We have five senses, right? Sight, hearing, smell, taste, touch. Plus that one that tracks where your body and limbs are in space, particularly useful for the likes of gymnasts and platform divers. And maybe one for pain. Seven in total. Ten tops.
But humans may, in fact, have as many as 52 senses. And the ones we have, rather than being the also-rans of the animal kingdom, are incredibly sensitive. Thanks to our big brains, we can detect maybe 10 million colours, and perhaps a trillion smells. With our fingers, we can detect a layer of difference one molecule thick. We have neurons in our noses and our muscles, and taste receptors right through our body. Our senses are actually extraordinary.
How might we have dozens of senses? Take touch, for instance, which might include perception, pain, heat and cold.
“There are so many receptors that makeup touch that, strictly speaking, they are different receptors,” says Ashley Ward, a professor of animal behaviour at the University of Sydney. “It’s almost like an orchestra of different senses which come together to give the perception of touch.”
Our senses actually compare pretty well to those of many animals, says Ward, the author of a new book, Sensational: A new story of our senses. Take dogs’ legendary olfactory abilities. “Obviously, we’ve relied on dogs’ noses to find all sorts of things that appear hidden to us. But actually, dogs are really good at smelling things which relate to potential prey, or animal-derived substances; we’re pretty good at smelling things which are relevant to us, such as corruption in our food, or certain vegetable stuffs.” In tests of the smelling abilities of canines and humans, dogs beat us a fair few times, but we actually beat dogs a few times also. Although elephants are probably the best smellers, he adds. “Part of the reason we thought we were bad actually stems from the philosophies of people during the Enlightenment and immediately afterwards, who were keen to assert that we were superior beings who relied on direct observation and not this animalistic sense that we call smell.”
Likewise, among mammals at least, we have pretty good eyes. Our visual acuity is good and our colour vision is not bad either. “Based on medical guidelines in many countries, cats will be right on the threshold of being declared legally blind. They don’t see with the same kind of detail as we do. They’re more entrained to movement. Also, of course, we can see colours that are trichromatic and they’re dichromatic.” This means they see, as far as we know, a narrower range of colours, mainly yellow and blue. “We don’t compare quite so well to birds, and in some cases even to fish in terms of our sense of sight.”
The range of our hearing is pretty good too, but humans are visual creatures: sight consumes more of the brain’s resources than all the rest of our senses combined.
Animal leaders
We are sensorially deficient in some ways. We can see only a tiny part of the electromagnetic spectrum, not into the infrared or ultraviolet ranges. Reindeer, by comparison, can see in ultraviolet. So they can not only see lichen better in the tundra, but urine glowing in the snow, of their herd and of predators like wolves.
Birds have four types of colour receptors and can also see ultraviolet light, meaning birds of prey in flight can spot lines of rodent urine. Pollinators can see amazing colours and patterns on flowers. Some animals can also see into the infrared spectrum, including some fish, which helps them navigate through turbid waters.
Even though they can’t see into the infrared spectrum, vampire bats can detect heat, through a special sensor on their nose. Mosquitoes track the release of CO₂ before switching to infrared to find a patch of warm exposed skin, says Ward. Some snakes can see in infrared, as can fish and frogs, whose chromophores in their eyes give them the equivalent of night-vision goggles.
Humans can’t see polarised light, says Ward, which is light that vibrates at only one angle.
“There are really three components to vision which animals use, one of which is obviously just brightness or intensity. One is what broadly we call colour, and the third is polarised light.” Mantis shrimps, which also have 12 or more colour receptors, can see this. Polarised light is arguably more important in the water, at least in the shallows. And it means they can not only detect their prey better, but communicate with each other through visual signals much more readily. Our more immediate ancestors, by comparison, were small shrew-like creatures. Even after mammals filled the gaps left by the dinosaurs, “their sense of vision, and, in particular, the relatively poor colour perception, is the legacy of more than 100 million years of evolution in the night”.
The sniff test
Can humans detect pheromones? Many animals are able to via what’s known as the vomeronasal organ (VNO). Horses often curl their upper lip and pull strange faces. Male giraffes sometimes take a sip of female giraffe urine and inhale through their mouths. It’s called the flehmen response and exposes the VNO to pheromones. “There’s no evidence as yet that’s universally accepted that humans can detect any pheromones at all.”
