The petals don't conjugate verbs or use adjectives to tell a bee it looks nice when it flies by, at least not as far as we know. But the plants do respond to the airborne sounds of a pollinator buzzing past and greet the bees with increased production of a sweet nectar, hoping to entice the bee to land. It's a poetic and practical tongue that increases everyone's chances of survival.
Scientists increasingly believe that trees and plants communicate with each other, various living things, and the environment, responding to acoustic vibrations. Now there's additional evidence thanks to a new study on “natural language," researchers from three Tel-Aviv University schools—plant sciences and food security, zoology, and mechanical engineering—collaborated on a study that measures how evening primroses, or Oenothera drummondii, respond to sound. The Dec. 28 study on bioRxiv, a preprint server for biology papers, hasn't yet been peer-reviewed, but its findings are in line with recent discoveries on the inner life of plants. And Ed Yong at The Atlantic asked plant biologists unassociated with this paper to assess the findings, noting, “Almost unanimously, they loved the paper asserting that plants can hear."
In this study, the scientists compared plants' response to different sounds at various frequencies and used laser vibrometry to measure the vibration of the flowers' petals. They also evaluated pollinators and flowers interacting in the field. In four different experiments using grafts from hundreds of flowers that originally grew naturally outdoors, as well as films of flowers and pollinators in the wild at night and during the day, the researchers found a kind of communication at play.
The research team played sounds like that of bees' wings beating to evening primroses. The flowers vibrated when they heard the sounds. It's as if they were listening. But the flowers didn't respond the same way to every sound, or to silence. When scientists played recordings of pollinators flying past, and other sounds at similar frequencies to those bees make, the flowers answered by quickly producing a sweeter nectar within three minutes.
Other sounds, played at different frequencies, didn't lead to the same result. The flowers vibrated, but didn't increase the sweetness of their nectar production. Each flower was emptied of its nectar before the experiment, and then the sugar concentrations of the nectar produced were measured before and after to compare production based on various sounds' frequencies. The average sugar concentration was 20% higher in flowers exposed to pollinator-like frequencies, but remained stable at the sound of higher frequency recordings and silence. The researchers write, “Our results document for the first time that plants can rapidly respond to pollinator sounds in an ecologically relevant way."
These findings led the researchers to argue that flowers function as a plant's ears, informing the plant as a whole of what's going on nearby and when it's time to woo a pollinator. When the researchers tested leaves and stems to see if they, too, responded to sounds, while covering the flowers sound-proof glass, the scientists saw no changes. They hypothesize that the petals of other flowering species may have also evolved to detect sound, and that plants with “noisy" pollinators—such as bees, moths, and birds—have evolved to have large, ear-like flowers that make them extra-sensitive to these creatures' flight.
Plants have to be sensitive to the sound of pollinators because they have a symbiotic relationship with one another. The plants rely on the pollinators for reproduction, so they produce a sweeter nectar as a kind of seduction. And the bees eat the nectar, which means they'll be attuned to what the flowers are doing and drawn to the sweeter food.
Listening carefully—for plants as for people—ensures efficiency and saves resources. Both the flowers and bees benefit from the communication, according to scientists. It pays for flowers to be able to distinguish between the sound of bats as compared to bees, for example, and for the pollinators, it's worthwhile to find the best food in the least amount of time. The researchers explain: Plants could allocate their resources more adequately, focusing on the time of pollinator activity; pollinators would then be better rewarded per time unit; flower shape may be selected for its effect on hearing ability, and not only on signaling; and pollinators may evolve to make sounds that the flowers can hear.
Notably, the results also indicate that plants are likely sensitive to the noise humans make. While researchers have extensively measured plant sensitivity to aspects of the environment—like light and touch—there have been few previous studies of how flowers respond to airborne sounds. This latest work indicates that plants are relying on their listening skills, responding to acoustic vibrations to attract pollinators, and that they can distinguish between human chatter, for example, and the much more useful sound of a bee buzzing past. And it may well be that in noisy environments, flowers struggle to hear their pollinators and respond appropriately.
While this study is just a first step in understanding how plants respond to airborne acoustics, and studies on other species must be done next to better understand how flowers listen, the researchers believe there's more to the auditory story they have begun to uncover. They conclude,"[P]lants' ability to hear has implications way beyond pollination: plants could potentially hear and respond to herbivores, other animals, the elements, and possibly other plants."
