Many believe that aromatic aphrodisiacs exist, sparking fierce competition in the fragrance industry to discover the 'Holy Grail' of perfume. Science suggests chemicals related to mating may be more relevant to insects, like the giant silkworm above, rather than mammals. The research continues...

1.7 Pheromones & Other Smell Oddities

From the minute we're born, our sense of smell allows us to form an attachment to our parents. We smell our mother and her milk, we know when we are being held by her even when our eyes are closed. We remember the smell of our father and associate certain feelings with him too, which are triggered whenever we smell that bodily odour.

Each of us, with the exclusion of identical twins, have their own unique scent, which changes subtly, dependent on our moods and bodily state.'The smell of fear' is not just a saying, it is a fact. For instance, a study by Denise Chen of Rice University showed that the scent of sweat taken from the armpits of men who had been watching a frightening movie caused bias in a group of women who were asked to rate the expressions of various facial expressions whilst simultaneously sniffing samples of the men's sweat. The scent had no effect on the ratings of those faces which were obviously happy or sad etc, but it did have an effect on the rating of the more neutral faces - the women were more likely to rate these as being in a 'fearful' state. How does our brain pick up on these signals though? What is the nature of the chemicals that are behind these signals?

In addition to our main olfactory system, a second, more primitive system, termed the 'accessory' olfactory system has generated much interest in the past two decades. In mammals, this system has been shown to respond to various chemical signals emitted by others of their species. Specifically, signals of the pheromone kind. A key function of pheromones is to indicate sexual availability/maturity, and has even been shown to be a trigger for puberty and menstruation in mammals.
Pheromone detection and usage has been widely observed in animals and, in particular, insects. For instance, female flies are able to attract between 500 to 1000 male flies from 150 feet away in just 5 minutes with just 0.004 micrograms of a particular pheromone. While, silkworm moths are able to detect pheromones from other moths up to 11km away.
Behaviours of a similar, but somewhat less dramatic kind have also been observed in mammals such as hamsters and mice. A virgin male hamster whose accessory olfactory system has been removed will not mate with a receptive female. If, however, the hamster had mated prior to the removal of the system, only a slight decrease in sexual activity was observed. This perhaps indicates a learned identification of visual or scent-based cues from the female and/or the usage of the main olfactory system.

In mice, pheromones released by a female will cause a male mouse to instantly desire to mate. Whereas, different pheromones released by another male mouse cause aggression. It has also been observed that some chemicals released by other mice will cause the onset of puberty in young male mice or cause a female mouse to go into oestrus.

But, what exactly is the 'accessory' olfactory system and is it active in humans too? It has been found that this system is made up of more primitive structures than the main olfactory system. (2) The front-end of this system are a series of nerve cells, which are packed in a pair of tiny, cigar-shaped sacks called vomeronasal organs (VNOs), which are located just behind the nostrils. In rodents, signals received by the VNO travel directly to a brain region named the 'accessory olfactory bulb', rather than to the main olfactory bulb discussed previously. The signals are then routed to parts of the brain which control reproduction and maternal behaviours. No wonder pheromones make creatures go a little crazy - and it appears that we may have some form of VNO's too.

An otolayngologist at the University of Colorado at Denver named Bruce Jafek and a second researcher, David Moran, found evidence of VNOs in humans. However, it is still not known how or even if signals are routed to the brain. Certainly, if we are affected by pheromones as other mammals are, the results are far more subtle than what has been observed in non-human species.  

One study undertaken in 1971 by researcher Martha McClintock(3) suggests that there may be some connection with chemical signals from the body and women's fertility. She showed that a women's menstrual cycle is sensitive to the cycles of other women in close proximity, that is, chemical signals exchanged between women can cause their cycles to synchronise with each others.

Another study, also involving Martha McClintock(4), showed that humans, females in particular, are quite sensitive to the smell of a potential mate. In this study, a group of females were asked to rank the smell of various non-human scents along with the scent of t-shirts which had been worn by male subjects in terms of four key attributes: familiarity, intensity, pleasantness and spiciness.