We have no VNO, no snout, and our brains have developed in a different way. And yet, we are exquisitely attuned to the smells of other humans, Ward says. Everyone has a unique scent, the result of our genetic makeup, our metabolism and the colonies of bacteria on our skins.
Mothers and babies can recognise each other by smell. Children can similarly identify siblings, and identical twins smell so similar that tracking dogs can be confused.
Our chemical signature is particularly affected by our major histocompatibility complex, or MHC, a suite of genes – often shared by relatives – that play a critical role in our immunity to disease.
And the role of smell in sexual attraction? A famous study done in 1995 by Swiss researcher Claus Wedekind asked female volunteers to rate the scent of T-shirts that had been worn for two days by male students who hadn’t used deodorant. The women showed a strong preference for those worn by men who had different MHCs from themselves.
Not only does this make biological sense – it helps us avoid inbreeding – but having children with a person who has a different MHC spreads your genetic risk and potentially produces offspring with a sturdy immune system. This may even be why we kiss, Ward says – it helps us sample a potential partner’s smell and taste. If the T-shirt-raters were taking oral contraceptives, however, they preferred the smell of men with similar MHCs.
This, says Ward, suggests that hormones influence mate-choice decisions and because the pill essentially tricks the body into thinking it’s pregnant, it might also cause women to be drawn towards their social ingroup, perhaps including nurturing relatives.
Multiple follow-up studies have been conducted in the nearly 30 years since – and the results have been mixed. Human attraction is a complex business. It seems that, overall, heterosexual people slightly prefer those with different MHCs – and slightly avoid those with similar ones. Regardless of whether we actually detect such things, surveys of both sexes say that smell can be a drawcard or a deal-breaker.
If certain smells instantly throw you back into past memories, you are not alone. The olfactory bulb is right next to a part of the brain which deals with memory and especially long-term memory. “It’s been suggested by some that this proximity between organising sense of smell and the sort of memory centres of our brain is what underlies that.” For many animals, scent is likely to play a major role in the organisation of their brain into a kind of three-dimensional spatial map. “It seems that this sense of smell is integral to that sense of learning ‘home’, where we come from and our place in relation to it, and how to navigate around that.”
Matter of taste
Taste, to a large degree, is smell. We typically think of smell by way of our nostrils, called orthonasal in the jargon, but there’s also retronasal. As we chew, we break up the food and its aromas flow up via the nasopharynx to our olfactory receptors. It’s what the French epicure Jean Anthelme Brillat-Savarin called “the chimney of the mouth”. The weird thing, says Ward, is that scans of brain activation show the experience of smelling food appears to happen in the nose, but if the food is in our mouths, it seems to happen there. “When we have a sip of wine, we might sense all kinds of flavours, but all that taste is contributing is perhaps a slight sourness, because wine tends to be acidic, and possibly some sweetness. Everything else is coming from the nose – it just seems like the flavour is in the mouth because of a trick played by the brain.”
It turns out that, rather than just having them in our mouths, we have taste buds throughout our bodies, including men’s testicles. This finding triggered a brief craze of young men dipping their testicles in soy sauce. Of course, the taste buds there can’t actually taste anything, says Ward, because they’re not wired to the brain in the same way. They are, however, playing a fundamental role in detecting the chemical environment within the testes, which is important to sperm production. “All through our organ systems there are taste buds doing their job in a slightly different way to the way they do in the tongue, but really just monitoring the chemical environment. Taste cells in our respiratory system are sampling the chemical environment, potentially with a view to detecting the presence of pathogens. And that, I guess, all comes into interoception, this continuous monitoring of the body by the brain and indeed of the brain by itself.”
In terms of mouth tastes, we’ve only just got used to the fifth one, umami. But we might have a sixth, kokumi, a kind of “mouthfulness”. “There was a fair degree of sales resistance to even the idea of umami joining the four previous members of the taste club.” Those would be sweet, salt, sour, and bitter. “We can experience umami – you get a particularly good tomato, for instance, or the nori that wraps the sushi we eat, or mushrooms are packed with umami.” Kokumi, like umami discovered in Japan, is much harder to nail down. It effectively acts as the frame in which the other tastes are experienced, he says. It’s the taste that rounds out hanging meats and maturing cheeses.
