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Body odour and sexual attraction

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Research has shown that certain body odours are connected to human sexual attraction.[1][2] Both fluctuating asymmetry (FA)[3] and the human leukocyte antigen (HLA)[4] are connected to scent. HLA is the human version of the major histocompatibility complex (MHC), a protein complex that plays a role in an individual’s immunities and self- versus nonself-recognition.[5] FA affects body symmetry, which in turn affects body size and stress tolerance.[3] Studies have not suggested that one scent caused by a particular HLA is more attractive to all individuals, but they have demonstrated attraction between individuals of genetically dissimilar HLA types.[6] Research has also shown that the scent of low FA person is universally more attractive,[3] and that facial attractiveness is connected to attractiveness of scent.[7]

Scent and sexual attraction

Through scent, humans can subconsciously tell if a potential mate will pass on favourable traits to their offspring[citation needed]. Research on human mating has found the effect of scent on males and on females to differs.[8] Part of this difference is caused by the different motives each gender holds for mating. Males, in order to pass on genes, subconsciously notice and are attracted to traits that indicate fertility in females, such as a voice of a higher pitch, a specific hip-to-waist ratio, and a certain body odour [citation needed]. Evolutionarily, females have two main motives for mating: to pass on genes and to find a partner who can provide adequate support for herself and future offspring. As a female reaches the fertile stage of her menstrual cycle, the desire to pass on favourable traits to offspring increases in importance and the female becomes more attracted than usual to males with favourable traits.[8] Many such traits are subliminally detected through scent.[1]

Scent and HLA

It is unclear exactly how HLA affects scent, though there are a range of hypotheses:

  • The vomeronasal organ, an olfactory bulb used to detect pheromones, can detect social information about gender, status, and individuality. Research on mice has shown that a particular sequence of exposures to MHCs induces a specific response in the vomeronasal organ.[9]
  • Certain molecules within the HLA are responsible for an individual's unique body odour. Research on rats and mice has shown that variation in MHC composition can be detected by smell.[10]
  • Variations in MHC may allow the growth of certain organisms that influence body odour.[5][clarification needed]

Human Biology

Research has attempted to clarify the relationship between sexual attraction and various aspects of human biology and genetics, including: HLA, dissimilar MHCs, heterozygosity of MHC, and rare alleles for MHC.

HLA

Men and women are attracted to the pheromones of the opposite sex. Males produce androstenol and androstenone. Androstenol is produced by fresh male sweat and is most attractive to women, while androstenone is produced once the sweat is exposed to oxygen and is highly unpleasant to women. Women are able to detect a single HLA difference, and are more attracted to the HLA with more matching elements of their genetics. Studies have shown that women who are at the most fertile stage of their menstrual cycle prefer the smell of men that have higher testosterone levels[citation needed].

Mens arousal can be influenced by a variety of odours. For example; pumpkin pie, liquorice, doughnuts, and lavender can increase penile blood flow consequently causing arousal.[11]

MHC

See also: Major Histocompatibility Complex and Sexual Selection

MHC (Major Histocompatibility Complex) are a group of immune related genes, of which there are millions of different combinations of alleles. MHC is also the source of unique body smells, therefore similarity or otherwise indicates a difference in the individuals' genotypes.[12]

It has been found that women prefer men with differing MHC genes from themselves, creating an ideal biological match. An evolutionary explanation as to why women are more attracted to men with differing MHC genes, is that these genes are a combination of both parents' genes which leads to a stronger immune system. When women are using hormonal contraceptives, their bodies are fooled into thinking that their ideal match is someone who is biologically very similar to them. Studies have shown that a woman's relationship with her partner is capable of changing based on whether or not she is taking contraceptives.[13] This creates an interesting dynamic based on whether or not the woman was taking contraceptives at the time she began a relationship with her partner.[14]

Apocrine glands are secondary sexual characteristics that are stimulated by androgens and are active once a person hits puberty. They produce a 'chemical soup' which encourages bacteria growth leading to a variety of odiferous chemicals. These glands are located in underarms, around the genitals, and around the areola. The smell they produce differs between sexes and individuals. Studies have shown that the perceived attractiveness of a woman's body odour is positively correlated with their physical attractiveness and body symmetry. These bodily variations are also affected by the menstrual cycle[citation needed].

