Facial attractiveness is related to women's cortisol and body fat,but not with immune responsiveness

Recent studies suggest that facial attractiveness indicates immune responsiveness in men and that this relationship is moderated by stress hormones which interact with testosterone levels. However, studies testing whether facial attractiveness in women signals their immune responsiveness are lacking. Here, we photographed young Latvian women, vaccinated them against hepatitis B and measured the amount of specific antibodies produced, cortisol levels and percentage body fat. Latvian men rated the attractiveness of the women''s faces. Interestingly, in women, immune responsiveness (amount of antibodies produced) did not predict facial attractiveness. Instead, plasma cortisol level was negatively associated with attractiveness, indicating that stressed women look less attractive. Fat percentage was curvilinearly associated with facial attractiveness, indicating that being too thin or too fat reduces attractiveness. Our study suggests that in contrast to men, facial attractiveness in women does not indicate immune responsiveness against hepatitis B, but is associated with two other aspects of long-term health and fertility: circulating levels of the stress hormone cortisol and percentage body fat.     

Darwinian aesthetics: sexual selection and the biology of beauty

Current theoretical and empirical findings suggest that mate preferences are mainly cued on visual, vocal and chemical cues that reveal health including developmental health. Beautiful and irresistible features have evolved numerous times in plants and animals due to sexual selection, and such preferences and beauty standards provide evidence for the claim that human beauty and obsession with bodily beauty are mirrored in analogous traits and tendencies throughout the plant and animal kingdoms. Human beauty standards reflect our evolutionary distant and recent past and emphasize the role of health assessment in mate choice as reflected by analyses of the attractiveness of visual characters of the face and the body, but also of vocal and olfactory signals. Although beauty standards may vary between cultures and between times, we show in this review that the underlying selection pressures, which shaped the standards, are the same. Moreover we show that it is not the content of the standards that show evidence of convergence--it is the rules or how we construct beauty ideals that have universalities across cultures. These findings have implications for medical, social and biological sciences.     

Familiarity adds to attractiveness in matters of siskin mate choice

There is currently considerable controversy in evolutionary ecology revolving around whether social familiarity brings attraction when a female chooses a mate. The topic of familiarity is significant because by avoiding or preferring familiar individuals as mates, the potential for local adaptation may be reduced or favoured. The topic becomes even more interesting if we simultaneously analyse preferences for familiarity and sexual ornaments, because when familiarity influences female mating preferences, this could very significantly affect the strength of sexual selection on male ornamentation. Here, we have used mate-choice experiments in siskins Carduelis spinus to analyse how familiarity and patterns of ornamentation (i.e. the size of wing patches) interact to influence mating success. Our results show that females clearly prefer familiar individuals when choosing between familiar and unfamiliar males with similar-sized wing patches. Furthermore, when females were given the choice between a highly ornamented unfamiliar male and a less ornamented familiar male, half of the females still preferred the socially familiar birds as mates. Our finding suggests that male familiarity may be as important as sexual ornaments in affecting female behaviour in mate choice. Given that the potential for local adaptation may be favoured by preferring familiar individuals as mates, social familiarity as a mate-choice criterion may become a potential area of fruitful research on sympatric speciation processes.     

A relationship between attractiveness and performance in professional cyclists

Females often prefer to mate with high quality males, and one aspect of quality is physical performance. Although a preference for physically fitter males is therefore predicted, the relationship between attractiveness and performance has rarely been quantified. Here, I test for such a relationship in humans and ask whether variation in (endurance) performance is associated with variation in facial attractiveness within elite professional cyclists that finished the 2012 Tour de France. I show that riders that performed better were more attractive, and that this preference was strongest in women not using a hormonal contraceptive. Thereby, I show that, within this preselected but relatively homogeneous sample of the male population, facial attractiveness signals endurance performance. Provided that there is a relationship between performance-mediated attractiveness and reproductive success, this suggests that human endurance capacity has been subject to sexual selection in our evolutionary past.     

