Three years ago, we reported on the “shocking” discovery that flowers decorate their petals with negative charges that bumblebees can detect. The patterns of charge are species-specific, as if to tell the positively charged insects to come on in for a treat. At the time, the methods bees use to detect the charges were unknown, but the news created quite a buzz in the media (see National Geographic). Now, the organs of electrical sensing in insects are coming to light.
In a recent open-access paper in the Proceedings of the National Academy of Sciences (PNAS), Sutton et al. locate the sensing in the tiny hairs, called filiform hairs, that cover the bumblebee’s body. It was known that the hairs respond to motion and sound, and that they are innervated at the base for transmission of information to the brain. The new findings add another function to these multipurpose sensors:
Electroreception in terrestrial animals is poorly understood. In bumblebees, the mechanical response of filiform hairs in the presence of electric fields provides key evidence for electrosensitivity to ecologically relevant electric fields. Mechanosensory hairs in arthropods have been shown to function as fluid flow or sound particle velocity receivers. The present work provides direct evidence for additional, nonexclusive functionality involving electrical Coulomb-force coupling between distant charged objects and mechanosensory hairs. Thus, the sensory mechanism is proposed to rely on electromechanical coupling, whereby many light thin hairs serve the detection of the electrical field surrounding a bumblebee approaching a flower. [Emphasis added.]
The “electromechanical coupling” means that the hairs respond to the presence of static electricity by moving toward or away from one another. That motion among “many light thin hairs” creates patterns in the nerve endings that the bee can use for information on the nectar quality in the flower. Apparently the antennae are less sensitive to electrical movement than the body hairs, for bumblebees at least. Honeybees may make more use of antennal deflections.
If another positively charged pollinator visited recently, the flower will have fewer negative ions due to charge cancellation. It will take a few minutes for the flower to recharge itself. In the meantime, the pollinator can move on to better sources, not wasting time where the nectar has already been taken.
Commenting on this discovery in PNAS, Harold H. Zakon finds it intriguing that insects get information from static “noise.” We humans feel static electricity (also called triboelectricity) primarily when we touch a doorknob after scuffing our shoes on the carpet; otherwise we are not aware of it (unless it gets strong enough to make our hair stand on end). That brief shock on the fingertip is noise to us, not information. It’s just an epiphenomenon to us, Zakon says. Imagine, though, if sensors on our body hairs detected patterns of charge in a candy store, leading us directly to the best-advertised treats? What if our friends could read and interpret our electrical charge patterns without our having to say a word about where we have been?
Whatever differences may exist between honeybees and bumblebees, Zakon says, “the bigger take-home message of all of these studies is that insects have a triboelectric sense mediated by mechanoreceptors.” Note, however, that having hairs that deflect in the presence of static electricity is not enough. The nerves have to know the difference between electrically-induced motions and wind or sound motions. The brain has to be able to interpret the patterns of sensations coming in. Then, the brain has to activate instinctive responses, with muscles and nerves, to make use of the information. Unless the whole system works together, static electricity is useless as a signal.
Most of us have watched bees pollinating flowers all our lives without knowing there’s a hidden communication system going on between plant and insect using invisible forces. That’s fascinating enough, but the case of bumblebees hints at widespread electrical signaling in the biosphere. Sutton et al. remark, “This finding prompts the possibility that other terrestrial animals use such sensory hairs to detect and respond to electric fields.”
What other animals might use static information? Zakon offers some possibilities.
Is there a triboeletric sense in other insects? If accumulation of charge on an insect’s body is as widespread as appears likely, is it an epiphenomenon or even a nuisance in some species — perhaps even suppressed centrally as noise — but used in others? Have other insect pollinators — such as wasps, moths, butterflies, flies, and beetles — also evolved to interact electrically with their flowers? It has been known for over 100 y that charge is held on the hair of mammals and feathers of birds. Similar to bees, pollen may be electrostatically attracted to approaching hummingbirds. Might hummingbirds and other nectarivorous pollinating birds, or perhaps some pollinating mammals such as bats, have evolved a similar triboelectric sense?
We’re used to hearing that all kinds of complex systems “have evolved.” But how sensible is that when a triboelectric sense requires detectors, nerves, brains, and muscles to utilize an information source? How much less sensible to say they “have evolved” multiple times in unrelated animals as diverse as birds, butterflies, and fish? How did members of the plant kingdom get involved?
A phenomenon is not a “signal” unless it is detected by a creature looking for it, equipped with an appropriate receiver. A signal is not “information” unless a creature can use it for a function. Mars has static electricity, but the only ones who care are humans who sent intelligently designed rovers there to measure it when dust devils passed by. The moon has static electricity, too; it was a nuisance to Apollo astronauts when it made dust cling to everything. It only became “information” when scientists investigated its properties to gain insight into the origin of the lunar regolith, partly to plan for dealing with it in case a lunar base is ever built.
We’ve pointed to other invisible sources of information used by animals, including the earth’s magnetic field. Earlier this month we reported how deer are magnetically equipped; now we can add another mammal: the wart hog. No kidding; scientists from Uppsala University reporting in Mammal Review now say that “wild boars and wart hogs may have an internal compass.”
“The fascinating findings add on to a well growing body of evidence for a magnetic sense in mammals. The interesting questions that arise now are how they are able to sense the magnetic field and whether they really use it for navigation” said Dr. Pascal Malkemper, senior author of the Mammal Review study.
In Living Waters, Illustra Media showed sea turtles using the magnetic field for information, salmon using odor molecules for information, and dolphins using sound for information. In each case, the animals are equipped with extremely sophisticated machines to detect, transmit, interpret, and utilize the information. In each case, furthermore, the mechanisms appear irreducibly complex — incapable of explanation by blind, unguided processes.
Now we can add static electricity to the growing list of information sources utilized by living things. Without well-designed receivers and interpreters, static electricity is a mere epiphenomenon of no functional consequence. In short, it’s noise. Only intelligence knows how to extract signal out of noise and use it to get things done.