Female sexual ornaments

Example of a female modelling her body

Female sexual ornaments are features which are used by women to attract mates. Some theories suggest their purpose is sexual selection. However, it has also been claimed they are merely evolutionary by-products.[1] Typically males use sexual ornaments to compete for females, but in human females sexual ornaments can also highlight reproductive value to males.

Theories of female ornaments

Photograph of Charles Darwin in 1874 taken by his son Leonard Darwin

Darwin's theory

In general, Charles Darwin focused his theory of sexual ornaments on males. Despite this, he also noticed there were some exceptional cases where females were more decorated, explicit and powerful than males.[2] However, he considered these female ornaments as non-adaptive, non-functional, secondary product of sexual selection compared with the functional ornaments in males.[2][3] He expressed that sexual ornaments in females were only due to a 'shared genetic architecture'.[4]

Although Darwin's theory has been questioned many times, it provides a strong evolutionary base for research on female sexual ornaments.[3][4]

Parental investment

Parental investment is any effort that increases quality of offspring. This could be things such as spending of time, energy and resources to help offspring to survive. Traditionally, females have greater parental effort while males have greater mating effort, therefore female ornaments exist to signal fertility and incite male competition, allowing for only the fittest male to finally mate[5]. Some theories suggest that female sexual ornaments exist largely where benefits of the ornaments outweigh the direct investment of the resources into the offspring. This is common in sex role reversed species such as the pipefish,[6] where males have greater parental effort. Therefore in these situations female sexual ornaments exist to allow the female to better compete for the male and his investment.

Honest signals

Olive baboon, also know as the anubis baboon

Across a wide variety of species, many traits that are deemed attractive have evolved to communicate mate quality to the opposite sex. Often ornaments act as honest signals and are a reliable indicator of mate quality. This is because sexual ornaments are indicators of good genes and heritable viability, as they are costly to an individual's survival to maintain and produce.[7] Therefore, honest signals cannot be faked as a weak individual would not be able to survive the cost. Typically, males are more elaborately ornamented than women.[8] However, it has been hypothesised that female sexual swellings in primates function to signal female quality.[9] Males that preferred exaggerated displays gained a reproductive benefit and thus, a preference for female exaggerated displays such as sexual swellings evolved. Research has suggested that male olive baboons show increased competition and grooming towards females with greater ornamentation.[10] In addition, the reproductive success of female olive baboons across a lifetime positively co-varied with their degree of ornamentation.[10]

Factors affecting honest signalling

Female sexual ornaments reflect current estrogen levels and thus, if ornaments are honest signals than estrogen must also reliably reflect quality.[11] As a result, a female will show more attractive ornamentation at certain times in her life when her estrogen levels are high, such as during estrus and when she is at her most reproductive age (around 20).[12] Ornaments also give men cues about the number of offspring a female already has, as breasts lose their firmness, signalling to males that if they mate with this female, any resources may also go to offspring that is not their own.[1]

Nutrition also influences female ornamentation as women who cannot consume enough calories do not have the energy to expend on producing their ornamentation as this is too costly.[1] A minimum level of stored energy is needed for females to ovulate and those who do not consume the nutrients they need often have low levels of fertility.[13] For example, malnutrition in females suffering from anorexia nervosa can lead to amenorrhea (absence of menstruation).[13]

Functions of female sexual ornamentation

Long term pair bonding

Female ornaments were likely selected to facilitate long term pair bonding.[14] This involves males being attracted to females who are fertile and capable of repeated reproduction, leading to mating and creating a long-term bond. Females with selected sexual ornaments are markers to males of future superior quality offspring, making males more willing to provide many non genetic material benefits and services to the female which could in turn benefit offspring.[1]

Ornamentation as a form of female-female competition

One theory on female-female competition states that females use their sexual ornaments to compete indirectly against each other (intrasexual selection).[3] This behaviour has two possible causes:[3][4][15]