The females did not know which scent originated from a human or non-human source, nor did they know anything about the males in the study. As it turns out, the males were selected based on their genetic make-up, or specifically, their HLA (human leukocyte antigen) gene sequence. This genetic sequence plays a key role in determining our immune system response, as it allows our body to discriminate between those cells which are part of our body, and those which are potentially harmful (such as viruses and parasites). The HLA sequence is different in all humans, aside from identical twins.  
Once the test had been completed, the researchers on the team analysed the HLA of the top-ranking male scents chosen by each female. The results were rather interesting. It was shown that the females had preferred the scents of those males whose HLA were neither too similar nor too dissimilar to their own, but somewhere in between.

In a similar vein to this, a 2007 study undertaken by Johan Lundstrom of the Monell Chemical Senses Centre in Philadelphia(5)showed that women were able to differentiate the body odours of themselves, a close friend and a complete stranger. Furthermore, brain scans taken of the sniffing women showed that the scent of the stranger activated the amygdala and the insula, both of which process emotions of fear and disgust  (amongst other emotions), whereas the scent of the close friend produced a response in the retrosplenial cortex, an area of the brain which is triggered by familiar objects.

So, there does seem to be some evidence that humans are able to pick up subtle chemical cues from other humans, albeit at a much lower level than many other animals. However, it's not clear as to the exact extent of our detection abilities, nor why we rely far less on pheromones than our animal neighbours. Perhaps this is due to human's superior visual (and to a slightly lesser extent audio) based classification abilities that pheromonal-type signals are just not necessary for our survival, and thus, strong signals have been 'evolved out' of us. Furthermore, given that our life-spans are long in comparison to most creatures on this planet, we don't need the fast fertility-detection system that pheromones provide. So, any human thinking of buying a pheromone 'instant attraction' spray is probably wasting their money - you would be better off trying to attract a mate by being a decent person, eating healthy food and undertaking a regular exercise regime.

On this subject, it's fascinating to note how large a market there is for actively disguising natural human smells. Soaps, deodorants, perfumes, hairsprays, aftershaves infused with synthetic scents of flowers, 'green tea and cucumber' or 'incense' or, most curiously 'ocean spray' make the wearer smell more 'attractive' to other humans by making them smell less human. Yet babies don't come out of the womb smelling of lavender, and adults are not meant to attract insects for the purposes of pollination (which is the primary function of a flower's perfume.)

If we think that to smell human is somehow bad, perhaps it's because it reminds us that we are animals of a kind. Unmasked by all sorts of sanitary devices, humans have been shown to use smell to detect blood related kin (mothers and children but not husbands and wives); A woman smells differently at different stages of her menstrual cycle, which, in animals, is used as an indicator of fertility. When you think of a woman's smell being used in such a way, do you think it's disgusting? What is the future of smell censorship? Will we in the west continue down our path of deliberate scent illusions?

1. 'A novel multigene family may encode odorant receptors: A molecular basis for odor recognition.' Linda Buck and Richard Axel. In: Cell,  "" Volume 65, Issue 1, 175-187, 5 April 1991.
2. 'The Olfactory System.' Michael T. Shipley, Matthew Ennis and Adam C. Puche. In: Neuroscience in Medicine, 3, 2008. Pages 611-622.
3. 'Menstrual Synchrony and Suppression.' Martha K. McClintock. In: Nature, Volume 229, January 22, 1971.
4. 'Human Body Scents: Conscious Perceptions and Biological Effects.' Martha K. McClintock, Susan Bullivant, Suma Jacob, Natasha Spencer, Bethanne Zelano and Carole Ober. In:  HYPERLINK "" Chemical Senses Vol. 30 No. supplement 1, 2005.
5. 'The Neuronal Substrates of Human Olfactory Based Kin Recognition.' Johan N. Lundström , Julie A. Boyle, Robert J. Zatorre, Marilyn Jones-Gotman. In: Inter Science, Volume 30, Issue 8, Pages 2571-2580.