And the number of tastes may not stop at six. Ward suggests we are likely to have receptors for fat, water, calcium and metallic tastes. Umami effectively represents the kind of proteins we need to eat. “Fat would be an obvious contender to join the taste club. We can detect salt, sugar – it’s obviously the most important as it can indicate some kind of carbohydrate.” Though it’ll take a while before fat starts appearing in textbooks. Debate on this subject is quite extraordinarily fierce, Ward says.
Another aspect of how our taste and smell overlap – another sense, in effect – is chemesthesis. It enables us to “taste” menthol and capsaicin in chillies, not only giving us the flavours of mint and spiciness but making us feel cooler or hotter without any change in temperature. “In fact, since the chemicals in question stimulate the nervous pathways that are engaged in tactile, pain and thermal sensations, chemesthesis has most in common with our sense of touch.”
Touch me
There are two types of touch, says Ward, called discriminative and emotional, and the two have a separate nerve architecture. Discriminative is what babies do best, exploring the shapes and textures of the world with their hands and mouths. The signals we get from it are fast-routed to the brain through Type A nerve fibres for rapid response. Signals from emotional touch, our hugs and caresses, travel up to 50 times slower along C-tactile fibres.
And the two are treated differently in the brain through distinct neural networks. We’ve long underappreciated the body’s dedication to emotional touch, which may have something like three times as many nerve fibres devoted to it.
Emotional touch provides the foundation for our social interaction and relationships from the time we leave the womb, says Ward, triggering the release of endorphins, oxytocin or adrenalin. We also have two types of skin, hairy and glabrous, the first primarily for letting us know when something comes into contact with us, the latter supplied with highly sensitive receptors for touching.
Touch is a complex network of systems. There are Merkel cells, capable of detecting differences in a surface a hundredth that of the diameter of a human hair. Many people are able to detect a layer on a substance that is one molecule thick, says Ward. “We can detect it better when we run our finger along something – which is what we do instinctively. It gives us a much better sense of how something feels.”
Meissner corpuscles detect movement against our skin. Then there are Pacinian corpuscles, which respond to deep pressure and vibrations, and Ruffini endings, which tell the brain what our body is doing.
We underplay touch, says Ward. “We tend to touch each other less, for fear of rejection, being misconstrued, much of which has a very reasonable and rational basis. But obviously, the pandemic really accelerated it. And then people described the sense of touch starvation, where they were no longer even just shaking hands, or high-fiving, or hugging. And it plays an incredibly important part in our lives.”
Magnetic attraction
While humans don’t have a VNO for smelling pheromones, we do have magnetite, which in other creatures helps sense direction and detect magnetic fields. Magnetite, an iron oxide, can be found in birds’ beaks, bees’ brains and fishes’ noses – in fact, in the heads of nearly all animals that display impressive navigational abilities.
Magnetite is extremely sensitive, aligning with the Earth’s magnetic field. The magnetite in our bodies tends to be concentrated along the front of our foreheads and at the top of our noses, says Ward, which is exactly where you’d expect it to be in a forward-facing animal that moves around. What we seem to lack is the ability to respond to it appropriately.
And yet there’s the case of Robin Baker, a researcher in Manchester who, some decades ago, convinced a group of students to get on to a bus and be blindfolded and driven in circles out to the moors. When they got there, they were asked to point to where they’d come from. And they got it right to an eerily accurate extent, says Ward. When they took off their blindfolds, this directional ability disappeared – as it did with those who’d had a magnet placed on their forehead. “The evidence suggests that is probably just like a historical echo of what once we were capable of, but we have at least some of the hardware.”
Migratory birds also have a protein called cryptochrome in their eyes, which seems to let them “see” the Earth’s magnetic field in some way, making flying long-distance child’s play. Humans have cryptochrome, too, but its role in most species is setting the internal clock, through the detection of blue light.