Both sexes commonly use artificial scents to enhance their perceived sexual appeal. Many of these share a similar chemical profile with naturally produced body chemicals.[15] Several infertility issues come from the fact that people often use perfumes or scented body washes that erase their natural scent, hindering (women in particular) being able to detect if their partner is genetically comparable.

It has been proven that couples who have many failed attempts at conception share a considerably larger amount of genes than those who are able to conceive without problems. If, for these genetically similar couples, there is a successful conception, the babies are often born early or underweight.[16]

Dissimilarity

MHC is expressed codominantly; a more diverse set of MHCs leads to a stronger immune system. Females are thus likely to be more attracted to males with MHC alleles different from their own to provide their offspring with a stronger immune system .[17] A study confirming this had males wear a t-shirt for two consecutive nights without adding any artificial scents, then had females smell the shirts and classify the odour as attractive or unattractive. Females not using contraceptives were more attracted to the scent of males with dissimilar MHCs, whereas females on contraceptives preferred the scent of men with MHCs similar to their own.[4] Further studies, while not confirming the female attraction to the scent of a dissimilar MHC, have also shown females on hormonal contraceptives were attracted to similar MHCs. This study also showed that males are more attracted to the scent of females with a dissimilar MHC.[6] Research has also shown that the scent of a low FA person is universally more attractive,[3] This increased attraction between people of dissimilar MHCs is also hypothesised as a prevention of incest and subsequent birth defects.[4][6]

Heterozygosity

See also: Heterozygote advantage

Heterozygosity of HLA can also be detected through scent: in this case, heterozygosity confers greater ability to recognize a wider variety of antigens. [17] Females, especially when not using hormonal contraceptives, are more attracted to the scent of males heterozygous for HLA. The same attraction is not held by males for heterozygous females. Males are, however, more attracted to the scent of females with rare HLA alleles.[6]

Body Odour and Menstrual Cycle Changes

Women’s fertility levels shift dramatically throughout the menstrual cycle, so the period surrounding ovulation is extremely important from a reproductive standpoint because it represents the peak period of reproductive fertility.[18] As conception is most likely to occur during a woman’s brief fertile period, evolutionary theories suggest that men possess adaptations designed to maximize their reproductive success during this period.[18] Women’s fertility shapes male mating behaviour, many studies have shown that being exposed to the scent of women’s fertility led men to display greater implicit accessibility to mating-related concepts and males judge the odours of women during the follicular (ovarian) phase as more pleasant and 'sexy' than odours during the luteal (non-ovulatory) phase.[18][19] Olfaction, therefore, transmits information relevant to human mate selection, by which men are capable of detecting shifts in women’s fertility.[18][19]

There is now also considerable evidence from psychological studies that women’s preferences for varied male traits change across the menstrual cycle.[20] Hormonal fluctuation across the menstrual cycle explains temporal variation in women’s judgment of the attractiveness of members of the opposite sex as the psychological processes that shape the formation and maintenance of human romantic relationships are influenced by variation in hormonal levels.[21]

Due to their level of hormones, oral contraceptives have the potential to alter women’s partner preference for a range of male traits -this could have important consequences on sexual relationships, as it alters women’s attraction to their partner and, potentially, to other men.[20] If a woman’s use of oral contraceptives is congruent, meaning she was using oral contraceptives when she met her partner, her current preference will more closely match the preference that shaped her partner choice in the first place, and the desire is higher than that of a woman whose use of oral contraceptives is in-congruent.[20] The resulting factor is that women’s partner preferences are influenced by oral contraceptives use, meaning that attraction towards an existing partner changes over time if a woman initiates or discontinues oral contraceptive use.[20]

