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Book Review #003: Sentient - What Animals Reveal About Our Senses

Higgins, Jackie, What Animals Reveal About Our Senses (London: Picador, 2021) 

The understanding that human beings have five senses was set out by Aristotle in 350 BCE, and remains widely accepted to this day. But now, neurobiologists such as Colin Blakemore argue that, instead of five, we might have to count more than 30, each with its own dedicated receptors. 

In Sentient: What Animals Reveal About Our Senses, Jackie Higgins explores twelve of those senses – the five we know, plus seven more: our senses of colour, balance, desire, time, direction, pleasure and pain, and body. Each chapter is dedicated to a specific sense, which is discussed through an exploration of a particular animal’s specialised skill, one that has arisen from its unique anatomy. 

Higgins offers stories of the goliath catfish, which is described as a ‘swimming tongue’, and has such sensitive taste receptors that it can detect one part amino acid in a billion parts water; of superfast cheetahs, whose speed would be for nothing were it not for their exceptional manoeuvrability which relies on an acute sense of balance; and bar-tailed godwits (that’s a bird, by the way), whose superior sense of direction unwittingly guided Captain Cook to New Zealand in 1769. More recently, a female godwit was tracked flying non-stop from Alaska to New Zealand, a journey that took eight days without food, rest or sleep, and was made possible by the bird’s internal magnetic compass.

It's all absolutely astonishing, and made meaningful by Higgins’s skill in applying what we learn from these creatures to human beings, our limitations, and our potential. 

One of my favourite chapters was on the common octopus and our sense of proprioception, which gives us the ability to automatically balance ourselves when we walk, to navigate unfamiliar terrain, and to react instinctively when we trip and fall. As Higgins says, “If our body is a puppet then proprioception is our internal puppeteer, orchestrating movement on our behalf.” 

To illustrate what happens when that internal puppeteer is damaged, Higgins tells the story of a British man who lost all proprioceptive abilities after an illness and “had no sense of where any part of his body was”. He was eventually diagnosed with acute sensory neuropathy syndrome and taught himself how to control a limb by visually focusing on it, just like the common octopus whose arms act independently, except when focused acts of vision switch on their proprioceptive sense and their bodies gain coherence

By studying the extraordinary proprioceptive sense of the common octopus, scientists hope to be able to build the world’s first soft-bodied robot, which could be deployed for surveillance, search and rescue, or in operating theatres, and which would be able to reach places inaccessible to “their stiff, steely edged counterparts, and do so without weakening the surrounding structures.” 

Some chapters can get a bit science-y, but Higgins has a knack for concluding each one in a way that is thought-provoking and affirming. For instance, her chapter on our sense of touch explains how that sense is really two – one with an emphasis on touching, the other with an emphasis on being touched – and that the latter relies on one particularly slow neuron which targets the part of our brain that processes emotion. “In all likelihood,” says Higgins, this touch nerve “tunes us to tenderness, it transforms touch into interpersonal glue and the skin into a social organ.” 

Higgins goes on to examine cases of people who experience extreme pain – and those who experience no pain at all – before concluding by emphasising the importance of physical touch, with a quote from Vladimir Nabokov: “‘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.’ As our world becomes more touch-averse than at any time in history, as the act of a touch becomes politicized…as we lean towards conducting our relationships online and older people are said to be silently enduring an epidemic of loneliness… scientific evidence warns us to ignore this sense at our peril. It is not simply our handle on reality, but the sense that, more than any other, makes us who we are.” 

Finally, in her discussion on our sense of time – our circadian rhythms – Higgins introduces us to Michel Siffre, a French geologist who, in 1962, entered the Scarasson Caves, deep beneath the French Alps. He stayed underground in complete isolation for two months, his only contact with others being to signal to his team when he woke up, ate and drank, and before he went to sleep. Siffre lost all sense of time – he wrote in his dairy that he was living ‘outside time’ – but when all the data were collated, researchers discovered that he had unwittingly adhered to a regular wake and sleep cycle, and so proved that humans have an internal body clock. 

This leads Higgins to her discussion or the trashline orbweaver spider, a creature with the shortest known body clock, with a duration of 18 and a half hours. Because the spiders’ body clocks are so short, there has to be a phase shift of more than five hours so they stay in sync with the solar day. “The scientists liken it to snapping out of five-hour jet lag…like travelling west from London to New York and readjusting instantly to being five hours behind Greenwich Mean Time.” Because of their unique body clock, scientists are studying the trashline orbweavers as they look for a way to help us beat jet lag or to help avoid the dangers of ‘time desynchrony’ often seen in shift workers. 

For me, this was one of the greatest strengths of the book: looking ahead to see how what we can learn from other creatures might help us develop in the future. That might be implants to cure blindness by enabling people to ‘see’ with their tongues or new types of sound-reducing plastic, inspired by the feathers of the great grey owl, which could be used to make stealthier submarines or quieter wind turbines. The US Navy is already working hard on that one. 

Although not always a pacy read, Sentient is beautifully written absolutely fascinating. If nothing else, it should remind us all to open our “eyes, ears, skin, tongues, noses and more to the everyday miracle of being sentient.”

This review was written by the awesome Sarah W.

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