals can eavesdrop on these communiqués. Plants can also influence one another through the network of fungi that connects their roots—a so-called wood-wide web. And they can respond to vibrations moving through their tissues: Many release pollen only when insects land on them and buzz at the right frequency, while others create defensive chemicals when they sense the rumbles of chewing insects.
To Hadany, one of the Tel Aviv University researchers, it seemed weird to think that plants wouldn’t also make use of sounds—airborne vibrations. “Plants have plenty of interactions with animals, and animals both make and hear noises,” she says. “It would be maladaptive for plants to not use sound for communication. We tried to make clear predictions to test that and were quite surprised when it worked out.”
First, two team members, Marine Veits and Itzhak Khait, checked whether beach evening primroses could hear. In both lab experiments and outdoor trials, they found that the plants would react to recordings of a bee’s wingbeats by increasing the concentration of sugar in their nectar by about 20 percent. They did so in response only to the wingbeats and low frequency, pollinator-like sounds, not to those of higher pitch. And they reacted very quickly, sweetening their nectar in less than three minutes. That’s probably fast enough to affect a visiting bee, but even if that insect flies away too quickly, the plant is ready to better entice the next visitor. After all, the presence of one pollinator almost always means that there are more around.
“This shows yet again that plants can behave in remarkably animal-like ways,” says Heidi Appel from the University of Toledo, who has studied plants’ responses to animal vibrations. Crucially, she says, the study is “ecologically relevant”—that is, it involves a sound (bee buzzes) and a response (nectar sweetening) that actually matter to the plant. It’s a far cry from past studies that showed plants reacting to sounds they would never normally encounter, such as classical music, in ways that are hard to interpret (certain genes might switch on or off, but to what end?).
Here, the plants’ responses make clear evolutionary sense. Sweeter nectar is more enticing to pollinators, and by attracting more pollinators, the plant increases its odds of making more plants. But it takes a lot of energy to make supersweet nectar, and the resulting brew could be degraded by microbes or stolen by non-pollinating thieves. Far better to sweeten the fluid when it most needs to be sweet—and the buzz of a bee is the perfect cue that the time is right.
But if plants can hear, what are their ears? The team’s answer is surprising, yet tidy: It’s the flowers themselves. They used lasers to show that the primrose’s petals vibrate when hit by the sounds of a bee’s wingbeats. If they covered the blooms with glass jars, those vibrations never happened, and the nectar never sweetened. The flower, then, could act like the fleshy folds of our outer ears, channeling sound further into the plant. (Where? No one knows yet!) “The results are amazing,” says Karban. “They’re the most convincing data on this subject to date. They’re important in forcing the scientific community to confront its skepticism.”
“It’s such a wonderful and exciting finding,” adds Monica Gagliano from the University of Sydney, a pioneer in the study of plant acoustics. She notes that one of the team members, Daniel Chamovitz, “was quite skeptical, or even dismissive, of the whole idea of plant bioacoustics only a few years ago. Now, in the spirit of good science, he is experimentally testing these ideas. That approach deserves to be applauded and encouraged.”
But she and the others are less impressed by the Israeli team’s second study, which looked at whether plants make sounds. For decades, scientists have known that plants give off popping noises, as air bubbles form and collapse in their stems—a botanical version of the bends, which is exacerbated by drought. But these pops have mostly been recorded by microphones placed directly onto stems. Hadany and Yovel wanted to know whether they could be heard from afar, through the air. If so, perhaps they could act as signals for animals—or, more enticingly, for other plants.
The team put individual tobacco or tomato plants inside soundproof boxes, in front of two sensitive microphones each. They then searched for noises they could attribute to a specific plant—sounds picked up by a plant’s two dedicated mics but not by those trained onto its neighbors. It worked: Every few minutes, the plants emitted short ultrasonic sounds, too high for humans to hear normally. But they were still relatively soft noises. At a distance of four inches, they had a volume of 60 decibels, roughly equivalent to normal conversation and perhaps insignificant to any other creatures. “Ultrasonic-sensitive creatures like moths and bats, going around a field, might be hearing lots and lots of sound,” Hadany says.
The team also found that dry or damaged plants produce noises more frequently. A computer could even learn to distinguish the sounds of ailing plants from those of healthy ones, with about 70 percent accuracy. And if software can do it, could an insect? Might a moth use sound to avoid laying eggs on a stressed plant? Could hungry bats head toward the noise of plants being besieged by insects? And could farmers use these pops to tell whether their crops need more water?
Without knowing exactly how the pops of harmed and healthy plants differ, it’s hard to know how informative they’d be, says Rafael Rodríguez Sevilla from the University of Wisconsin at Milwaukee, who has called for more careful interpretations of studies on plant acoustics. Would an eavesdropper be looking for some change in how frequent they are? They’re not that common. They’re also very brief, and liable to fade with distance. “Yes, in theory, animals might use the sounds to gain information about a plant’s condition,” says Carel ten Cate from Leiden University, “but how meaningful is it if a plant produces something like 20 soft 0.1-millisecond pulses per hour?” And since the sounds would likely vary with different types of damage, or degrees of dryness, how much specific information could an animal possibly glean from them?
Also, “there is no indication that the pops are specialized signals of stress rather than cues produced incidentally due to the damage,” Rodriguez Sevilla adds. Hadany acknowledges that but says, “If the sounds are out there, they’re informative, even if they’re not ‘intentional’ by the plant.”
To convince their critics, Hadany and Yovel clearly need to do more experiments. They’re already planning to repeat their study in a more natural outdoor setting to see whether those sounds carry amid ambient noise. “We also need to test specific relevant organisms to see if they respond,” says Hadany. “And, of course, the most exciting prospect for us is: Are plants capable of hearing the sounds of plants?”