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Beyond the five senses

by Jackie Higgins

Animals have shown us there are myriad senses. Can we rewild our own?

Some 500 years ago, Leonardo da Vinci observed that 'an average person looks without seeing, listens without hearing, touches without feeling, eats without tasting, moves without physical awareness, inhales without awareness of odour or fragrance'. 

We remain guilty of underestimating the ways in which we sense the world. Most of us still labour under a misapprehension dating back to Aristotle that we have only five senses, even though a human sixth sense is no longer deemed pseudoscientific fiction but scientific fact; so too, a seventh, eighth and ninth. Some neuroscientists count as many as 33, served by dedicated sensors. We take these senses for granted because they circumscribe our every waking moment. The evolutionary biologist Richard Dawkins has talked of how familiarity dulls the senses and anaesthetises us to the wonder of existence. I propose we shake off the anaesthetic and recapture the wonder with a rewilding of our senses.

Rewilding has become a fashionable notion in ecology and conservation, in no small part thanks to Isabella Tree's book Wilding, in which she recounts the remarkable rewilding project at the Knepp Estate, where the land, plants and animals are being left to recover a natural equilibrium. I am not advocating a restoration of our senses to some uncultivated state, but a return to nature of another sort. We are part of an immense, sprawling, evolutionary family, interconnected by billions of years of shared history - other species, our distant relatives, can offer fresh perspective on ourselves and an intellectual distance to more fully appreciate the ways in which we sense, even make sense of, the world. By seeing through their eyes, hearing with their ears, feeling through their skins, tasting and smelling with their tongues and noses, we might re-engage with our own wildness and shed light on that most pressing of philosophical questions: what does it mean to be human?

We celebrate sight as our most important sense and presume to understand it, but its definition fractures beneath the gaze of a peacock mantis shrimp. This crustacean, the most colourful on the Great Barrier Reef, eyeballs its surroundings with the most diverse array of light sensors on the planet. It can see ultraviolet as well as polarised light and is the only animal known to see circularly polarised light, giving it a secret channel of communication invisible to others. It boasts so many colour sensors that its world is reputed to be 'a thermonuclear bomb of light and beauty'. Shrimp sight may be beyond our wildest imaginings, but it reminds us that light contains infinite information, and our eyes support more than one way of seeing. Similarly, our ears, tongue, nose and skin support unexpected ways of hearing, tasting, smelling and touching and, sometimes, these organs conceal further talents, what the late neurologist Oliver Sacks called 'secret senses'.  

Comparisons with far-flung branches of our evolutionary family tree - from orb-weaving spiders to bar-tailed godwits - suggest that our eyes enable us to sense time as well as space and possibly even direction, like a navigational compass. Our inner ears hear but also give us a sense of balance, enabling us to stand tall and walk. Inspired by snakes, scientists have found that we smell with our tongue and, to further confound assumptions, that our nose tastes and might detect - as the largest European moth, the giant peacock of the night, suggests - airborne messages that don't even have a smell. As researchers uncover the hidden potential of our eyes, ears, tongue and nose, their focus has shifted to the largest sense organ. Our skin has become science's last great sensory frontier. 

The accolade of the fastest eater in the animal kingdom does not go to the cheetah, but to a mammal that barely fills a human hand, seldom ventures above ground, and poses little threat unless you are an earthworm. The star-nosed mole can identify, capture, and consume its prey faster than the blink of an eye, thanks to its nose, topped with a 22-tipped star that doesn't so much smell as feel its way through subterranean tunnels. Ken Catania, the neurobiologist who made it his mission to study this star, found that it has six times the sensitivity of a human hand, in an area no larger than a fingertip. He discovered it was primed with the same sensors found in our skin, Merkel cells. These are most abundant in our fingers and enable such nuance that we can discern features mere fractions of millimetres apart. They are how we gauge the topography of the world; how we distinguish the smooth roundness of a ball bearing from the corrugated roughness of a walnut. Their arrangement on the mole's nose casts light on a timeworn battle of the sexes: who has the more sensitive touch?

Scientists have long known that some of us have a heightened sense of touch. Convention would have us believe that women prevail here; typically they outperform men in tactile acuity tests. Psychologists might argue that it has something to do with the ways male and female brains work, but the neuroscientist Daniel Goldreich suspected a simpler solution. Focusing on the skin sensor prevalent on the mole's star, he found it in similar numbers on the fingertips of both men and women. He decided to plot touch acuity against fingertip area. Every person on the graph landed on the same line. Men with slender digits displayed more tactile acuity than some women. In other words, touch proficiency is dictated not by sex, but by hand size. The star-nosed mole is living proof of what happens when sensors are crammed into a small area. 