“In some of these species, however, it has diversified and joined forces with other proteins to form what amounts to a biocompass.”
Sense and sensitivity
Among our other senses, we have equilibrioception, for balance, including acceleration sensors in our ears. The vestibulo-ocular reflex keeps our vision still like a steadycam, while motion sickness and intoxication are perceived by the body as potential poisoning, making us throw up.
Proprioception is our sense of body movement. Interoception is that sense that keeps a watch on how we feel inside and seems to be tightly woven with our psychology and mood. Regular exercise seems to make us more attuned to bodily signals, says Ward, ultimately improving emotional resistance and well-being.
Consciousness is effectively the sum of all our senses, he says.
“We really don’t know yet fully how our brain construes the messages that it gets from inside to create these experiences that we have of the outside world. I think that once we get a real handle on perception, which may not happen in my lifetime, we’ll be a long way towards understanding consciousness. Because really nothing that I’m looking at out of the window in Sydney here, buildings, trees, hearing cicadas buzzing, none of these things, in the strictest sense, exists; these are all things that my brain is interpreting. The sound of the cicada is just vibrations. The green of the trees is just a wavelength. But for me, it’s a very real experience.”
But because our senses are the interface between our inner selves and the outside world, they let us perceive beauty, appreciate a cold drink, the sound of laughter, the touch of a lover. “Senses are, in short, what makes life worth living.”
Fun facts
- It’s likely health and lifestyle affect our smell, but so might our personality. Sniffers of worn T-shirts proved remarkably good at identifying where people sat on three scales – extraversion-introversion, neurotic-emotionally stable and social dominance-submissiveness – probably through hormones and neurotransmitters in our blood, says Ward. We can also smell fear or identify happy people.
- Carnivores like dogs have only about a quarter as many taste buds as humans, house cats have fewer still. Cows have about 25,000, probably originally to avoid eating something poisonous. Similarly, omnivore humans’ sensitivity to bitterness is 1000 times more acute than to sweet or salty flavours.
- Our sensitivity of touch declines with age. We lose about 8% of our skin’s nerve fibres each decade, most dramatically in our hands, face and feet. Similarly, after the age of 20, we lose about one hertz per day of our hearing range. If the upper range of our hearing when we’re young is around 20,000Hz, by our 50s we don’t hear much beyond 10,000Hz, because of deterioration in our inner ear.
- Plants can apparently detect movement and sound. In one experiment, the roots of plants were found to grow towards plastic pipes that had water running through them, even though it made no change to the moisture or temperature of the soil. Some plants, when they detect a caterpillar munching on their leaves, can pack them with unpleasant chemicals. They can also ramp up the amount of sugar in their nectar when they hear insects like bees in the vicinity.
- Dogs may be able to detect the Earth’s magnetic fields. One study found they tend to line up along a north-south axis when defecating.
Female v Male
- The senses of women are, on average, better than men’s across the board, says Ashley Ward. There’s one area in which men do better than women – motion detection. By contrast, women are better at colour reception, some even getting four types of colour cones in their eyes, thanks to a genetic mutation. One hypothesis from evolutionary anthropology is that men as hunters needed to see movement and women as gatherers needed to see slight differences in the brightness of fruit.
- Women have a better sense of touch, which is probably explained by the fact that men tend to have larger hands, so women have more touch receptors by area. Every single square centimetre of our skin has about 15 sensory nerve fibres on average, making about 230,000 for the entire body. They are most densely packed in the fingertips, followed by the lip area. Multiple studies show women also have more sensitivity to pain, being far less insulated against it by their hormones; early exposure to testosterone ultimately makes males less so.
- Something like one in 12 men experience red–green colour blindness. It’s comparatively rare among women because some of the genes involved are located on the X chromosome. Since women have two copies of this chromosome, it makes it much less likely that they will be affected by the condition.
- Women are much better at smell, and therefore taste, probably because they carry fetuses so need to be hyper-alert to any potential threat that might come through diet. Aesthetic taste aside, men need to listen to women’s views. “If men are going to get into an argument about how something looks or feels or sounds or smells,” says Ward, “then we are probably better to defer to women in some sense.”