Body Odour and Fluctuating Asymmetry

Fluctuating Asymmetry (FA) is a type of biological asymmetry, which refers to the extent to which small random deviations occur in perfectly bilaterally paired structures and hence cause a deviation from perfect symmetry in such structures.[22] FA acts as an index for measuring developmental instability as it is a clear indicator of the possible environmental and genetic stressors affecting development.[23] It is thought that having a preference for a symmetrical face may offer some adaptive value as such symmetry may signal an individual's ability to cope with environmental challenges.[24] FA shares an inverse relationship with certain desired traits; a low FA is correlated with higher stress tolerance, larger body size in males, smaller body size in females, and higher facial attractiveness.[25]

FA is detectable through the olfactory senses and has an effect on sexual attraction.[26] Females during their fertile phase have repeatedly been shown as being more attracted to the body odours of more symmetrical males [27] and of males whose faces they rate as attractive.[28] Women who are highly fertile find the scent of low FA as attractive but this scent is not necessarily attractive to other women.[26] For example, it has been found that normal cycling women near their peak fertility tended to prefer the odour of shirts worn by symmetrical men and yet women at low fertility in their menstrual cycle or those using contraception showed no preference for the odour of shirts of symmetrical men compared to those of asymmetrical men.[29]

Body Odour and Sexual Attraction in Animals

Insects

In insects, sex pheromones, usually released by the female to lure a male, are vital in the process by which insects locate each other for mating. The main purpose of releasing these sex pheromones is to attract a partner from a distance, however the sex pheromones also serve to evoke a courtship response and sexually excite the male prior to copulation[30]. Male insects can also release sex pheromones, but this is only for the purpose of sexually exciting the female, making her more receptive to the male's advances. Generally, the majority of insects are sensitive and selective to the sex pheromone of their own species.

There is a significant amount of research supporting body odour and sexual attraction in insects. Observations and laboratory experiments of Culiseta Inornata, identified a chemical substance involved in mating behaviour, when exposed to this scent the male mosquitos were found to attempt sex with dead females and when exposed to the scent of virgin females, the males showed increased sexual activity through excited flight, searching and attempts to copulate with other males[31]. Further evidence comes from research on the commercial silkworm moth, Bombyx mori, a chemical produced in the abdominal sac of the female adult moth is released shortly after it's emergence from the cocoon, male moths were found to be immediately attracted to this scent demonstrated by a characteristic wing flutter and attempts to copulate [32].

Mammals

When it comes to sex, mammals use chemical signals to convey information to one and other. Mammals use sex pheromones to arouse, attract, and elicit specific behavioural responses from the opposite sex[33]. For an odour to be exclusively result in sexual behaviour, it must not only be perceived and preferred, but when absent there should be a decrease or complete elimination of sexual activity. This exclusivity has only been shown in golden hamsters[34] and the rhesus monkey[35].

There is vast evidence for the use of pheromones in mating behaviours. For example when boars become sexually aroused, they salivate profusely dispersing pheromones into the air. These pheromones attract receptive sows, causing it to adopt a specific mating posture, known as standing, which allows the male boar to mount it and therefore copulate[36].