Cuticular hydrocarbons influence female attractiveness to males in the Australian field cricket,Teleogryllus oceanicus

Sexual dimorphism is thought to result from directional sexual selection acting on male signal traits, with female signal traits given little, if any, attention. Here, we examine male mating preferences in the Australian field cricket, Teleogryllus oceanicus. Using a multivariate selection analysis approach, we found that male preferences have the potential to exert selection on female cuticular hydrocarbons, chemical compounds widely used as sexual signals in insects. In addition to finding both stabilizing and disruptive preference gradients, we also found weak negative directional preference for female cuticular hydrocarbons. We contrast our results with a recent study examining sexual selection via female choice on male T. oceanicus cuticular hydrocarbons and suggest that differences in the form and intensity of sexual selection between the genders may provide part of the net selection differential necessary for the evolution of sexual dimorphism in this species.     

Facultative adjustment of the offspring sex ratio and male attractiveness: a systematic review and meta‐analysis

Females can benefit from mate choice for male traits (e.g. sexual ornaments or body condition) that reliably signal the effect that mating will have on mean offspring fitness. These male‐derived benefits can be due to material and/or genetic effects. The latter include an increase in the attractiveness, hence likely mating success, of sons. Females can potentially enhance any sex‐biased benefits of mating with certain males by adjusting the offspring sex ratio depending on their mate's phenotype. One hypothesis is that females should produce mainly sons when mating with more attractive or higher quality males. Here we perform a meta‐analysis of the empirical literature that has accumulated to test this hypothesis. The mean effect size was small (r = 0.064–0.095; i.e. explaining <1% of variation in offspring sex ratios) but statistically significant in the predicted direction. It was, however, not robust to correction for an apparent publication bias towards significantly positive results. We also examined the strength of the relationship using different indices of male attractiveness/quality that have been invoked by researchers (ornaments, behavioural displays, female preference scores, body condition, male age, body size, and whether a male is a within‐pair or extra‐pair mate). Only ornamentation and body size significantly predicted the proportion of sons produced. We obtained similar results regardless of whether we ran a standard random‐effects meta‐analysis, or a multi‐level, Bayesian model that included a correction for phylogenetic non‐independence. A moderate proportion of the variance in effect sizes (51.6–56.2%) was due to variation that was not attributable to sampling error (i.e. sample size). Much of this non‐sampling error variance was not attributable to phylogenetic effects or high repeatability of effect sizes among species. It was approximately equally attributable to differences (occurring for unknown reasons) in effect sizes among and within studies (25.3, 22.9% of the total variance). There were no significant effects of year of publication or two aspects of study design (experimental/observational or field/laboratory) on reported effect sizes. We discuss various practical reasons and theoretical arguments as to why small effect sizes should be expected, and why there might be relatively high variation among studies. Currently, there are no species where replicated, experimental studies show that mothers adjust the offspring sex ratio in response to a generally preferred male phenotype. Ultimately, we need more experimental studies that test directly whether females produce more sons when mated to relatively more attractive males, and that provide the requisite evidence that their sons have higher mean fitness than their daughters.     

Women's mating strategies

What does a woman want? The traditional evolutionist's answer to Freud's famous query is that a woman's extensive investment in each of her children implies that she can maximize her fitness by restricting her sexual activity to one, or at most, a few high-quality males. Because acquiring resources for her offspring is of paramount importance, a woman will try to attract wealthy, high-status men who are willing and able to help her. She must be coy and choosy, limiting her attentions to men who are worthy of her and emphasizing her chastity so as not to threaten the paternity confidence of her mate. The lady has been getting more complicated of late, however. As Sarah Hrdy¹ predicted, we now have evidence that women, like other female primates, are also competitive, randy creatures. Women have been seen competing with their rivals using both physical aggression^2,3 and more subtle derogation of competitors.⁴ While they are still sometimes coy and chaste, women have also been described recently as sexy and sometimes promiscuous creatures, manipulating fatherhood by the timing of orgasm^5,6 and using their sexuality to garner resources from men. The real answer to Freud's query, of course, is that a woman wants it all; a man with the resources and inclination to invest, and with genes that make him attractive to other women so that her sons will inherit his success. Her strategies for attaining these somewhat conflicting aims, and her success in doing so, are shaped by her own resources and options and by conflicts of interest with men and other women.     

