Smell is important to allow us to taste our food, and can even trigger memories. But in some animals, special molecules called pheromones can also trigger certain types of behaviour, such as mating. Kate Lamble spoke to Greg Jefferis from the Cambridge Medical Research Council Laboratory of Molecular Biology, or the MRC LMB, investigates the pathways between smell and behaviour in the fruit fly
Greg - A pheromone is, I guess, a specialised smell molecule that's used to communicate between individuals of a species. So, it's produced by one member of a species and used to signal for example to a member of the opposite sex, is one of the classic cases, so sex pheromones. So, there's normally a specialised detection process as well and we're particularly interested in what happens within the brain when those odours are detected.
Kate - If we looked at the pheromones, would it look any different from a normal smell molecule?
Greg - No, not particularly and in fact, there are some pheromones which can be detected by our main olfactory epithelium, so the nose, that can have very specific meanings for certain species.
Kate - So, if they don't look any different, I suppose it's about how it is received. How does smelling a pheromone get translated into behaviour in animals?
Greg - So, like any odour, the first thing is that this small molecule needs to diffuse up to a receptor at the top of your nose and actually bind to a receptor there. And one of the key advantages of using insects is that these receptors and molecules have been identified for some time. So, once it binds to the receptor it then makes the neuron on which that receptor sits electrically active and so, you've turned this chemical binding into an electrical signal which can then talk to the rest of the brain and trigger behaviour responses.
Kate - And you study this in fruit flies. Why are fruit flies such a great animal to look at this in?
Greg - So, various reasons. There are lots of, people who have been studying fruit flies for years. Obviously, there are all sorts of powerful tools. There's also an issue of complexity. Fruit flies have only 50 receptor genes whereas a mouse has 1300 olfactory receptor genes. So, it's a bit easier to find the receptor for a particular smell in a fruit fly than a mouse. Also, once you found the receptor, it's a lot easier to do experiment in a fly to try and figure out what the fly's brain is doing with this information and that's really the kind of work that my lab is doing.
Kate - So, what pheromones do fruit flies react to?
Greg - So, the one that's been best studied is a pheromone called cva cis-Vaccenyl Acetate. So, this is a molecule that's produced by male flies and signals to both males and females. So, it seems to be attractive for females. It makes them more ready to mate with the male, but it's repulsive for males. It actually makes them more aggressive, more likely to fight with each other. So, this is interesting, right? It's the same molecule but different effects on the two sexes.
Kate - That is interesting. We've got an email in from Teo Gibson who asks us, "Can humans smell pheromones or is it just a myth?"
Greg - I think it partly depends on your definition of pheromone. So, I think one of the big points at least classically has been that the pheromone should be a molecule that has some kind of unconditioned response. That is that the first time you smell it, it's going to make you produce some kind of behaviour and you're always going to produce that behaviour when you smell the pheromone. Now of course, we expect most things to be highly contextual especially in humans, there are lots of signals that interact and also, that there's lots of learning. Most of the sort of work on human pheromones has found it very hard to tease apart sort of learned associations from something that might be innate as it were.
Kate - I can imagine with a human, so much of will comes into it as well, that a fruit fly would automatically respond to something whereas we're able to stop ourselves to a certain extent. Teo goes on to say, she read an article a while back about a woman judging a man's breed worthiness based on the smell of a sweaty shirt. Is that true?
Greg - So, maybe we should bring in Darren (Logan from the Wellcome Trust Sanger Institute) here. I think he's got lots of experience at least in dealing with that kind of question, if not with smelling smelly t-shirts, but...
Darren - So, it is true that the study was done that showed that people prefer the smell of people who are unrelated to them at certain of their menstrual cycle, a woman does. This was classically done by making people wear no toiletries and wearing a white t-shirt and sleeping in that t-shirt and then sniffing it. It's not the most pleasant of experiments. So, there's quite a long history of these types of experiments. Indeed, there appears to be some sort of effect. Quite how it works and what people are smelling, we don't yet know.
Ginny Smith - Now Darren I think you told me earlier that you actually have an example of a human pheromone with you. Is that right?
Darren - So this is a chemical called androstenone and it's first identified in the saliva of male pigs and it drives female pigs wild. Subsequently, it was found in male sweat and so, it was studied because of its pheromonal properties in a pig as a putative human pheromone. What's really interesting about it is it smells very differently to different people. So, if you'd like to have a sniff and tell me if you can smell it...
Kate - This is one of those quizzes that tells you about your personality. You don't want to get it wrong
Darren - So, a lot of people can't smell it at all.
Kate - I can smell something. It's not driving me wild. It's quite sweet, but quite subtle to me.
Darren - So, that's interesting. What about you Ginny?
Kate - He's definitely worked something out about my personality that I don't want to be revealed on air.
Ginny - I'm not smelling anything.
Kate - You can't smell anything.
Ginny - Nothing at all.
Darren - So, most people describe this as urinous or sickening or sweaty in some way. But a small proportion of people describe it as sweet and a lot of people also can smell it. I can't smell it, and so we have one person that can smell it that quite likes it. I think the other interesting thing about this chemical is irrespective of its potential pheromonal properties is that it is perhaps the most variable odour that has the most variable responses to smell to it. So, whether it's a pheromone, the jury is out I would say, but it certainly is an interesting smell.