Human skin has many types of sensor beyond Merkel cells. They register subtly different aspects of the environment and are named for the scientists behind their discoveries. Egg-shaped Meissner's corpuscles, found in our epidermis alongside Merkel cells, detect feather-light pressure and low vibration, and have been called 'kissing receptors'. Pacinian and Ruffini corpuscles are buried deeper, in our dermis. The first, layered like an onion, reacts to high frequency vibration; the latter detects the skin distortion and stretching sensed when squeezing a hand into a tight leather glove. Over the last century, scientists have established that these cells convey the infinite variety of the world at our fingertips, combining to create what scientists now call 'discriminative touch'. Today, attention has switched to the skin elsewhere on our body - to our back, chest, neck, shoulders, scalp - and there is mounting evidence of a whole other sense of touch.

As our hands explore an object, the cells of Merkel, Meissner, Pacini and Ruffini send electrical impulses through fat-insulated nerve motorways at speeds that surpass 100 miles per hour.  Under this rapid touch system, signals reach our brain in milliseconds. Meanwhile, our skin also contains fibres with little to no insulation. These country-lane fibres fire at a far more leisurely pace, travelling at a few miles per hour and arriving a second or two later. Three-quarters of our skin's touch nerves belong to this slow system. If discriminative touch relays the topographic detail beneath our fingers to our brain with instant gratification, these more prevalent slow sensors build the emotional tone of a touch. They confer a tactile sense that emphasises feelings over facts

The vampire bat offers insight into this sense. Blood contains so little fat that it only takes two nights of flying home on an empty stomach for an animal to starve. Colonies survive because the bats are not the malevolent vampires of legend but caring, sharing creatures. Those that have feasted regurgitate and donate their spoils to others. Evolutionary biologists have leapt on this behaviour as a case of selflessness at work among selfish genes: an example of reciprocal altruism that works because there is a trade in favours. A bat gives life-saving blood to another in exchange for being groomed and in the expectation that one day it will be the hungry one. This honourable code of conduct is mediated through hours and hours of social grooming. We too succumb to the touch of another because of the way it makes us feel. Recently, scientists uncovered a nerve in our skin that reacts to a low force, moving slowly at body temperature: in other words, the same pressure, speed and skin-on-skin warmth of a caress. A sensor that tunes us to tenderness. 

Vampire bats also help us to understand the ways in which we feel pain. Through the lens of a thermal-imaging camera, a vampire bat's face appears crimson, suffused with warmth, while its nose-leaf is bright blue, protruding like a pig's snout on top of dense connective tissue, nine degrees cooler than the rest of the bat's body. Packed with temperature sensors, this nose-leaf enables the bat to feel its prey's body heat from a distance and, on landing, to home in on the places where blood pulses just beneath its prey's skin. When scientists analysed these sensors, they found the ingredient that keys them to warmth is thermo-transient receptor potential proteins. These are found throughout the animal kingdom. They sensitise us to the blistering heat of boiling water and smouldering embers, so we feel pain and pull away. Then also fire on contact with the venom of a Trinidad chevron tarantula or the spiciest chilli pepper, or if we lounge too long under blazing summer skies. In reacting to sunburn, stings and searing coals, they blur the distinction between our senses of temperature and pain.  

Today, touch is the most overlooked and undervalued of the five senses identified by Aristotle. He excluded pleasure and pain, declaring them instead 'passions of the soul'. This thinking remains widespread, but examinations of the star-nosed mole and the vampire bat expose the philosopher's error. Pleasure and pain are very much part of our sensory experience and since they are wedded neurologically through the slow-touch system, arguably they encompass a sense distinct from discriminative touch: the yin and yang, the light and dark, of emotional touch. When Aristotle asserted that the aim of the wise is not to secure pleasure but to avoid pain, we know now that the wise would do well to listen to all the emotions of touch, because this sense is a guardian angel: while pain protects us from harm, pleasure encourages us towards behaviours that secure our survival. Vladimir Nabokov wrote: 'It is strange that this tactile sense, which is so infinitely less precious to men than sight, becomes at critical moments our main, if not only, handle to reality'. If discriminative touch is this handle, emotional touch is the sense that, maybe more than any other, makes us human.  
 
In Elif Shafak's latest novel, The Island of Missing Trees, she creates a wise and sentient fig tree, which wonders whether 'humans deliberately avoid learning more about us, maybe because they sense, at some primordial level, that what they find out might be unsettling'. One might imagine such sentiments being echoed by the animals in my book Sentient. Indeed, when we do take time to look, the parallels we uncover between humans and other animals are endless and unexpected. Their message is not one of fear, however, but promise, possibility, and hope.  Their senses open our minds to new worlds and enable us to grasp the complexity of our own experience.  We have spent too long imposing our attributes and assumptions on other species, anthropomorphising them. It is time to turn the tables. If we look to the animal kingdom to rewild our senses, our selves, perhaps we can better comprehend what it means to be human.
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Jackie Higgins

Jackie Higgins is the author of Sentient: What animals reveal about our senses. She read zoology at Oxford University as a student of Richard Dawkins, then worked at Oxford Scientific Films, making wildlife documentaries for National Geographic, BBC Natural world and The Discovery Channel. Moving to the BBC Science Department, she made films predominantly for BBC Horizon. 
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