How many senses do humans have?
If you had the same kind of education as me, you were probably taught at a young age that there are five human senses: vision, hearing, touch, taste and smell.
This is a lie.
I mean that in the sense of an over-simplification, the kind that’s necessary when you’re explaining a difficult concept to a small child. But it’s an over-simplification that doesn’t usually get corrected unless you happen to study psychology at university, and even then it’ll depend on who your lecturers or professors are.
The true number of human senses is definitely more than five, though I wouldn’t like to give you an exact number, because our bodies have more plot twists than Memento. For example, in both 2018 and 2021, researchers published studies on organs that we hadn’t previously known about. One is the interstitium, a set of fluid-filled compartments spread throughout the body that might act as a shock absorber. The other is the tubarial gland, a saliva gland above the top of the throat.
So – we’re going to do a whirlwind tour through a few of the human senses that don’t get as much attention as the big five, some senses that other animals have but we don’t, and an attempt to use technology to create a new sense. It’s gonna be awesome.
What other senses do humans have?
Just like the big five senses, some of us will have sharper abilities in some of these senses than others. It doesn’t say anything about your worth as a human being if you are better or worse at any of these, it’s just natural human variability. However, if the level of ability or sensitivity you have with any of these senses is causing you a problem, I’d recommend seeing your GP.
Pain
You probably won’t be surprised to learn that you have nerve cells that specialise in sensing pain all over your body. These come in two kinds: polymodal nociceptors, which sense all kinds of pain from high temperatures to inflammation to capsaicin (the hot chemical in chillis), and high threshold mechanoreceptors, which specialise in the pain that results from high pressure, like pinching and cutting.
You might think it would be nice never to experience pain, but it has the incredibly useful function of warning you if something you are doing or experiencing is dangerous, or, if you are already in pain, to prevent you from actions that would cause further damage. Very rarely, someone will be born without the ability to sense pain, often because they have a mutation in a particular gene – and they are likely to suffer many kinds of small and large injuries, simply because they do not experience the warning signs of pain.
Proprioception
Now for something cheerier. I want you to hold both hands above your head and – without looking – touch your thumbs together. (If you don’t have two thumbs, think of two other smallish body parts that it’s physically possible to bring into contact with each other and do that without looking.)
Congratulations, you have just made use of proprioception! This is your sense of where your body parts are in relation to each other, which you know about because of nerve cells in and around your muscles, joints, and tendons that sense what those parts of your body are up to. Most of the time you won’t be aware of these sensations, you’ll just casually go about your life moving your arm the exact right amount to catch a ball, or scratching a spot on the back of one leg with the other foot with total accuracy.
Proprioception also forms part of another sense…
Balance
Balance is – basically – the sense of whether you are going to stay upright or fall over. Given that your body can move in all kinds of ways, from plunking down on the sofa to responding to an unexpected jostle in a crowd, it relies on lots of different inputs. This means it’s really a meta-sense, made up of contributions from vision, proprioception, and the vestibular system. What’s the vestibular system? Glad you asked. It’s a set of organs deep inside your ear: the otolith organs, which sense linear movements like the forward motion of a train that you’re sitting in, and the semicircular canals, which sense rotational movements like nodding.
Interoception
How thirsty are you right now? How full is your bladder? Are you at a comfortable temperature? How fast is your heart beating?
How do you know these things? The answer is you’re using your interoception, the sense of what is going on inside your own body. Like balance, it’s a complex meta-sense made up of many different bits of information coming from all over the body. Originally, researchers thought its purpose was to help you with homeostasis, the process of keeping your bodily processes in balance so that you don’t have too much or too little iron, copper, calcium, or sodium, don’t get too hot or too cold, expend roughly the same amount of energy as you take in, and a whole host of other variables. However, recently it’s become clear that interoception can also help us with things like motivation, emotion and even self-awareness – ‘trust your gut’ isn’t just a saying.
What senses do other animals have that we don’t?
I was regularly overtaken by a sense of wonder at the universe while writing this bit, so please adjust your expectations about my levels of excitement to Very High.
Electroreception
The often-asked question, “What is the coolest animal and why is it a platypus?” has many answers, including:
LOOK AT IT
It’s got a bill
It lays eggs (also the answer to the question “What’s a hen do?”)
It can cause you extreme pain with its venom
It’s one of the few mammals that can sense electrical fields
Today I would like to talk to you about number 6 on this list. Delightfully, the ability of platypi to detect electrical fields seems to have been noticed in the 1980s by scientists who found that they would “seek out and attack batteries that were immersed and otherwise invisible”. It turned out that platypodes have nerves in their snouts that can sense electrical currents. Since batteries are not the natural prey of platycats, scientists have since concluded that their sense of electroreception evolved to detect the electrical fields created by the muscle movements of their actual prey, i.e., small water animals.