See also

Notes

  1. ^ a b Spehr, Mark; Kelliher, Kevin R.; Li, Xiao-Hong; Boehm, Thomas; Leinders-Zufall, Trese; Zufall, Frank (15 February 2006). "Essential role of the main olfactory system in social recognition of major histocompatibility complex peptide ligands". Journal of Neuroscience. 26 (7): 1961–1970. doi:10.1523/JNEUROSCI.4939-05.2006. PMID 16481428.
  2. ^ Singh, D.; Bronstad, P.M. (22 April 2001). "Female body odour is a potential cue to ovulation". Proceedings of the Royal Society B. 268 (1469): 797–801. doi:10.1098/rspb.2001.1589. PMC 1088671. PMID 11345323.
  3. ^ a b c d Manning, J.T.; Koukourakis, K.; Brodie, D.A. (1997). "Fluctuating asymmetry, metabolic rate and sexual selection in human males". Evolution and Human Biology. 18: 15–21. doi:10.1016/s1090-5138(96)00072-4.
  4. ^ a b c Wedekind, Claus; Seebeck, Thomas; Bettens Florence; Paepke, Alexander J. (22 June 1995). "MHC-Dependent Mate Preferences in Humans". Proceedings of the Royal Society B. 260 (1359): 245–249. doi:10.1098/rspb.1995.0087. PMID 7630893.
  5. ^ a b Havlicek, Jan; Roberts, S. Craig (May 2009). "HLA-correlated mate choice in humans: A review". Psychoneuroendocrinology. 34 (4): 497–512. doi:10.1016/j.psyneuen.2008.10.007. PMID 19054623.
  6. ^ a b c d Thornhill, Randy; Gangestad, Steven W.; Miller, Robert; Scheyd, Glenn; McCollough, Julie K.; Franklin, Melissa (2003). "Major histocompatibility complex genes, symmetry, and body scent attractiveness in men and women". Behavioral Ecology. 14: 668–678. doi:10.1093/beheco/arg043.
  7. ^ Rikowski, Anya; Grammer, Karl (1999). "Human body odour, symmetry and attractiveness". Proceedings of the Royal Society B. 266: 869–874. doi:10.1098/rspb.1999.0717. PMC 1689917. PMID 10380676.
  8. ^ a b Thornhill, Randy; Gangestad, Steven W. (1999). "The scent of symmetry: A human sex pheromone that signals fitness?". Evolution and Human Behaviour. 20 (3): 175–201. doi:10.1016/S1090-5138(99)00005-7. PMID 16403409.
  9. ^ Leinders-Zufall, Trese; Brennan, Peter; Widmayer, Patricia; Chandramani, Prashanth; Maul-Pavicic, Andrea; Jäger, Martina; Li, Xiao-Hong; Breer, Heinz; Zufall, Frank; Boehm, Thomas (2004). "HLA Class I Peptides as Chemosensory Signals in the Vomeronasal Organ". Science. 306: 1033–1037. doi:10.1126/science.1102818.
  10. ^ Wobst, Birgit; Zavazava, Nicholas; Luszyk, Dagmar; Lange, Katrin; Ussat, Sandra; Eggert, Frank; Ferstl, Roman; Müller-Ruchholtz, Wolfgang (1999). "Molecular forms of soluble HLA in body fluids: potential determants of body odor cues". Genetica. 104: 257–283. PMID 10386394.
  11. ^ "The Smell Report - Sexual Attraction".
  12. ^ Breedlove, Marc; Watson, Neil. Biological Pchology (Seventh Edition). Sunderland, Massachusetts: Sinauer Associates Inc. p. 372. ISBN 978-0-87893-927-5.
  13. ^ Wedekind, C.; et al. (1995). "MHC-dependent preferences in humans". Proceedings of the Royal Society of London. 260 (1359): 245–49. doi:10.1098/rspb.1995.0087. PMID 7630893.
  14. ^ "Scents and Sensibility".
  15. ^ "The Smell of Love - Psychology Today".
  16. ^ "When a Woman Smells Best".
  17. ^ Molinero, L.L.; Marcos, C.Y.; Mirbaha, F.; Fainboim, L.; Stastny, P.; Zwirner, N.W. (2002). "Codominant expression of the polymorphic MICA alloantigens encoded by genes in the HLA region". European Journal of Immunogenetics. 29: 315–319. doi:10.1046/j.1365-2370.2002.00274.x.
  18. ^ a b c d Miller, S. L., & Maner, J. K. (2011). Ovulation as a male mating prime: subtle signs of women's fertility influence men's mating cognition and behavior.Journal of personality and social psychology, 100(2), 295.