Males show a greater preference toward physically attractive females (sexually ornamented), enabling these women to obtain more resources. Attractiveness may also help females to achieve a better job or salary in Western civilisation.[1] As data show attractive female applicants are assessed as more suitable for a feminine-type job where physical attractiveness is important e.g., secretary, dietician,  human resource  manager, and significant, albeit less suitable when attractiveness wasn’t so important (e.g., nurse, social worker, bank teller). Moreover, woman’s attractiveness also increased her suitability to masculine-type job, however the one, that requires physical appearance (for instance a car salesperson, office  manager, sales manager).[16][17] Jackson in his book Physical Appearance and Gender: Sociobiological and Sociocultural Perspectives writes that physical attractiveness of the candidate for a job also raises assumed salary.[18] The difference in wages was also proved in further studies.[17][19]

Ornamentation as a signal to incite male competition

Female sexual ornaments act as signals of fertility. A female with the best ornamentation is likely to be attractive to males and will incite male-male competition.[1]

Female sexual ornaments allow them to find the best mate and increase the quality of their offspring.[15] Males prefer females with greater ornaments because these females have greater reproductive value. Therefore, males who mate with these females will have offspring with a greater chance or survival and better genes.[4] This creates competition between males over the mate with the best ornamentation.

Examples in humans

Breasts

Breasts are gynoid fat reserves used to provide nourishment to offspring. Gynoid fat acts as an indicator of fertility and future reproductive value[20] as a higher proportion of gynoid fat predicts the likeliness of a successful pregnancy.[1] Cross-culturally males show a preference for breasts which appear youthful and firm, not necessarily those which are the largest.[21][22] Breasts which contain higher levels of android fats may also be large, however these are deemed less attractive by males.[1]

Waist to hip ratio

Waist to hip ratio

A female's waist to hip ratio (WHR) is calculated by dividing her waist measurement by her hip measurement. A lower WHR (around 0.7) is deemed more sexually attractive as it indicates a preferred ratio of gynoid to android fats[23] signalling high reproductive value in females. Women with a low WHR (0.7–0.79) have optimal levels of estrogen and significantly higher pregnancy rates than those with higher WHRs.[24] In addition, females with high distribution of android fat experience impaired in-vitro fertilisation (IVF) embryo transfer. This leads to a lower pregnancy rate, compared with females who have a preferred ratio of gynoid to android fats.[25]

However, in nutritionally stressed environments BMI is a better predictor of neonatal birth weight and male attraction than WHR. In these circumstances, males prefer any form of stored fat, gynoid or android.[26]

Facial ornamentation

Men are most attracted to women with typically female facial features, such as a small jaw and chin, open and large eyes, and high cheekbones.[27] Facial ornamentation changes with age – for example, the brow bone becomes more defined and lips become smaller, making the face less attractive.[1] Studies also suggest greater facial symmetry reflects good health.[28] Facial attractiveness has correlates with estrogen levels, both of which reduce with age. Facial ornamentation is therefore seen as a signal of reproductive value to males, based on age, estrogen levels, and health.[12]

Skin

Women have skin that is finely textured, smoother and less hairy than men, which, as age increases, becomes less attractive. They also have more subcutaneous fat, a layer just beneath the skin which stores energy and is used as padding [29].Studies manipulating skin have shown smoother skin is rated more attractive by males, indicating good fertility and health and signalling reproductive value, making it vitally important in female sexual signalling.[30] .

