Sexual selection and physical attractiveness

Sexual selection processes have received much attention in recent years, attention reflected in interest in human mate preferences. Among these mate preferences are preferences for physical attractiveness. Preferences in and of themselves, however, do not fully explain the nature of the relationships that individuals attain. A tacit negotiation process underlies relationship formation and maintenance. The notion that preferences for physical attractiveness evolved under parasite-driven “good genes” sexual selection leads to predictions about the nature of trade-offs that individuals make between mates’ physical attractiveness and investment potential. These predictions and relevant data are explored, with a primary emphasis on women’s preferences for men’s qualities. In addition, further implications of trade-offs are examined, most notably (a) the impact of environmental variations on the nature of mating and (b) some effects of trade-offs on infidelity and male attempts to control women. The ideas in this paper were substantially influenced by discussions with Kim Hill and Hilly Kaplan following a preliminary presentation of work contained herein at a UNM Human Evolutionary Ecology Program colloquium. Steven W. Gangestad is an associate professor of psychology at the University of New Mexico. His recent research includes work focused on sexual selection in humans and its implications for general relationship phenomena. His other recent research concerns the impact of developmental instability on functional asymmetries, interpersonal orientations, and individual differences in the control of emotional expression.     

Face and voice attractiveness judgments change during adolescence

Attractiveness judgments are thought to underpin adaptive mate choice decisions. We investigated how these judgments change during adolescence when mate choice is becoming relevant. Adolescents aged 11–15 evaluated faces and voices manipulated along dimensions that affect adults' judgments of attractiveness and that are thought to cue mate value. Facial stimuli consisted of pairs of faces that were more or less average, more or less feminine, or more or less symmetric. The adolescents selected the more average, symmetric, and feminine faces as more attractive more often than chance, but judgments of some facial traits differed significantly with rater age and sex, indicating a role of development in judgments of facial cues. Vocal stimuli consisted of pairs of voices manipulated to raise or lower perceived pitch. The older but not younger girls selected the lower-pitched male voices as more attractive at rates above chance, while the younger but not older boys selected the higher-pitched female voices as more attractive. Controlling for rater age, increased pubertal development was associated with increased selection of lower-pitched boys' voices by girls and decreased selection of feminized male faces by boys. Our results are the first to demonstrate that adolescents show somewhat similar attractiveness judgments to adults in age-matched stimuli and that age, sex, and pubertal development have measurable effects on adolescents' attractiveness judgments. They suggest that attractiveness judgments in humans, at least for some traits, are facultatively calibrated to the individual's life stage, only reaching adult values upon sexual maturity when mate choice decisions become relevant.     

Populational differences in attractiveness judgements of male and female faces: Comparing British and Jamaican samples

In the UK and Japan, both men and women prefer somewhat feminised opposite-sex faces, especially when choosing a long-term partner. Such faces are perceived as more honest, caring, and sensitive; traits that may be associated with successful male parental investment. By contrast, women prefer less feminised faces for short-term relationships and when they are near ovulation. As genetic quality may be associated with facial masculinity, women may ‘trade-off’ cues between genetic quality and paternal investment in potential partners. No analogous trade-off has been suggested to influence men's preferences, as both attributions of prosociality and potential cues to biological quality are associated with facial femininity in female faces. Ecological and cultural factors may influence the balance of trade-offs leading to populational differences in preferences. We predicted that Jamaican women would prefer more masculine faces than British women do because parasite load is higher in Jamaica, medical care less common (historically and currently), and male parental investment less pronounced. Male preferences, however, were predicted to vary less cross-culturally, as no trade-off has been identified in female facial characteristics. We constructed masculinised and feminised digital male and female face stimuli of three populations (Jamaican, Japanese, and British) and presented them to men and women in Jamaica and in Britain. The results demonstrated that Jamaican women preferred more masculine male faces than their British counterparts did. Jamaican men tended to prefer more masculine female faces than did British men did, but this effect was complicated by an interaction suggesting that more feminised faces were preferred within culture.     