  19. ^ a b Singh, D., & Bronstad, P. M. (2001). Female body odour is a potential cue to ovulation. Proceedings of the Royal Society of London B: Biological Sciences, 268(1469), 797-801.
  20. ^ a b c d Roberts, S. C., Cobey, K. D., Klapilová, K., & Havlíček, J. (2013). An evolutionary approach offers a fresh perspective on the relationship between oral contraception and sexual desire. Archives of Sexual Behavior, 42(8), 1369-1375.
  21. ^ Roberts, S. C., Little, A. C., Burriss, R. P., Cobey, K. D., Klapilová, K., Havlíček, J., ... & Petrie, M. (2014). Partner Choice, Relationship Satisfaction, and Oral Contraception The Congruency Hypothesis.Psychological science, 25(7), 1497-1503.
  22. ^ Tomkins, Joseph L; Kotiaho, Janne S (2002). "Fluctuating asymmetry". eLS. doi:10.1038/npg.els.0003741.
  23. ^ Møller, A.P.; Pomiankowski, A (1993). "Fluctuating asymmetry and sexual selection". Personality and Individual Differences. 89 (1–3): 267–279. doi:10.1007/BF02424520.
  24. ^ Fink, B.; Neave, N.; Manning, J.T.; Grammer, K (2006). "Facial symmetry and judgements of attractiveness, health and personality" (PDF). Personality and Individual Differences. 41 (3): 491–499. doi:10.1016/j.paid.2006.01.017.
  25. ^ Manning, J.T.; Koukourakis, K.; Brodie, D.A. (1997). "Fluctuating asymmetry, metabolic rate and sexual selection in human males". Evolution and Human Biology. 18: 15–21. doi:10.1016/S1090-5138(96)00072-4.
  26. ^ a b Thornhill, Randy; Gangestad, Steven W; Miller, Robert; Scheyd, Glenn; McCollough, Julie K; Franklin, Melissa (2003). "Major histocompatibility complex genes, symmetry, and body scent attractiveness in men and women". Behavioral Ecology. 14: 668–678. doi:10.1093/beheco/arg043.
  27. ^ Thornhill, Randy; Gangestad, Steven W. (1999). "The scent of symmetry: A human sex pheromone that signals fitness?". Evolution and Human Behaviour. 20 (3): 175–201. doi:10.1016/S1090-5138(99)00005-7.
  28. ^ Rikowski, Anya; Grammer, Karl (1999). "Human body odour, symmetry and attractiveness" (PDF). Proceedings of the Royal Society B. 266: 869–874. doi:10.1098/rspb.1999.0717.
  29. ^ Gangestad, S.W.; Thornhill, R. (1998). "Menstrual cycle variation in women's preferences for the scent of symmetrical men" (PDF). Proceedings of the Royal Society of London B. 265 (1399): 927–933. doi:10.1098/rspb.1998.0380.
  30. ^ Jacobson, Martin (1972). Insect Sex Pheromones. New York: Academic Press. p. 1.
  31. ^ Kliewer, J. W.; Miura, T.; Husbands, R. C.; Hurst, C. H. (1 May 1966). "Sex Pheromones and Mating Behavior of Culiseta inornata (Diptera: Culicidae)". Annals of the Entomological Society of America. 59 (3): 530–533. doi:10.1093/aesa/59.3.530.
  32. ^ Butenandt, A; Beckmann, R; Stamm, D (1961). "Über den Sexuallockstoff des Seidenspinners, II. Konstitution und Konfiguration des Bombykols". Hoppe-Seyler´s Zeitschrift für physiologische Chemie. 324: 84-87.
  33. ^ The Neurobiology of Olfaction. Boca Raton, FL: CRC Press/Taylor & Francis. 2009.
  34. ^ Murphy, M. R.; Schneider, G. E. (16 January 1970). "Olfactory Bulb Removal Eliminates Mating Behavior in the Male Golden Hamster". Science. 167 (3916): 302–304. doi:10.1126/science.167.3916.302.
  35. ^ Michael, R. P.; Keverne, E. B.; Bonsall, R. W. (28 May 1971). "Pheromones: Isolation of Male Sex Attractants from a Female Primate". Science. 172 (3986): 964–966. doi:10.1126/science.172.3986.964.
  36. ^ Dorries, Kathleen M.; Adkins-Regan, Elizabeth; Halpern, Bruce P. (1997). "Sensitivity and Behavioral Responses to the Pheromone Androstenone Are Not Mediated by the Vomeronasal Organ in Domestic Pigs". Brain, Behavior and Evolution. 49 (1): 53–62. doi:10.1159/000112981.
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