Last week a chemical spill polluted the drinking water of Charleston West Virginia. Over 7,000 gallons of a chemical used to process coal were dumped into the Elk river just upstream of the city’s water supply intake pipe. The leak was detected because the water took on the smell of licorice.
Among the traditional five senses; sight, hearing, touch, taste and smell, smell is unique. As it is an eminently chemical sense.
When you see a bowl of onion soup or for that matter a rotting possum by the side of the road, your perception is due to light reflected off the surface of these objects. Smelling either of these is chemical as it requires the movement of molecules from the object through the air and up into your nostrils. The molecules bind at specific sites, called receptors, which then causes the sending of signals through nerves to a specific area of the brain.
Different things smell differently because the molecules given off have different shapes and so bind at different receptors. Humans are though to have about 5 or 6 million receptors compared with rabbits, about 100 million, or dogs with over 200 million.
Smell is also unique in that it is difficult to quantify. How bright a light is can easily be measured in units of lumens and how loud a noise is in decibels. No such quantifiable scale exists for smells. We can talk about something smelling strongly or not but that is about all. And when it comes to qualitative measures it is difficult to get two people even to agree if something is an agreeable or a disagreeable odor.
Strangely the qualitative descriptor for a particular substance can depend on the concentration, that is, how many receptors bind with a substance. An extreme example is a molecule called skatole, which comes from feces, especially that of carnivores. Humans are very sensitive to this odor, probably because evolutionarily those individuals insensitive to the smell of the excrement of a saber-tooth tiger didn’t survive to produce offspring. Those sensitive individuals knew to clear out, and were more successful at producing progeny.
Here’s the crazy part. This same molecule known as skatole which is the essence of smelly animal poop, is used in perfumery! In very low concentrations it is perceived as an agreeable odor. Weird huh?
Smell is all about communication, from the simplest single celled organisms up to and through the plant and animal kingdoms. Sexual activity among animals is frequently modulated by chemical signals, called pheromones, from females to males.
The same is true in the arthropod world. Molecular trickery even exits. There are spiders which have evolved to produce the same pheromone as a prey moth. The spider sits in its web, emits the moth pheromone and then waits for the unsuspecting males moths looking for a good time.
Humans are no exception, but communication is on an unconscious level. Brain scans show that men and women react differently when exposed to the odor of certain hormones, even though they are not consciously aware. It’s been suggested that the use of perfume or cologne is not so much to smell nice as it is to disguise our true smells.
An odd feature of unconscious odor communication is the fact that women living in close physical proximity synchronize there menstrual cycles. There must be an evolutionary advantage but it is hard to imagine what that would be. One possibility considering that humans lived in small hunter-gathering societies 10 or 15 thousand years ago: Could it be that once a month all the men said -whoa boys, it’s that time of the month, maybe we should go off hunting for a few days.