Animal Senses #2: Smell

In Immense World, Ed Yong points out that smell seems to be the one universal sense among almost all species – every living thing can detect some chemicals. That makes sense (pun intended) when you think a bit. After all:

“(Chemicals have been there forever and thus) are the most ancient and universal source of sensory information.”

 

A famous subcategory under the heading of smells is “pheromones”:

“It refers to chemical signals that carry messages between members of the same species.”

Think of them as smells that convey “standardized messages” – their use and meaning does not vary between individuals of the same species.

 

The famous ant pheromones are of three types: (1) Lightweight chemicals that rise easily into the air are used to “rapidly summon mobs” or to sound the alarm. (2) Medium weight chemicals that don’t disperse so quickly are used to mark trails; and (3) Heavyweight chemicals (so heavy they don’t even leave the body) are used as markers to help identify who is/isn’t a member of the group.

 

Smell is the super-strong sense for ants and it holds so much sway that it can lead to ridiculous and even self-harming behavior even when the other senses warn of a problem. For example, army ants will follow the pheromone trail even if it, by mistake, loops back into a circular path. In such a scenario, ants will enter the “death spiral” where they keep marching until they die of exhaustion. Or take the experiment where they dabbed a chemical on an ant that indicates death – its sister ants treated it like a corpse and carried it to the colony’s garbage pile, even though the ant was alive and visibly kicking. Because, hey, it smelled dead.

 

The most massive smelling organ we know of is the trunk of the elephant. Elephants can detect their favorite plants, even when hidden in lidded boxes. Put unequal amounts of food in two identical tubs that are covered, and the elephant can identify which tub has more food, by smell alone (Even dogs can’t do that). Thankfully, elephants can smell TNT – which is why they rarely get blown up by landmines strewn across so many parts of war-torn Africa.

 

Snakes smell with their tongue (That is why snakes keep flicking their tongues – each flick is a sniff). It also explains why their tongues are forked:

“The fork allows snakes to smell in stereo.”

What does that mean? A forked tongue allows the snake to compare the smell at two different points in space – if the smell on the right is slightly stronger, then the prey (or mate) must lie to the right.

 

We think of smell as being based on molecules carried in the air (gaseous state). Not fully correct. The smell receptors are always covered by a thin layer of liquid, so odorant molecules must be dissolved to be detected (liquid state). Taste and smell seem distinct, except they are not! Both require contact in liquid state. And contrary to our intuitions, taste is the simpler sense. After all, taste serves just a few purposes (Eat or not? Good or bad?) whereas smell serves so many different purposes. In fact, the boundary between smell and taste is not anywhere near as clear as it seems intuitively. After all:

“Food seems bland when you have a cold: its taste is the same, but the flavour dims because you can’t smell it.”

 

Studying smell is very hard. Light and sound can be defined by “clear and measurable” properties like frequency or wavelength. Whereas smell is like color – how (or whether) it is perceived depends on how the sense organ translates it into information and how the brain interprets it. Smells can’t be “captured”, unlike sound or a picture. And they are not at all easy to reproduce. All of this makes the study of smelling sense of animals particularly hard. And that is if anyone thinks of testing the olfactory sense at all – by default, as humans, we tend to design experiments based on senses that we use the most (light, sound and touch). Which means we may not fully understand what other info animals could be extracting from smell – identity? Emotional state? Illness?