Actually, we should probably start by noting that all humans have more than five senses. The ancient Greek philosopher and scientist, Aristotle (384-322 BC), first described the five senses of sight, hearing, touch, taste, and smell — and, even though we’ve learned a lot since then, this is still the way things are taught at most high schools. As I noted in an earlier column, How Many Senses Do Humans Have?:

In reality, we have at least nine senses, and possibly as many as twenty or more. In addition to sight, hearing, touch, taste, and smell, we also have the following four for sure:

• Thermoception: Also known as thermoreception, this is the sense by which we perceive temperature. Even if you are blindfolded, for example, if you hold your hand close to something hot, you can feel the heat in the form of infrared (IR) radiation. Similarly, if you hold your hand over a block of ice, you can detect the lack of heat.
• Nociception: Also known as nocioception and nociperception, this is the sense that allows us to perceive pain from our skin, joints, and internal organs.
• Proprioception: This is the sense of the relative position of one’s own parts of the body, and also the amount of effort being employed in movement.
• Equilibrioception: This is the part of our sense of balance that is provided by the three tubes in our inner ear (balance is the result of a number of body systems working together, including sight, proprioception, and equilibrioception).

Is this all? I fear we haven’t even scratched the surface. It can be argued that hunger and thirst are senses. We also have some senses that don’t operate at the conscious level. For example, the stomach has taste receptors of its own. Although these aren’t hooked up to our conscious brain, they do form a part of some kind of sensory response system.

But none of the above is what I wanted to talk about here. While I’m travelling to and from work listening to the national public radio (NPR), my mind tends to wander, bouncing around from topic to topic. This morning, I started to think about our five senses.

Let’s start with the fact that I cannot imagine any form of viable multi-celled lifeforms that could exist without a sense of touch. Similarly, even a single-celled amoeba has a sense of smell (sort of) in that it can detect chemical gradients in the surrounding medium and move towards or away from the area of highest concentration. It’s easy to see how this might develop into our senses of smell and taste. Having said that, suppose we had only a sense of taste but no sense of smell, how would we ever know that we were lacking this ability?

In the case of the senses of sight and hearing, it’s easy to see how they both greatly increase an organism’s ability to survive, but what if our species were lacking one of them. How would we ever know? I cannot imagine how to explain hearing to a deaf person or sight to a blind person — so if we were all lacking one of these senses — how could we conceive of their existence.

At this point, I started to ponder those animals that have abilities beyond our own. Dogs have around 300 million olfactory receptors in their noses (we have about 6 million) and the part of their brains devoted to analyzing smells is about 40X greater than ours (proportional to the relative sizes of our brains). I’ve seen visualizations depicting dogs as “seeing the world” through smell, but I cannon imagine what it’s like.

What about the audio echolocation capabilities of bats and the sonar echolocation capabilities of dolphins? Although very clever, in some ways this is just “more of the same” in that blind people can use echolocation to detect objects based on the sounds reflected from the tapping of their canes. Furthermore, some blind people can detect the location of other people or objects in the room — even if those entities aren’t moving or making a sound — by their acoustical shadow (the way in which they absorb ambient sound).

In the case of sight, there are certainly creatures that can do some things better than us and/or have additional capabilities. Take the American kestrel, which has far more rods and cones in its foveas (65,000 per square millimeter) than do humans (38,000 per square millimeter), thereby resulting in spectacular long-distance vision. There’s also the fact that many birds can see a wider spectrum of colors that humans, expanding into the ultraviolet (UV) in some cases.

Or take the case of the mantis shrimp. As I wrote in my paper on The Evolution of Color Vision:

[These little rascals] are said to have the most complex eye known in the animal kingdom. The intricate details of their visual systems (three different regions in each eye, independent motion of each eye, trinocular vision, and so forth) are too many and varied to go into here. Suffice it to say that scientists have discovered some species of mantis shrimp with 16 different types of photoreceptors: 8 for light in (what we regard as being) the visible portion of the spectrum, 4 for ultraviolet light, and 4 for analyzing polarized light. In fact, it is said that in the ultraviolet alone, these little rapscallions have the same capability that humans have in normal light.

