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Role of Smell

What is smell and what does the sense of smell do for us?

Orange blossom

Hazard warning | Memory prompt | Chemical signalling | Antidepressant | Flavour detector |
Appetite control | Mate selector (HLA detector)
also: Smell and the pill

Hazard warning
Dog rose
Smell is one of the chemical senses, the other being taste (see "Taste" page). They are so called because they sense chemicals, and smells are, of course, chemicals. With these senses we sample our environment for information. We are continuously testing the quality of the air we breathe (this will alert us to potential dangers, e.g. smoke) as well as using this sense to inform us of other relevant information, such as the presence of food or another individual. The chemicals detected by our sensory systems need to have certain properties. For instance, odour molecules must be small enough to be volatile (less than 300-400 relative molecular mass) so that they can vapourise, reach the nose and then dissolve in the mucus. This tells us that smell, unlike taste, can signal over long distances (an early warning device). We appear to have an innate ability to detect bad, aversive smells. One-day old babies give facial expressions that indicate rejection when given fish or rotten egg odour.

Memory prompt

But, is our olfactory system doing more than just giving us warnings? Yes, of course. Smell is the backdrop to our every waking moment, it is part of our consciousness and, even in sleep (see Smell and dreams) the sense of smell keeps working. It also serves a recognition function. We all have our own unique smell (some more pleasant than others! - but that's another story, see Smelly you on the "Smell - What & How" page) and can recognise and be recognised by our smell. Smell evokes memory and, to a greater extent than the other senses, evokes the mood and emotion associated with the memory. The brain regions dealing with smell (piriform cortex) and memory (hippocampus) are close together and there is a two way highway of connections between them. The amygdala is also part of this collection of brain centres, called the limbic system, and sets the the appropriate emotional tone in response to memories.

Chemical signalling

Dogs can distinguish between the smell of T-shirts worn by non-identical twins (they couldn't tell the difference between identical twins - presumably because they smell identical!). Children can distinguish between the smell of their siblings and other children of the same age. Babies recognise their own mothers' smell and mothers recognise their own babies' smell. Emotion can be communicated by smell. Dogs and horses are very sensitive to the smell of fear in humans and recently it has been demonstrated that humans can detect fear. Dr Mujica Parodi used fMRI to show that "fear sweat" (from skydivers) activates the amygdala in other people whereas gym sweat does not. Research has shown that a panel of women can discriminate between armpit swabs taken from people watching "happy" and "sad" films. Men were less good at this. Mum's learn the smell of their own babies and babies learn the smell of their own mothers. The emotions of others, for example fear, contentment, sexuality, may therefore be experienced and communicated by smell. Memory is often associated with smell. Smell and memory are intimately linked - although this phenomenon is not well understood.

We certainly underestimate the importance of smell to our well-being - ask an anosmic (someone who has lost some or all of their sense of smell). There are suggestions that smell can influence mood, memory, emotions, mate choice,

the immune system and the endocrine system (hormones). In fact the sense of smell could be said to be at the mind-body interface working in the background to modulate our response to the outside world.

A proportion of those who lose their sense of smell become depressed. So you could say that smell acts as an antidepressant. This works both ways. More detail on smell and depression can be found in my page on "Smell and Mood" and I look into new, non-drug treatments for depression in "Therapeutic use of smell".

Smell and flavour

Flavour is SMELL + TASTE, but mainly smell. Without smell food tastes really dull, so does drink. When we are hungry our appetite increases and our sense of smell becomes more acute, we smell things more intensely and this eases off once we've eaten. Those that loose their sense of smell often have difficulty in controlling their weight either eating too much to compensate or not eating at all because they have lost the pleasure in food.

Smell and mate selection

I have always believed that the prime function of smell is mate selection and that this has driven (human) evolution. We like the smell of some people and don't like the smell of others. Our sense of smell is subconsciously identifying the immune system of others. It is our MHC genes (called HLA in humans), part of of our immune system, that confer our own unique body odour and we like the smell of those with very different HLA genes and do NOT like the smell of those with similar HLA genes. The attraction to the smell of those with different HLA alleles encourages outbreeding, increases disease resistance and prevents incest. It is not so much that we actively sniif out our perfect partner more that we are unlikely to end up with someone whose smell we can't stand. The more scientific details about this are given below.

HLA and mate selection - non random pairing

One of the major functions of our sense of smell is as an HLA (Human Leucocyte Antigen) detection system (Wedekind et al, 1995; Penn, 2002 (review); Pause et al., 2006). Human leucocyte antigen (HLA) is the human equivalent of the vertebrate major histocompatibility complex (MHC). MHC genes control our immune responsiveness and are the most polymorphic genes known in vertebrates. One of the main hypotheses explaining this polymorphism is disassortative mating preferences (Penn and Potts, 1999) where unlike genotypes are attracted to one another. This results in non-random pairing in which pairs with similar genotypes are underrepresented in the population. Evidence for this has been found in mice (Yamazaki et al., 1976; 1978; Beauchamp et al., 1988), humans (Ober et al., 1997; Jacob et al., 2002; Chaix, Cao and Donnelly, 2008), birds (Freeman-Gallant et al., 2003), reptiles (Olsson et al., 2003) and fish (Landry et al., 2001). Individuals tend to select mates with appropriate genetic compatibility, neither too similar nor too dissimilar at the MHC-loci. This mating preference acts to increase disease resistance (Hughes and Nei, 1992) and prevents kin mating (incest avoidance; Weisfeld et al., 2003). Mate choice appears to be mediated by immune-dependent odour cues referred to as odourtype. There are four hypotheses as to how MHC genes influence odourtype (individual body odour; reviewed by Penn, 2002): (1) MHC molecules may provide the odours themselves, (2) MHC molecules may act as carriers for volatile odorants (the carrier hypothesis), (3) metabolites of MHC-bound peptides may provide the source of volatile odours (the peptide hypothesis), and (4) MHC genes may influence the odour indirectly by shaping an individual’s personal microflora composition (the microflora hypothesis).