You know what else has an electrical field? A flower! Bees can sense these electrical fields and use them to figure out which flower might have the most pollen, though they have to use a very different system from platypuses because electricity does not travel as well through air as it does through water. Honey bees detect electrical fields with their antennae, while bumble bees, being much fuzzier, make use of their hairs.
Bees also build up a positive electrical charge as they go about their business. Flowers, including pollen, typically have a negative electrical charge. This might mean – I am very excited about this – that part of how a bee picks up pollen is by static electricity, much like when you rub a balloon on your hair and then use it to pick up little bits of paper.*
Infrared sensing
Not to terrify you or anything, but did you know that rattlesnakes don’t need to be able to see you in order to attack you? That’s because they have pit organs, tiny little dents on either side of their head which contain heat-sensitive membranes that – I cannot believe I am writing this – are like inbuilt thermal vision goggles. Only they’re not exactly thermal vision goggles, because pit organs are not eyes, so snakes aren’t seeing heat, they’re sensing it in some other way that may be impossible for humans to imagine, in the same way as it’s impossible for most of us to imagine having the rare condition of tetrachromacy, in which an extra type of light-sensitive cell in your eye means you can see colours that most people can’t.
This is a problem which comes up again and again in research on consciousness, to the extent that there’s a famous philosophical paper called ‘What is it like to be a bat?’ – in which the author concludes that a human can’t truly answer this question because they’d be attempting to do it with a mind that cannot accurately imagine the experience of flying or echolocation.
Though… it turns out that at least some humans can echolocate, and I will write a blog about that at some point because it’s super cool. But as far as I know, no humans have the final sense I’ll tell you about today…
Magnetoception
Magnetoception is the ability to detect magnetic fields, and it’s seen in many animals including sea turtles and homing pigeons. It’s brilliant for wayfinding, because an animal with this sense is always able to tell which direction magnetic north is in, unless someone comes along with a large electromagnet and confuses the issue. Which, incidentally, is how scientists showed that sea turtles have magnetoception.
No-one’s exactly sure how magnetoception is achieved, or even if different animals use the same method, but there are three main theories:
Electromagnetic induction, i.e., when something that can conduct electricity moves through a magnetic field, the voltage in the conductor changes. In this case, the magnetic field is the one created by the Earth, and the thing that can conduct electricity is the animal. The animal needs to be able to sense changes in electrical fields as well, and we know some animals can do that. The real problem with this theory is what’s around the animal. Air and fresh water are not very good at conducting electricity, so homing pigeons definitely can’t use this method. Sea water is good at conducting electricity, but tends to move a lot and has electrical currents of its own, both of which would disrupt the animal’s ability to sense the Earth’s magnetic field.
Chemical magnetoception, which relies on molecules in an animal’s body passing electrons back and forth at different rates. Possibly these molecules are in the light-sensitive cells in animals’ eyes, but as yet the only ones that have been found to do the electron switcheroo only do so in response to magnetic fields much stronger than that created by the Earth.
Magnetite, a magnetic mineral found in many animals that are known to have magnetoception. This seems more plausible than the other two theories based on the evidence, though it’s still pretty weak.
Humans may be able to detect magnetic fields through chemical magnetoception – we have a protein in our eyes which is sensitive to them, but it’s not clear if our bodies actually make use of that information and, as I pointed out before, no-one’s found a chemical that can pick up the Earth’s magnetic field yet.
So… what if we tried a technological means of creating human magnetoception?
I don’t mean a compass, though compasses are brilliant – they’re designed to be checked every now and then, not to provide a constant flow of information about direction in the way that a built-in magnetoception sense can. Let’s talk about an attempt to do that.
A feelSpace belt is a loop of vibrating panels that can be worn around the waist. It relies on your sense of touch, but it doesn’t contain the same information that touch does (for example, how hard you are pushing something). Instead, the one that is closest to magnetic north always vibrates – you’ll feel the one in the middle of your stomach vibrating if you are facing north, but if you turn to face south, the vibration will move around your waist to the small of your back. Some people wearing these belts for a period of weeks start to experience it as a new sense that made them aware of their relative location in a large space that extended well beyond what they could see.
Is this what it’s like for a bird or a turtle to experience magnetoception? Probably not, because we’re still trying to do it with a mind that’s not the right shape in other respects. But this does open up some fascinating possibilities about extending the senses we currently have. If we can design a technology to detect something like magnetic fields, electricity, or radioactivity, and we can work out a way to map that on to our existing senses, then there’s probably no limit to the number of things that we can sense.
*Note for pedants: this is technically the reverse of the bee situation, since rubbing a balloon on your hair will actually cause it to build up a negative charge and it will then pick up positively charged items.