The evolution of filial cannibalism and female mate choice strategies as resolutions to sexual conflict in fishes

Filial cannibalism (the consumption of one's own viable offspring) is common among fish with paternal care. In this study, I use a computer simulation to study simultaneous evolution of male filial cannibalism and female mate choice. Under certain conditions, selection on parental males favors filial cannibalism. When filial cannibalism increases a male's probability to raise the current brood successfully, filial cannibalism also benefits the female. However, when egg eating is a male investment into future reproduction, a conflict between female and male interests emerges. Here I investigate how female discrimination against filial cannibals affects evolution of filial cannibalism and how different female choice criteria perform against filial cannibalism. The introduction of discriminating females makes the fixation of filial cannibalism less likely. I introduced three different female choice criteria: (1) females who could discern a male's genotype, that is, whether the male was going to eat eggs as an investment in future reproductive events; (2) energy-choosing females that preferred to mate with males who had enough energy reserves to live through the current brood cycle without consuming eggs; and (3) females that preferred to mate with already mated males, that is, males with eggs in their nest. Genotype choice never coexisted with filial cannibals at fixation and filial cannibals were unable to invade a population with genotype-choosing females. Energy choice was successful only when males had high energy reserves and were less dependent on filial cannibalism as an alternative energy source. The egg choosers frequently coexisted with the cannibals at fixation. When the female strategies were entered simultaneously, the most frequent outcome for low mate sampling costs was that both the cannibals and the egg choice was fixed and all other strategies went extinct. These results suggest that sexual conflicts may not always evolve toward a resolution of the conflict, but sometimes the stable state retains the conflict. In the present case, this was because the egg-preference strategy had a higher fitness than the other female strategies. The outcome of this simulation is similar to empirical findings. In fish with paternal care, male filial cannibalism and female preference for mates with eggs commonly co-occur.     

An evolutionary perspective on physical attractiveness

Everyday experience suggests that physical attractiveness is important in personal—and especially sexual—relationships. This impression is confirmed by a large body of social psychological research.^1,2 Cross-cultural surveys and ethnographic accounts show that concern with the attractiveness of potential mates is also common in non-Western societies and in tribal and peasant cultures.³ However, social psychologists and anthropologists have often had a hard time explaining why attractiveness should count for so much, or why some features rather than others should seem particularly attractive. The theoretical difficulties in accounting for physical attraction are brought out in a Brazilian saying, “Beleza nâo pôe na mesa” (“Good looks don't put anything on the table”), which points to the absence of any evident practical advantage to choosing an attractive mate. Faced with these difficulties, a growing number of researchers in biology, psychology, and anthropology have turned to the modern theory of sexual selection, which has been highly successful in explaining nonhuman animals attractions to traits of no direct ecological utility. In this article, I survey recent efforts to apply the theory of sexual selection to human physical attraction.     

Male courtship attractiveness and paternity success in Photinus greeni fireflies

Although female mate choice and male sperm competition have separately attracted much attention, few studies have addressed how precopulatory and postcopulatory episodes of sexual selection might interact to drive the evolution of male traits. In Photinus fireflies, females preferentially respond to males based on their bioluminescent courtship signals, and females gain direct benefits through male nuptial gifts acquired during multiple matings over several nights. We experimentally manipulated matings of P. greeni fireflies to test the hypothesis that postcopulatory paternity success might be biased toward males that are more attractive during courtship interactions. We first measured male courtship attractiveness to individual females using field behavioral assays. Females were then assigned to two double-mating treatments: (1) least attractive second male-females were first mated with their most attractive male, followed by their least attractive male, or (2) most attractive second male-females mated with males in reverse order. Larval offspring produced by each female following these double matings were genotyped using random amplified polymorphic DNA (RAPD) markers, and male paternity was determined. Contrary to prediction, firefly males that were more attractive to females based on their bioluminescent courtship displays subsequently showed significantly lower paternity, reflecting possible male trade-offs or sexual conflict. Differences in male paternity were not related to male body condition, testes or accessory gland mass, or to variation in female spermathecal size. Additionally, this study suggests that changes in phenotypic selection gradients may occur during different reproductive stages. These results indicate that it is crucial for future studies on sexual selection in polyandrous species to integrate both precopulatory and postcopulatory episodes to fully understand the evolution of male traits.     

