Many scientific reports show inexplicable behaviours of plants that seem to be analogs to animal senses, behaviours, and perhaps even intellect.

This observation of complex behaviour in plants would seem to be impossible given the fact that plants don’t have the diversification of their bodies and biology into sensory organs, nervous systems, and brains, but the facts remain. I’ve reported on some of these oddities associated with ocean phytoplankton and bacteria. My post on the finding of sentient life in the ocean micro-drifters, the plankton tells part of that story.

Another post in my collection showing the capacity of life at what we improperly call a primitive level is about ocean bacterial communication mats that have the ability to communicate complex information across vast networks made up of mats made up of those bacteria.

Now, researchers at the University of Missouri, in a collaboration that brings together audio and chemical analysis, have proven that plants hear sounds. They not only listen for, they respond to the sounds that caterpillars make when they are biting and eating those plants and that the plants respond with complex defenses.

“Previous research has investigated how plants respond to acoustic energy, including music,” said Heidi Appel, senior research scientist in the Division of Plant Sciences in the College of Agriculture, Food and Natural Resources and the Bond Life Sciences Center at MU.

Arabidopsis sp. a favourite food of the terrifying caterpillar

“However, our work is the first example of how plants respond to an ecologically relevant vibration. We found that feeding sounds of caterpillars attacking plants signal changes in the plant cells’ metabolism, creating more defensive chemicals that can repel attacks from caterpillars.”

Appel collaborated with Rex Cocroft, professor in the Division of Biological Sciences at MU. In the study, caterpillars were placed on Arabidopsis, a small flowering plant related to cabbage and mustard. Using a laser and a tiny piece of reflective material on the leaf of the plant, Cocroft was able to measure the movement of the leaf in response to the chewing caterpillar.

Cocroft and Appel then played back recordings of caterpillar feeding sounds to one set of plants, but for a control played back only silence to the other set of plants. When caterpillars later fed on both sets of plants, the researchers found that the plants previously exposed to feeding soounds produced more mustard oils, a chemical that is unappealing to many caterpillars. Clearly those plants retained a memory of the sound and knew what to do when they heard it again.

“What is remarkable is that the plants exposed to different vibrations, including those made by a gentle wind or different insect sounds that share some acoustic features with caterpillar feeding vibrations did not increase their chemical defenses,” Cocroft said. “This indicates that the plants are able to distinguish feeding vibrations from other common sources of environmental vibration.”

MU Scientists Appel and Cocroft – “The field is somewhat haunted by its history of playing music to plants. That sort of stimulus is so divorced from the natural ecology of plants that it’s very difficult to interpret any plant responses,” Cocroft said. “We’re trying to think about the plant’s acoustical environment and what it might be listening for, then use those vibrational sounds to figure out what makes a difference.”

Appel and Cocroft say future research will focus on how vibrations are sensed by the plants, what features of the complex vibrational signal are important, and how the mechanical vibrations interact with other forms of plant information to generate protective responses to pests.

“Plants have many ways to detect insect attack, but feeding vibrations are likely the fastest way for distant parts of the plant to perceive the attack and begin to increase their defenses,” Cocroft said.

“Caterpillars react to this chemical defense by crawling away, so using vibrations to enhance plant defenses could be useful to agriculture,” Appel said. “This research also opens the window of plant behavior a little wider, showing that plants have many of the same responses to outside influences that animals do, even though the responses look different.”

Another example of the same sort of intelligent plant behaviour is found in another post of mine: “The leaf and the caterpillar, slow thinking on sentient life.”

The point of these examples on the nature of plants is to show how life on this blue planet both animal and vegetable has in common a billion years of evolution. Just why we insist on believing that only certain animal life found sentience a useful evolutionary path is beyond me. This thing we like to think of as our unique sentience is in fact not at all unique rather it is just the opposite.

We are not alone.

The Missouri study, “Plants respond to leaf vibrations caused by insect herbivore chewing,” was funded in part by the National Science Foundation and was published in Oecologia.

For a longer version of the story of plant hearing MU provides this page.

Next time you bite down on your favourite vegetable it might be best to first plug your ears lest its dying screams ruin your meal.