This odour-based mate selection encourages disassortative pairing and there are penalties if it is subverted. Couples sharing MHC alleles have a higher rate of spontaneous abortion (Ober, 1995), and significantly lower success at achieving pregnancy following artificial reproductive therapy (reviewed in Ober and van der Ven, 1997) and some studies have suggested that females reject MHC-similar sperm or zygotes (Rulicke et al., 1998). Consanguineous married couples in China have been found to have low fertility and it has been suggested that they fail to initiate pregnancy due to the increased incidence of HLA-sharing (Liu et al., 2005). In another Chinese study, the odds of being in a consanguineous marriage were two to three times higher for infertile couples compared to fertile couples (Cheng, 1988). Ober et al. (1992) concluded that fertility is reduced in couples sharing HLA-DR alleles. MHC-dissimilar mate preference may influence the psychology of sexual attraction. As MHC sharing increased, women’s sexual responsiveness to their partners decreased, their number of extrapair sexual partners increased, and their attraction to men other than their primary partners increased, particularly during the most fertile period of their menstrual cycles (ovulation) (Garver-Apgar et al., 2006).

From the above it can be suggested that it is important to make the appropriate, genetically-compatible choice when selecting a partner. Clearly this is not an active, conscious process in humans. More likely, it exerts a negative influence, viz. couples who find the smell of their partner continuously unpleasant would be less likely to stay together. To confound this biological selection mechanism, use of the oral contraceptive interferes with womens’ ability to select HLA-compatible partners, reversing their preference, and they have been found to prefer the smell of HLA-similar men (Wedekind et al., 1995; Craig Roberts et al., 2008). Is this the end of human evolution? Actually, not a silly question and no-one has looked into it. They should.

Smell and the pill

How does taking the oral contraceptive pill alter your ability to smell and your own body odour?

Beauchamp,G.K., Yamazaki, K., Bard, J. and Boyse, E.A. (1988) Preweaning experience in the control of mating preferences by genes in the Major Histocompatibility Complex of the mouse. Behav. Genet. 18, 537-547.
Cheng, Y.M. (1988) An epidemiological study of infertility. Beijing Medical Sciences Bulletin: Family Planning Volume 4, 6.
Doty, R.L., Snyder, P.J., Huggins, G.R. and Lowry, L.D. (1981) Endocrine, cardiovascular, and psychological correlates of olfactory sensitivity changes during the human menstrual cycle. J. comp. physiol. Psychol. 95, 45-60.
Freeman-Gallant, C.R., Meguerdichian, M., Wheelwright, N.T. and Sollecito, S.V. (2003) Social pairing and female mating fidelity predicted by retriction fragment length polymorphism similarity at the major histocompatibility complex in a songbird. Mol.Ecol. 12, 3077-3083. (doi:10.1046/j.1365-294X.2003.01968.x)
Good, P.R., Geary, N. and Engen, T. (1068) The effect of estrogen on odor detection. Chem. Senses Flavor 2, 45-50.
Garver-Apgar, C.E., Gangestad, S.W., Thornhill, R., Miller, R.D. and Olp, J.J. (2006) Major histocompatibilty complex alleles, sexual responsivity, and unfaithfulness in romantic couples. Psychol. Sci. 17, 830-835.
Hughes, A.L. and Nei, M. (1992) Models of host-parasite interaction and MHC polymorphism. Genetics 132, 863-864.
Jacob, S., McClintock, M.K., Zelano, B. and Ober, C. (2002) Paternally inherited HLA alleles are associated with women’s choice of male odor. Nature Genetics 30, 175-9.
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Ober, C., Elias, S., Kostyu, D.D. and Hauck, W.W. (1992) Decreased fecundability in Hutterite couples charing HLA-DR. Am. J. Hum. Genet. 50, 6-14.
Ober, C. and van der Ven, K. (1997) Immunogenetics of reproduction: an overview. In L.B. Olding (ed.) Current Topics in Microbiology and Immunology, pp. 1-23. Berlin: Springer-Verlag.
Ober. C. (1995) HLA and reproduction: lessons from studies in the Hutterites. Placenta 16, 569-577.
Ober, C., Weitkamp, L.R., Cox, N., Dytch, H., Kostyu, D. and Elias, S. (1997) HLA and mate choice in humans. Am. J. Hum. Genet. 61, 497-504.
Craig Roberts, S., Gosling, L.M., Carter, V. and Petrie, M. (2008) MHC-correlated odour preferences in humans and the use of oral contraceptives. Proc. R. Soc. B 7 275 , 2715-2722.
Olsson, M., Madsen, T., Nordby, J., Wapstra, E., Ujvari, B. And  Wittsell, H. (2003) Major histocompatibility complex and mate choice in sand lizards. Proc. R. Soc. B 270 (Suppl.), S254-S256. (doi:10.1098/rsbl.2003.0079)
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