Voice

Voice is a sexually dimorphic ornament, which means it differs between the sexes. This is due to the fact it seems to be dependent on sex hormone levels (i.e. estrogen and androgen) which vary between males and females.[31] For example, on average, women have higher pitched voice due to increased levels of estrogen. Research shows female opera singers who sang more 'femininely' (soprano), had a more estrogenized voice and lived longer than those who sang lower with an androgenized voice.[1] Furthermore, women with a relatively symmetric face were rated as having more attractive voices than these more asymmetric faces.[32] Men prefer faces of women with higher pitched voices who are typically younger, highlighting their fertility.[33]

Other ornaments

Typical Male hand where the index finger is shorter than the ring finger

Other ornaments include gait, a sex-typical finger length and size of foot. However, these are not always honest signals and can be deceptive.[1]

Gait

If females walk smoothly, in a straight line, it's seen as a good predictor of reproductive value.[1] Gait is also positively correlated with a females attractiveness.[34]

Finger length

Males' index fingers should be shorter than their ring fingers, whilst females show the reverse. Individuals with typical – for their sex – length of fingers were found to be more attractive.[1]

Size of feet

Research has found that women with smaller feet are more attractive than those with larger feet.[35] Some cultures highlight this preference through practices such as foot binding in China.

See also

References

  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Thornhill, R.; Gangestad, S. W. (2008). The Evolutionary Biology of Human Female Sexuality. Oxford:NY: Oxford University Press.
  2. 1 2 Darwin, C. (1871). The Descent of Man, and Selection in Relation to Sex. London, UK: John Murray. pp. 253–320.
  3. 1 2 3 4 5 6 7 Amundsen, T. (2000). "Female Ornaments: Genetically Correlated or Sexually Selected?". Animal Signals: Signalling and Signal Design in Animal Communication. Trondheim. Trondheim, Norway: Tapir Academic Publishers: 133–154.
  4. 1 2 3 4 5 Tobias, J.A.; Montgomerie, R.; Lyon, B.E. (2012). "The evolution of female ornaments and weaponry: social selection, sexual selection and ecological competition.". Philosophical Transactions of the Royal Society B. 367: 2274–2293.
  5. ↑ Trivers, R. (1972). "Parental investment and sexual selection". Sexual Selection and the Descent of Man. New York: Aldine de Gruyter. pp. 136–179.
  6. ↑ Berglund, A.; Rosenqvist, G.; Bernet, P. "Ornamentation predicts reproductive success in female pipefish". Behavioral Ecology and Sociobiology. 40 (3): 145–150. doi:10.1007/s002650050327.
  7. ↑ Zahavi, A (1975). "Mate selection - a selection for a handicap". Journal of theoretical biology. 53(1): 205–214.
  8. ↑ Rubenstein, D; Lovette, I (2009). "Reproductive skew and selection on female ornamentation in social species". Nature. 462: 786–789.
  9. ↑ Pagel, M. D (1994). Eggleton, P; Vane-Wright, R. I., eds. The adaptationist Wager. In Phylogenetics and ecology. London: Academic Press. pp. 29–51.
  10. 1 2 Domb, L. G.; Pagel, M (2001). "Sexual Swellings advertise female quality in wild baboons". Nature. 410: 204–206.
  11. ↑ Thornhill, R; Gangestad, S.W. (1993). "Human facial beauty: Averageness, symmetry and parasite-resistance". Human Nature. 4: 237–269.
  12. 1 2 Law Smith, M.J.; Perrett, D. I.; Jones, B.C.; Cornwell, R. E.; Moore, F.R.; Feinberg, D. R. (2006). "Facial appearance is a cue to oestrogen levels in women". Biological Sciences. 273: 135–140.
  13. 1 2 Frisch, R; McArthur, J (1974). "Menstrual cycles:Fatness as a determinant of minimum weight necessary for their maintenance or onset.". Science. 185: 949–951.
