What is a smell? A smell is the sensory response to chemicals known as odourants, which are carried by the air around us. Both single chemicals and complex mixtures of many chemicals produce the sensation of a single smell or odour.

For instance, airborne molecules of acetic acid are responsible for the characteristic smell of vinegar. However, different types of vinegar, such as malt vinegar and wine vinegar smell different because they give off different mixtures of other chemicals. Some odours such as fresh coffee are extremely complex mixtures of hundreds of different odourants. We are able to sense these chemicals because they bind to protein receptors on cells in our nose. These receptors are a bit like locks that can only be opened by certain chemical keys. There are a variety of different receptors in the nose, which respond to different types of chemicals and produce the sensation of different smells.

Each receptor protein is produced from a gene. Our evolutionary ancestors only possessed a few genes for different receptors, and consequently had a limited sense of smell. Over many generations these genes have been duplicated and small random changes have occurred. This has produced families of slightly different receptors, which increase the range of chemicals to which the nose is sensitive. The largest number is found in rodents, such as mice, which possess genes for around 1000 different receptor types. However, changes to the receptor genes can also stop them working. For humans only about 350 genes are thought to produce functional receptors out of the 900 possible receptor genes that have been identified in the human genome.

What does this mean for the abilities of mice and humans to sense smells? The repertoire of human odourant receptors covers as wide a range of chemical types as is covered by the receptors of mice, but not in as much detail. In other words we get a much coarser grained view of the odour world than mice. We’re a bit like a colour-blind person having difficulty picking out an orange rucksack against the green background of a field. Humans find that certain chemicals smell similar to each other and may merge into the background, whereas they would stand out a mile to a mouse. Many animals have a greater number of odourant receptors than humans, and therefore a better sense of smell. This is probably why dogs are so much better at tracking scents than we are, although come to think about it, another reason may be that their nose is that much closer to the ground!

Peter Brennan, PhD, Cambridge University, UK