Fixed and dilutable benefits: female choice for good genes or fertility

Benefits accruing to females who exercise mate choice have been defined to be either 'direct' or 'indirect'. We suggest an alternative distinction: benefits can be considered 'fixed', meaning they are on average equal to all females mating with the same male (e.g. good genes' benefits) or 'dilutable', meaning they are shared between females mating with the same male, so that the more mates a male has, the lower the average benefit to each (e.g. fertility benefits or many forms of direct benefit). Using a simple model, we show that this distinction has a major effect on the form of female preference. We predict that mating skew will be far greater in species where the benefits are fixed when compared with those where the benefits are dilutable.     

Female mate choice across mating stages and between sequential mates in flour beetles

Few studies have examined how female premating choice correlates with the outcome of copulatory and post-copulatory processes. It has been shown that polyandrous Tribolium castaneum females discriminate among males before mating based on olfactory cues, and also exert cryptic choice during mating through several mechanisms. This study tested whether a male's relative attractiveness predicted his insemination success during copulation. Bioassays with male olfactory cues were used to rank two males as more and less attractive to females; each female was then mated to either her more attractive male followed by less attractive male, or vice versa. Dissections immediately after second copulations revealed a significantly higher percent of successful inseminations for females that remated with more attractive males compared with those that remated with less attractive males. These results indicate that cryptic female choice during copulation reinforces precopulatory female choice in T. castaneum, and suggest that females could use cryptic choice to trade up to more attractive males, possibly gaining better phenotypic or genetic quality of sires.     

The mating system of the brown bear Ursus arctos

• 1 Research on mating systems and reproductive strategies is valuable for providing ethological knowledge, important for the management and conservation of a species, and in a broader sense, important for biodiversity conservation.
• 2 We reviewed the literature to document the mating system of the brown bear Ursus arctos. We determined that many aspects of the reproduction of the brown bear remain unclear, including (i) biological aspects, such as hormone and oestrous cycling, sperm competition, mate choice, sexually selected infanticide, etc. and (ii) human impacts on the mating system, occurring when humans alter population size and structure, through, for example, hunting or habitat degradation.
• 3 We considered three mating system classification frameworks from the literature ( Emlen & Oring 1977 , Clutton‐Brock 1989 , Shuster & Wade 2003 ) and applied various brown bear populations to them. We did this (i) to document the plasticity of the mating system of the brown bear, and (ii) to find commonalities among the reported mating system classifications in order to provide a general and common classification of the brown bear's mating system.
• 4 The mating system of the brown bear can, in general, be classed as ‘polygamous’. Subclassifications can nevertheless be valuable on smaller spatial scales.
• 5 Within the polygamous mating system of the brown bear, biological aspects and human impacts can influence reproductive strategies at the individual and population level. Mating system classification frameworks often lack a common terminology, which contributes to the variety of published classifications of the mating system of the brown bear.

     

Color Assortative Mating in a Mainland Population of the Poison Frog Oophaga pumilio

Assortative mating is a reproductive strategy used by a diversity of animals, in which individuals choose a mate that shares similar characteristics. This mating strategy has the potential to promote the evolution of various sexual signals and has been a proposed mechanism driving and maintaining color variation in the anuran family Dendrobatidae. Most studies have examined this reproductive strategy in the polytypic poison frog, Oophaga pumilio, in the Bocas del Toro archipelago in Panama. Little attention, however, has been given to ancestral populations across this species’ mainland range, where dramatic color polytypism appears to lack. Additionally, most studies are exclusively experimental and investigate mate choice between allopatric populations, neglecting the behaviors of naturally occurring mates. This study observed natural mating pairs within a population of O. pumilio on mainland Costa Rica and tested the prediction that color phenotype of mating females and males would be correlated. Naturally occurring pairs were found to share similar coloration, suggesting that color assortative mating operates in nature, and in a mainland population. Our results indicate that coloration is an important trait in driving the natural mate choices of female O. pumilio, which provides valuable insight into realistic mate selection tactics of this dendrobatid frog.     

