As you read these words, let me cast your attention to seeing this page, feeling its weight, even hearing it rustle and ask: how many senses do you think you have?  Most of us reel off the list we learned at nursery.  Sight, touch, hearing, smell and taste.  After all, this sensorium has weighty provenance.  It was set out over two millennia ago, in 350 BCE, by Aristotle in his treatise De Anima (On the Soul), in which he claimed, ‘There is no other sense distinct from the five.’  The idea, powerful in its simplicity, took root and thrived.  Today, it is a belief expressed across cultures, in everyday conversation and scientific literature.  However, Aristotle was mistaken.

My interest in zoology has always been to better understand myself.  I look at life on our planet as one vast, sprawling family tree.  We are distant cousins not only of one another, indeed all human beings present and past, but also of every animal and, for that matter, every plant.  Zoology is a mirror that we can hold up to see ourselves more clearly, more comprehensively.  My book – Sentient: What Animals Reveal about our Senses – exposes the notion of our five senses as a Greek myth. The reality is so much more wondrous.

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 all other eyes.  Furthermore, 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 that our eyes support more than one way of seeing.  Similarly, Sentient shows that our ears, tongue, nose and skin support unexpected ways of hearing, tasting, smelling and touching.  Moreover, some of 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 feathered bar-tailed godwits – illustrate 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 grant us a sense of balance to stand tall and walk.  Inspired by snakes, scientists have found that we smell with our tongue and to further confound assumptions, 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 our largest sense organ.

Our skin delineates us, gives us shape and holds us together.  It shields us from infections, chemicals and the elements.  Also, as it is where we come into contact with the outside world, it harbours our sense of touch.  Miniscule egg-shaped Meissner’s corpuscles register feather-light pressure; Pacinian corpuscles, layered like onions, react to high frequency vibration; Ruffini corpuscles detect stretching on, say, squeezing a hand into a tight leather glove; Merkel cells gauge edge, shape and texture, so we can discern the smooth symmetry of a ball-bearing from the corrugated roughness of a walnut.  Together, these sensors convey the infinite variety of the world at our fingertips to our brain.  We have known about them for over a century, but relatively recently another was uncovered.

While researching Sentient, the neuroscientist Frances McGlone told me, ‘The discovery of this new nerve fibre in our skin has meant that we have had to work out what it is for’.  Studies have shown it is absent from the hairless skin on the soles of our feet, the palms of our hands, our fingertips and lips.  ‘Absent from the skin that uses discriminative touch to explore the world,’ he explained.  ‘This anatomy tells us something, gives us a clue to its function.’  Scientists have recorded it responding to all manner of prods, pokes and strokes. ‘We found that it reacts only to very low forces below five thousandths of a Newton; only to low-velocity movements around 3 to 5 centimetres per second; and optimally when the touching stimulus is at skin temperature.’  In other words, by firing to the same pressure, speed and skin-on-skin warmth of a caress, this sensor is uniquely primed to the caring touch of another.

Nearly all the research into our sense of touch has focused on our hands and how we grasp the lie of the land; what scientists call discriminative touch.  By contrast, this new sensor gathers feelings rather than facts and registers being touched rather than touching.  According to McGlone: ‘There is mounting evidence that touch has another dimension: a social and emotional side, whose circuits and systems we are only now uncovering.’  It runs the gamut from pleasure to pain.  Aristotle did not class these as senses, but ‘passions of the soul’.  Later, pain was deemed a sense in its own right, but perhaps more accurately it works alongside pleasure; together they are the yin and yang, the light and dark of emotional touch.  Maybe more than any other, this sense makes us human.  No wonder McGlone deems touch a biological necessity, not an indulgence.  He rails against its demonization by society; he fears the inevitable and as yet unknown repercussions of our long lonely stretches during the pandemic.  Our skin is becoming science’s last great sensory frontier.  Just beneath are sensors of yet another, even stranger variant of touch.

To tell this story, I chose the animal that bewitched the world with the film My Octopus Teacher.  This creature, more closely related to an oyster than a mammal, is evidence that evolution built minds twice over.  It is nick-named nature’s Houdini because it pours its entire mass through cracks and crevices, constrained only by the size of its parrot-like beak.  Its eight arms can bend at any point, in any direction, as well as elongate, shorten and twist.  All protean, it is the embodiment of otherness and relies on sensors under its skin, within its muscle, to control and coordinate such infinite possibility.  These stretch-sensors are found throughout the animal kingdom, embedded in those that soar, slither or simply walk.  Within us, they give rise to a sense so elusive that it was neither conceived of, nor named until the turn of the twentieth century.

The Nobel-winning neurophysiologist Charles Sherrington coined the term proprioception for our core sense of body: a sense that grants us knowledge of where our limbs are, allowing us to move without thinking.  Perhaps what Shakespeare’s Hamlet referred to when he said, ‘Sense, sure you have/ Else you could not have motion.’  It is also how we feel all our flesh as our property, but its sensations are so automatic, so familiar that we barely heed them.  Dictionaries list terms for blindness and deafness, but do not have a word for being blind or deaf to our body.  Sherrington said such an experience would be ‘singularly individual and doomed to remain indescribable.’  Yet, it was only when Oliver Sacks met a woman to whom this had happened – ‘the first of her kind’ – that the penny dropped.  He realised that without this sense ‘a body must remain unreal, unpossessed’ making it ‘more vital than any or all of the other five senses put together.’

Returning to 350 BCE, I picture Aristotle on the shores of the Aegean Sea, knee-deep in its warm waters, hands tentatively feeling submerged crags for octopuses.  He recounted one such encounter in Historia Animalium (The History of Animals) and was not impressed, declaring the octopus ‘a stupid creature’.  Aristotle dedicated his life to answering the deep philosophical questions of our existence and turned to nature for answers.  Modern science may have proven him wrong at times, but I like think he would have been delighted by the octopus’s shape-shift: that this animal is intelligent, sentient and perhaps even conscious, that it embodies a sense he never knew existed and this is one of many that would have toiled tirelessly beneath his awareness.  More than at any time in history, Aristotle would find his sensorium – like the octopus’s body – full of infinite promise and possibility.

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Consciousness, Creativity, Mind, Nature, Science