A sense of electricity
(This essay was published at Pregones)
If you ask a child what superpower they would like to have, many would choose the power to fly. If you ask for more details, they would say something like they would enjoy the feeling of soaring over the landscape like a bird and seeing the familiar landmarks like a house or a school from up above. But what they would visualise would be a version of their human selves flying in a bird body. A bird has a completely different sensory system, and the brain of the bird would process the information from its eyes in a way completely different to ours.
Every species lives in a different sensory world. A hundred years ago, a German biologist called Jakob Johann von Uexküll coined the term Umwelt to describe the sensory world that each species lives in, and posited that one cannot understand the world of any given species unless we have access to their sensory abilities and the brain to process these signals. We are limited and held back by our very human biases. An example from insects is useful here. We view the colours of flowers as a mixture of red, blue and green, together generating the vast number of shades, but bees view them as a mixture of ultraviolet, blue and green. Thus, the same flowers look completely different to bees -- the bright yellow colour of a Margarita flower is dim and dark to a bee. The difference in how something looks to different species is somewhat easy to imagine, after all, we are aware that some people have a condition called colour blindness or daltonism, where some hues such as red or green are not as they are seen by the majority of humans. But colours are just one aspect: imagining the entire spectrum of the sensory world from touch to scent to sound is a daunting task. And then, it is almost impossible to imagine a sense that we humans do not even possess: a sense of electricity.
We are surrounded by natural electricity, but we are barely aware of it. The difference in charge between the earth and the upper atmosphere creates a gradient that is constantly fluctuating through thunderstorms and flowing currents and raindrops. Plants and trees, because they are rooted to the ground and subject to this gradient, accumulate a negative charge, and animals, because they move around, accumulate a positive charge, accentuated by the kinds of hair and fur they possess. Our awareness of this sort of electric interaction only appears in the form of simple physics experiments, where rubbing a balloon with a cloth generates a charge that seemingly magically attracts strands of hair.
Perhaps the most famous animals that use electricity are the electric eels which hunt their prey by giving them shocks, but in this essay, I want to focus on smaller animals that use electricity in surprising ways. Animals such as ticks and mites need to find and climb on to larger mammals and birds. The problem is that these animals are often in motion and are relatively fast, so the ticks and mites have trouble climbing on to the fur. They solve this problem in a remarkably ingenious way: they fly to their target using static electricity. As these mammals and birds move around, they accumulate a charge by brushing against the undergrowth, or in the case of hummingbirds, just the action of flapping their wings generates this charge. Ticks and mites, when they come close enough to their target are attracted and propelled through the air. In this way, they can clamber abroad their hosts without expending more energy.
Looking at the daily lives of animals, and especially insects through the perspective of electric ecology has revealed many surprises. Treehoppers often have strange structural features: their bodies show an amazing diversity of twisted and pointy forms, whose function was a mystery. Now researchers have shown that treehoppers may use these structures to detect the electric field of approaching predators: they have special sensing structures that respond only to the electric field generated by a predator. Bees can use the electric field of flowers and detect if this particular flower has been visited by a competitor pollinator and therefore move onto another flower, thus saving energy.
Spiders use the electric field in two distinct ways; first in a behaviour known as ballooning, and second to attract insects into the web. Ballooning is a behaviour that spiders, usually very small ones, and especially juveniles, use to travel great distances. A spider that wants to disperse climbs up to a stalk or a twig, positions its legs together in a behaviour known as tip toeing, puts its abdomen in the air and from its spinnerets releases a dense mat of silk in the air. Silk, which is liquid in the glands, hardens on contact with air and forms a flexible extensible structure that catches the wind. The spider releases silk that in time, it acts like a kite and has sufficient lift to raise the entire spider off the ground and launch it into the air.
Spiders can reach surprising heights with this technique, they have been captured at a few km above ground, sometimes a thousand kilometres away from the nearest land, and the one of the most celebrated record of spiders ballooning was by none other than Charles Darwin himself. In 1862, he recorded spiders landing on his ship the HMS Beagle more than 100 kilometres off the coast of South America in the Atlantic Ocean. Ballooning is not restricted to tiny spiders and juveniles; even large social spiders can do this under special circumstances by building a triangular kite like structure. It was thought that wind was all it needed for a spider to get airborne; but recently researchers have shown that spiders exploit the electric field differential to get in the air in the first place. As the spider climb up to an exposed stalk, it generates a charge and when it reaches the top, the charge difference between the spider and the atmospheric electric field is sufficient to get it into the air. Once airborne, the winds act on the silk and the spider to carry it on to its eventual destination.
Now that we have a better intuition of how electric charges work in nature, we can visualise their effects without actually sensing it. To put ourselves in the mind of a small flying insect, we need to see the world and act on it by receiving sensory information. Information like the aroma from a favourite flower, the glimpse of the blurry shape of a bush telling us where to head for, the vibrations that our wings produce, the feel of the dense air and the vibrations of approaching threats and finally the inexplicable sense of the electric field nudging us in certain directions. Some nudges are useful, for mites it would be a helpful push on to a useful target, but other nudges can be dangerous. For an insect that approaches a spider web, there is the obvious danger of crashing into an invisible obstacle, which has all sorts of repercussions. The effect of such a collision could be as mild as just a momentary pause in the insect's itinerary, or much harsher -- physical damage to the wings, and if the insect doesn't escape from the glue droplets that coat the silk, death in the form of a waiting spider. Though the consensus is that insects see the world blurrily, because their vision is limited, new research is changing this view. Insects can double the information they receive because of their two compound eyes and in combination with the speed of processing and the motion of their bodies, increase their viewing power. Even small stingless bees are capable of flying through the gaps in a spider web. But what they can't do is avoid the electric field.
Spider webs, though electrically an insulator, are coated with a complex cocktail of chemicals, and attract water from the atmosphere, during which process electrostatically charge the entire web. This means that any charged particle in the vicinity is attracted towards the web. Any particle such as pollen or dust, flying through the air, is pulled onto the web, potentially reducing the effectiveness of the web. Perhaps this is why most spiders rebuild their webs every day. However, this applies to flying insects as well. These insects, if they find themselves close to a web, are pulled towards it, and more importantly, even the spider web deforms as it extends towards the insect. Pulled by these forces, the insect often comes to a stop mid-flight, held in mid-air and awaiting its eventual demise. Therefore, an insect that has a method to detect and use the fluctuations in the electric field that it is flying in will benefit by avoiding such dangerous encounters.
The field of electric ecology is still in its infancy and these studies are paving the way for a different understanding of the forces that surrounds us. Humans are very visual creatures; our sense of understanding of the world is driven by what we can see. But by exploiting the power of our imagination, we can approach and understand even those phenomena that we cannot see but perhaps we can imagine. By limiting our biases and accounting for different sensory systems, we can then arrive at a view of the world that other animals have. We can finally get a glimpse into the Umwelt of the other inhabitants of this shared physical world. And then, when we imagine soaring through the air as a bird, we can see the world not as a human inside a bird body, but as a bird would.
Further information England, S.J. and Robert, D. (2022), The ecology of electricity and electroreception. Biological Reviews, 97: 383-413. https://doi.org/10.1111/brv.12804 England, S.J. Electric Ecology: How Invertebrates Capitalise on Static Electricity. Brad Ashby Memorial Lecture at the London Natural History Society [https://www.youtube.com/watch?v=e3R5T1jVcfU]