Mate‐sampling costs and sexy sons

Costly female mating preferences for purely Fisherian male traits (i.e. sexual ornaments that are genetically uncorrelated with inherent viability) are not expected to persist at equilibrium. The indirect benefit of producing ‘sexy sons’ (Fisher process) disappears: in some models, the male trait becomes fixed; in others, a range of male trait values persist, but a larger trait confers no net fitness advantage because it lowers survival. Insufficient indirect selection to counter the direct cost of producing fewer offspring means that preferences are lost. The only well‐cited exception assumes biased mutation on male traits. The above findings generally assume constant direct selection against female preferences (i.e. fixed costs). We show that if mate‐sampling costs are instead derived based on an explicit account of how females acquire mates, an initially costly mating preference can coevolve with a male trait so that both persist in the presence or absence of biased mutation. Our models predict that empirically detecting selection at equilibrium will be difficult, even if selection was responsible for the location of the current equilibrium. In general, it appears useful to integrate mate sampling theory with models of genetic consequences of mating preferences: being explicit about the process by which individuals select mates can alter equilibria.     

Male Mate Choice in Lemur catta

Though females are generally more selective in mate choice, males may also derive reproductive benefits from exercising mate selectivity if one or more factors limit male reproductive success and females differ in reproductive potential. I used male mating effort as a proxy for male mate choice in ring-tailed lemurs (Lemur catta). I calculated mating effort as the rate of male-male agonism during each female's estrous period 30 min before and 30 min after the first and last mountings with intromission. I collected data on 1 free-ranging Lemur catta troop during 2 consecutive breeding seasons on St. Catherines Island, USA. In both yrs, male mating effort differed significantly among troop females once I adjusted male-male agonistic rates to reflect agonistic intensity, and I corrected for the number of observed mates per female (2000: χ² = 27.43, df = 3, p < 0.0001; 2001: χ² = 21.10, df = 3, p < 0.001). Results strongly suggest male mate choice. Contrary to expectation, males did not expend the greatest mating effort for females with the highest dominance status nor the highest reproductive success. Males preferred females that either: (1) belonged to the age class in which fecundity and infant survival is the highest at this site (4–9 yrs), or 2) were older females (≥10 yrs) with high reproductive success. Female reproductive potential appears to be an important variable determining male mating effort in Lemur catta.     

Sexuelle Selektion und die Evolution von Kopulationen au?erhalb des Paarbundes: Spielregeln der Weibchen