Some birds can “see” the Earth’s magnetic fields, which is an ability that comes in handy when navigating the course of a multi-thousand-mile journey (some migrating birds travel as far as 16,000 miles).

What about senses that are completely different to the standard five? For example, electroreception (or electroception) is the biological ability to perceive natural electrical stimuli. Electroreception is used in electrolocation (detecting objects) and for electrocommunication. In addition to detecting their pray over long distances using their phenomenal sense of smell, for example, sharks also have the ability to detect and home in on the small electrical fields generated by other animals.

As we read in the Sensory Biology Around the Animal Kingdom article on The-Scentist.com website, some creatures have the ability to detect gravity. I remember reading (a long time ago now) about an experiment in which some type of tiny sea creatures where placed in a large tank of water. When a large mass was maneuvered into position outside one side of the tank, the sightless creatures all moved to the other side.

Now, it’s true that things like electroreception work better in water, which (taking its mineral content into account) is a better conductor than air, but are there any other senses out there that we cannot conceive simply because we don’t already have them?

How about alien life on exoplanets? Did you ever read The Tripods by John Christopher? This is young adult science fiction at its best (I may re-read them all again this coming weekend). I won’t go into this too much here, but the general idea is that we have a post-apocalyptic scenario in which humanity has been enslaved by the “Tripods” — gigantic three-legged walking machines piloted by unseen alien entities. When our heroes eventually trick their way into one of the alien’s domed cities, they discover that the “Masters” are alien creatures. According to Will, the chief protagonist:

They stood much taller than a man, nearly twice as tall, and broad in proportion. Their bodies were wider at the bottom than the top, four or five feet around I thought, but tapered up to something like a foot in circumference at the head. If it was the head, for there was no break in the continuity, no sign of a neck. The next thing I noticed was that their bodies were supported not on two legs, but three, these being thick but short. They had matching them three arms, or rather tentacles, issuing from a point halfway up their bodies. And their eyes — I saw that there were three of those, too, set in a flattened triangle, one above and between the other two, a foot or so below the crown […]”

As you probably know by now, I’m a devotee of the British science fiction series Doctor Who. In the early days — with the notable exception of the Daleks, of course — the vast majority of the aliens the good doctor ran across looked strangely like standard humans (Thals, Moroks, Xeronsor, Drahvins, etc.) or like humans dressed in ill-fitting costumes (Aridians, Monoids, Vogans, Zygons, etc.).

Even many of the more recent characters like the Ood, the Judoon, the Sontarans, and Abzorbaloff exhibit bilateral symmetry and have a regular complement or arms, legs, eyes, and ears.

So, when (if) we do come into contact with reasonably large alien lifeforms — intelligent or otherwise — would we expect to see something that looks at least a bit like us (two arms, two legs, two eyes, two ears); something that is sort of similar but with notable differences, like two mouths (one for breathing and one for eating) or more arms, like the Moties in The Mote in God’s Eye by Larry Niven and Jerry Pournelle. (I must admit there have been occasions when four arms would have come in handy — no pun intended.)

More importantly (at least, in the context of this column), would we expect them to have a similar set of senses to ourselves. Alternatively, might they boast advanced capabilities like the ability to communicate using ultrasonic frequencies or changing the colors on a patch of their skin — something like this video of squids.

Or, might they have additional senses that we haven’t even dreamed of? One of the books on my Christmas “Wish List” is Imagined Life by James Trefil and Michael Summers. This book, which just came out a couple of months ago, is billed as, “A Speculative Scientific Journey among the Exoplanets in Search of Intelligent Aliens, Ice Creatures, and Supergravity Animals.”

I cannot wait to peruse and ponder this little beauty. In the meantime, as always, I welcome your thoughts on any of the topics we’ve discussed here.