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
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'.
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. 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.
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.
Factors affecting honest signalling
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). 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.
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. 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. For example, malnutrition in females suffering from anorexia nervosa can lead to amenorrhea (absence of menstruation).
Functions of female sexual ornamentation
Long term pair bonding
Female ornaments were likely selected to facilitate long term pair bonding. 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.
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). This behaviour has two possible causes:
- 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.
- Females fight for ecological resources for their offspring, (non-sexual resources). These include material benefits, food, care, protection, status etc. Female-female competition over non-sexual resources increases with age. In many species, also among humans, these kind of resources can be the most desirable.
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. For example, attractive female applicants are deemed more suitable for a typically feminine job where physical attractiveness is important (e.g., secretary, dietician etc...) On the other hand, they are deemed less suitable when attractiveness is not as important (e.g., nurse, social worker, bank teller). Moreover, woman’s attractiveness also increased her suitability for typically masculine roles, in which she can exploit her attractiveness (e.g. a car salesperson, sales manager). 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.
Female sexual ornaments allow them to find the best mate and increase the quality of their offspring. 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. This creates competition between males over the mate with the best ornamentation.
Examples in humans
Breasts are gynoid fat reserves used to provide nourishment to offspring. Gynoid fat acts as an indicator of fertility and future reproductive value as a higher proportion of gynoid fat predicts the likeliness of a successful pregnancy. Cross-culturally males show a preference for breasts which appear youthful and firm, not necessarily those which are the largest. Breasts which contain higher levels of android fats may also be large, however these are deemed less attractive by males.
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 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.
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. Studies also suggest greater facial symmetry reflects good health. 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.
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. 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 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. Furthermore, women with a relatively symmetric face were rated as having more attractive voices than these more asymmetric faces. Men prefer faces of women with higher pitched voices who are typically younger, highlighting their fertility.
Other ornaments include gait, a sex-typical finger length and size of foot. However, these are not always honest signals and can be deceptive.
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.
Size of feet
Research has found that women with smaller feet are more attractive than those with larger feet. Some cultures highlight this preference through practices such as foot binding in China.
- Biological ornaments
- Human sexuality
- Mate choice
- Multiple sexual ornaments
- Sex hormones
- Sexual dimorphism
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