Zusammenfassung Seit Darwins grundlegendem Werk von 1871 über sexuelle Selektion ist es kein Geheimnis mehr, daß der Pfau sein Rad schlagen kann, weil die Weibchen es so wollen. Anerkanntermaßen ist der Pfau polygam; dasjenige Männchen mit dem prächtigsten Gefieder hat die meisten Weibchen und den größten Reproduktionserfolg. Durch die Zuchtwahl der Weibchen (female choice) wurden die Männchen so schön, wie sie es heute sind. In monogamen Paarungssystemen konnte man auffällig gefärbte Männchen auf die gleiche Weise allerdings nicht erklären. Dies wurde erst möglich, seit sich die Erkenntnis durchgesetzt hat, daß auch bei monogamen Arten gewisse Männchen durch Fremdkopulationen mehr Nachkommen haben als andere. Erst seit wenigen Jahren ist bekannt, daß es häufig die Weibchen sind, die die aktive Rolle beim Vollzug von Seitensprüngen spielen. Welche selektiven Vorteile können Weibchen durch dieses Verhalten erlangen?Die basale Theorie der sexuellen Selektion wird auf der Grundlage klassischer Arbeiten besprochen. Darauf fußend wird ein überblick über die aktuelle Diskussion von Fremdkopulationen (extra-pair copulations [EPCs]) gegeben. Die für Weibchen möglichen Fitness-Gewinne, die zur Evolution von EPCs geführt haben können, werden zusammengefaßt. Ein Zusammenhang wird hergestellt unter dem Gesichtspunkt, wie sich EPCs jeweils auf den Gesamtfortpflanzungserfolg (total reproductive value) der Weibchen auswirken. Die Notwendigkeit wird erläutert, die theoretischen Ansätze aus einem übergeordneten Blickwinkel zu betrachten, ohne von vornherein die eine oder andere Erklärung auszuschließen.Als gelungenes Beispiel wird u. a. die Hypothese der eingeschränkten Weibchen (constrained female hypothesis) von Gowaty (1996) besprochen. Diese besagt, daß Weibchen zur Erlangung eines beliebigen zusätzlichen Fitness-Gewinns auf EPCs angewiesen sein können, da sie in ihrer Wahl eines passenden Partners oft behindert sind (z. B. durch die Männchen selber). Schließlich werden mögliche Ansatzpunkte zur Überprüfung der Hypothesen in Feldstudien aufgezeigt; zukünftig sollte vermehrt auch die Qualität der Weibchen als Grundlage für deren Verhaltensentscheidungen betrachtet werden.

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Sexual selection and the evolution of extra-pair copulation: rules of the game from the females' point of view

Summary In view of Darwin's fundamental thesis on sexual selection, which appeared in 1871, it is no longer a secret that a peacock can spread his tail because the females want it that way. It is generally acknowledged that the peacock is polygamous; whichever male has the most attractive plumage wins the most females and therefore achieves the greatest reproductive success. It is due to selective breeding by the females, i. e. female choice that the males have become what they are today. In monogamous mating systems, however, the source of striking colouring in males could not be explained the same way until evidence for the common occurrence of extra-pair copulation became more accepted. This mating behaviour offers an explanation of how certain males, even in monogamous species, can produce more offspring than others. Only in recent years was it recognised that it is often the females which play the active role in the initiation of extra-pair copulation. What fitness gains can the females expect to achieve through this behaviour?This review commences with an introduction to fundamental theories of sexual selection. Progressing from this, the current discussion of extra-pair copulation (EPC) is reviewed. Conceivable fitness gains for the females, which may have resulted in the evolution of EPC, are summarised. A connection is noted between the various possible fitness gains in their effect on the total reproductive value of the females. The necessity of considering all of these theories from a more general perspective, without having to dismiss any explanations from the outset, is made clear. Gowaty's constrained female hypothesis (1996) is one example in which this has been achieved. This hypothesis proposes that females can be obliged to engage in EPC in order to obtain any kind of extra fitness gain, since they are often constrained in their choice of a partner (e.g. by the males themselves). In conclusion, possible directions are suggested for the testing of these hypotheses in field studies; in future more emphasis should be put on intrinsic quality differences between the females while investigating their mating behaviour.

     

Ejaculate investment and attractiveness in the stalk‐eyed fly,Diasemopsis meigenii

The phenotype‐linked fertility hypothesis proposes that male fertility is advertised via phenotypic signals, explaining female preference for highly sexually ornamented males. An alternative view is that highly attractive males constrain their ejaculate allocation per mating so as to participate in a greater number of matings. Males are also expected to bias their ejaculate allocation to the most fecund females. We test these hypotheses in the African stalk‐eyed fly, Diasemopsis meigenii. We ask how male ejaculate allocation strategy is influenced by male eyespan and female size. Despite large eyespan males having larger internal reproductive organs, we found no association between male eyespan and spermatophore size or sperm number, lending no support to the phenotype‐linked fertility hypothesis. However, males mated for longer and transferred more sperm to large females. As female size was positively correlated with fecundity, this suggests that males gain a selective advantage by investing more in large females. Given these findings, we consider how female mate preference for large male eyespan can be adaptive despite the lack of obvious direct benefits.