The science of smell

Us humans get a lot from our sense of smell. But other animals, such as dogs, rely on their noses so much more. Why might that be? And why did we evolve to have the noses we do? New research in Science Advances sheds some light on those questions and Darren Logan from the Waltham Centre for Pet Nutrition joined Katie Haylor in the studio to chat all about it...

Darren - Our sense of smell is essentially a chemical scent. So the volatiles that's coming off the freshly baked bread - into the atmosphere - is what we're sensing through our sense of smell. And every time you take a deep breath in, the volatiles will rush through your nose and hit these molecular receptors on the surface of your nose called olfactory receptors and they exist in olfactory sensory neurons.

In a human nose we have about 300 different types of these neurons and each one detects a different combination of small molecules and it's the combination and the pattern of those together that your brain interprets as the smell of - in this case freshly baked bread.

Katie - 300 doesn't sound like an awful lot so how do we end up with the incredible amount of smells that we're able to recognize?

Darren - So that is the sort of the real trick of your olfactory system. So it's due to something called combinatorial coding, so each receptor - each of the 300 receptors - can detect a combination of different molecules and each molecule can activate a combination of different receptors. So when you multiply those together we think that we can detect up to a trillion different orders.

Katie - Okay. So how did you analyse smell in this study? What were you interested in?

Darren - So we took advantage of a particular quirk of the olfactory system - which is that each sensory neuron in the nose expresses just one olfactory receptor. And so because we knew that, we were able to use something called RNA sequencing to quantify the RNA of each receptor and that allows us to tell us the number of each type of neuron the nose. You might expect that of the 300 neural types, they'd all be equally represented. And what we found out - that wasn't the case. There were a very small number, actually, about 10 or 15 are very very highly represented in the nose and the vast majority are relatively lowly represented.

Katie - So what does this mean then? What do we take away from this?

Darren - So we were really interested about what these receptors are - there are these neurons that are there in very high abundance. And so, what we did is, we looked at what those neurons are detecting and what we found in the case of humans is that they're detecting what we call key food orders. So these are the orders that are produced by our food. So - as you mentioned - the orders in freshly baked bread.

When we looked in other species - we looked in mice, rats, dogs and a number of primates - we found that wasn't the case. And likewise, when we looked in mice, we found that the neurons that were very abundant actually detect pheromones - so cues that the mice used to sexual communicate with each other. So what we think that means is that each mammal has evolved to to have a nose that is very specific to its niche.

Katie - Can we take from that that sourcing food is particularly important to us but there may be other equally pressing matters for other animals. How would you pick that apart?

Darren - Yeah that's our hypothesis. There was a theory that our senses of smells were essentially not under evolutionary pressure. They're just drifting around and they can detect anything that we happen to run into. What this research - we think - suggests that's not true, that actually our noses are tuned and over time have been tuned to the things that are important to us - to promote our reproduction and our survival. In the case of humans, our sense of smell is particularly important for detecting food and scavenging for food. And that's why we think our noses are tuned the way they are.

Katie - So more receptors equals better smelling ability. Is that pretty much right?

Darren - This is a bit of a mystery in the olfactory field. Species like dogs or mice or indeed elephants, who we think have the most receptors, may be able to smell more, but we actually think at the moment that it's likely that they don't smell more - they just discriminate better. So they can tell subtle differences between things that we as humans - who are not the best smellers in the world - probably couldn't.

Katie - Having said we're not the best smellers in the world, I've got to say, I’ve named myself the bloodhound of the Naked Scientist office because I feel like my sense of smell is really good. Why would that be? Why would I be better at smelling than say Izzie, for instance?

Darren - Well there are people who are better at smelling than others and it probably down to genetic variation. We know there is a lot of variation in the olfactory receptors, however, we also know that people who are often deemed or described as better smellers are often more verbal - so are able to describe the scent, the smells that they detect better and that appears that they’re therefore better but they're actually better explaining it.

Katie - I'm still going to take credit for that one. Very briefly... what's the next step then with this particular piece of work?

Darren - We are doing two things - I guess - one is that we are looking to spatially identify the position of the neurons in the nose, rather than just the abundance of them. And this is important because when you smell, the air rushes through the nose and depending on which parts of the nose it hits, we might think it works differently. And secondly, we are particularly interested in those abundant neurons and finding out exactly what they detect.