Female sexual ornaments

Female sexual ornaments are features which are used by females of different species to attract mates. Some theories suggest their purpose is sexual selection. However, it has also been claimed they are merely evolutionary by-products. Typically, males have sexual ornaments that allow them to compete for females, but in some species, females have sexual ornaments that highlight reproductive value to males.
Theories of female ornaments
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. However, he considered these female ornaments as non-adaptive, non-functional, secondary products of sexual selection compared with the functional ornaments in males. He expressed that sexual ornaments in females were only due to a 'shared genetic architecture'.
Although Darwin's theory has been questioned many times, it provides a strong evolutionary base for research on female sexual ornaments. 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, 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
Across a wide variety of species, many traits that are deemed attractive have evolved to communicate mate quality to the opposite sex. Honest signals are a reliable indicator of mate quality. This is because sexual ornaments and some other attractive features such as body and face symmetry, are indicators of good genes and heritable viability, as they are costly to an individual's survival to maintain and produce. Typically, males are more elaborately ornamented than females. However, it has been hypothesised that female sexual swelling in primates functions to signal potential mate quality. Research has shown that male olive baboons show increased competition and grooming towards females with greater ornamentation. In addition, the reproductive success of female olive baboons across a lifetime positively co-varies with their degree of ornamentation. 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. For example, malnutrition in females suffering from anorexia nervosa can lead to amenorrhea (absence of menstruation). 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.
* Females fight for mating benefits (sexual resources). They do this to ensure they gain the fittest mate in order to produce the best quality offspring. In addition, attractive females also earn more than females deemed unattractive.
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. as a higher proportion of gynoid fat predicts the likeliness of a successful pregnancy. Breasts which contain higher levels of android fats may also be large, however these are deemed less attractive by males. 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. 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.
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.
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. Facial ornamentation changes with age - for example, the brow bone becomes more defined and lips become smaller, making the face less attractive. Facial attractiveness 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. Studies manipulating skin show 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.
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. 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. Men prefer faces of women with higher pitched voices who are typically younger, highlighting their fertility.
Other ornaments
Other ornaments include gait, a sex-typical finger length and size of foot. However, these are not always honest signals and can be deceptive.
Finger length
Males' index fingers are usually 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. Some cultures highlight this preference through practices such as foot binding in China.

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