  14. ↑ Buss, D. M.; Schmitt, D. P. (1993). "Sexual strategies theory: An evolutionary perspective on human mating". Psychological Review. 100: 204–232.
  15. 1 2 Clutton-Brock, T. (2009). "Sexual selection in females". Animal Behaviour. 77: 3–11.
  16. ↑ Johnson, S.K.; Podratz, K.E.; Dipboye, R.L.; Gibbons, E. (2010). "Physical Attractiveness Biases in Ratings of Employment Suitability: Tracking Down the "Beauty is Beastly" Effect". The Journal of Social Psychology. 150: 301–319.
  17. 1 2 Pfeifer, C. (2011). "Physical Attractiveness, Employment, and Earnings". Applied Economics Letters. 19: 505–510.
  18. ↑ Jackson, L.A (1992). Physical Appearance and Gender: Sociobiological and Sociocultural Perspectives. New York, USA: State University of New York Press.
  19. ↑ Fletcher, J.M. (2009). "Beauty vs. brains: Early labor market outcomes of high school graduates". Economics Letters. 105: 321–325.
  20. ↑ Gallup, G. G. Jr (1982). "Permanent breast enlargement in human females: A sociobiological analysis". Journal of Human Evolution. 11: 597–601.
  21. ↑ Grammer, K; Fink, B; Møller, A. P; Manning, J. T. (2005). "Physical attractiveness and health: Comment on Weeded and Sabini". Psychological Bulletin. 131: 658–661.
  22. ↑ Havlicek, J (2016). "Men's preferences for women's breast size and shape in four cultures". Evolution and Human Behaviour. 0.
  23. ↑ Singh, D (1993). "Adaptive significance of female physical attractiveness: Role of waist-to-hip ratio". Journal of Personality and Social Psychology. 65 (2): 293–307.
  24. ↑ Singh, D (2002). "Female mate value at a glance: Relationship of waist to-hip ratio to health, fecundity and attractiveness". Neuroendocrinology. 23: 65–75.
  25. ↑ Wass, P; Waldenstrröm, U; Rössner, S; Hellberg, D (1997). "An android body fat distribution in females impairs the pregnancy rate of in-vitro fertilization-embryo transfer". Human Reproduction. 12(9): 2057–2060.
  26. ↑ Marlowe, F; A, Wetsman (2001). "Preferred waist-to-hip ratio and ecology". Personality and Individual Differences. 30: 481–489.
  27. ↑ Rhodes, G. "The Evolutionary Psychology of Facial Beauty". Annual Review of Psychology. 57 (1): 199–226. doi:10.1146/annurev.psych.57.102904.190208.
  28. ↑ Fink, B.; Neave, N.; Manning, J. T.; Grammer, K. (2006). "Facial symmetry and judgements of attractiveness, health and personality". Personality and Individual Differences. 41: 491–499.
  29. ↑ Montagna, W. (1982-01-01). Chiarelli, Professor A. B.; Corruccini, Dr R. S., eds. Advanced Views in Primate Biology. Proceedings in Life Sciences. Springer Berlin Heidelberg. pp. 35–41. doi:10.1007/978-3-642-68300-8_4. ISBN 9783642683022.
  30. ↑ Fink, B.; Grammer, K.; Thornhill, R. (2001). "Human (Homo sapiens) facial attractiveness in relation to skin texture and color.". Journal of Comparative Psychology. 115: 92–99.
  31. ↑ Abitbol, J.; Abitbol, P.; Abitbol, B. (1999). "Sex hormones and the female voice". Journal of Voice. 13: 424–446.
  32. ↑ Feinberg, D.R.; et al. (2005). "The voice and face of woman". Evolution and Human Behavior. 26: 398–408.
  33. ↑ Collins, S.A; Missing, C. (2003). "Vocal and visual attractiveness are related in women.". Animal Behaviour. 65: 997–1004.
  34. ↑ Johnson, K.L.; Tassinary, L.G. (2007). "Compatibility of basic social perceptions determines perceived attractiveness". Proceedings of the National Academy of Sciences of the United States of America. 104: 5246–5251.
  35. ↑ Fessler, D.M.T; Haley, K.J.; Roshni, L.D. (2005). "Sexual dimorphism in foot length proportionate to stature". Annals of Human Biology. 32: 44–59.
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