Commitment to relationships and preferences for femininity and apparent health in faces are strongest on days of the menstrual cycle when progesterone level is high

Previous studies of changes in women's behavior during the menstrual cycle have offered insight into the motivations underpinning women's preferences for social cues associated with possible direct benefits (e.g., investment, low risk of infection) and indirect benefits (e.g., offspring viability). Here we sought to extend this work by testing for systematic variation in women's preferences for male and female faces and in their attitudes to their romantic relationship during the menstrual cycle. In Study 1, we found partnered women's reported commitment to their romantic relationship and preferences for femininity in male and female faces were strongest on days of the menstrual cycle when progesterone levels are increased (and fertility is low). Happiness in relationships did not change across the cycle. In Study 2, we found that the effect of cycle phase on women's preference for feminine faces was independent of increased attraction to apparent health in faces during the luteal phase. Collectively, these findings are further evidence that women's preferences for social cues associated with possible direct benefits and commitment to relationships are strongest during conditions characterized by raised progesterone level, while attraction to men displaying cues associated with possible indirect benefits is strongest when women are most fertile.     

Chronic exposure of cat odor enhances aggression,urinary attractiveness and sex pheromones of mice

To test whether predator odor exposure negatively affects the behavior of prey, we exposed three groups of male house mice (Mus musculus) to the odors of cat (Felis catus) urine, rabbit (Oryctolagus cuniculus) urine and water (control), respectively, for consecutive 58 days and investigated how the treatments affected the response, aggressiveness, dominance, urinary attractiveness to females and pheromone composition of male mice. Compared to mice exposed to rabbit urine or water, those exposed to cat odor did not show any response habituation to the cat odor and became more aggressive, increased mark urine production and were more attractive to females when the latter were tested with their urine. Furthermore, gas chromatography coupled with mass spectrometry analysis revealed coincident elevations of the well-known male pheromones, E,E-α-farnesene, E-β-farnesene, R,R-dehydro-exo-brevicomin or S-2-sec-butyl-dihydrothiazole. In addition, rabbit urine exposure increased urinary attractiveness to females and pheromonal levels of the males in comparison with the mice exposed to water. This could be related to olfactory enrichment of heterospecific chemosignals, suggesting that predator odors were more beneficial. In light of these anti-intuitional findings in the chemical interaction between cats and mice, we conclude that predator odor affects prey more profoundly than previously believed and that its impact may not always be negative.     

The relative importance of orange spot coloration and total length of males in female guppy mate preference

Female mate choice by multiple male traits is an important current topic in animal behavior. However, the relative importance among the multiple cues in female choice is not explored in most cases. Female guppies Poecilia reticulata use both the color saturation of orange spots and the total length of males as mate choice criteria. In the present study, we used digitally modified video playbacks to examine the relative importance of these two male traits to female mate preferences. We initially examined the effective difference in the color saturation of orange spots as well as that in total length between two stimulus male images. Females only showed a strong preference for a bright male image (compared to the dull image) when the difference in color saturation was large (91% versus 25%). Conversely, females only exhibited a preference for larger size when they were presented a choice between two relatively small male images (total length 26.0 mm versus 23.0 mm). When two male images in which both the two traits were modified were presented to females, they prioritized male images possessing higher color saturation of orange spots, indicating the color saturation of male orange spots to be a more important factor than the total length in their mate choice. The color saturation of orange spots may convey more reliable information about the males to the females than their total lengths. These findings imply that females may rank multiple male criteria depending on relative benefits or costs derived from their mate choice based on each criterion.     

Ecological context and the importance of body and gnathopod size for pairing success in two amphipod ecomorphs

Ecological context generates interspecific variation in mating behavior by imposing differential constraints on the action of sexual selection, but operation of these constraints in nature is not well understood. We used field and laboratory studies to examine the importance of body size and size of sexually dimorphic appendages, the gnathopods, for pairing success in two freshwater amphipod species within the Hyalella azteca species complex. We focused on a large-bodied species found in habitats where ecological factors tend to favor large body size, and a small-bodied species in habitats where small body size is favored by size-selective predation. A field study indicated that although male pairing success was greater for larger males in both species, pairing success increased throughout the range of variation in male size in the large species, whereas, in the small species, pairing success was low for smaller individuals, but roughly constant across intermediate to larger sizes. A laboratory mate choice experiment was consistent with the field study, finding that females of the large species exhibited a preference for larger males that was independent of absolute male size, but females of the small species were indifferent when choosing between males of intermediate to larger size. Species also differed in the direction of sexual size dimorphism in the field, with males being the larger sex in the large species but the smaller sex in the small species, a pattern consistent with the species differences in mate preference. Large gnathopod size relative to body size was associated with enhanced pairing success across all body sizes for the large species, but, in the small species, large gnathopod size enhanced pairing success only in smaller males. Species differences in mating behavior appear consistent with change driven by differing forms of the interaction between sexual and natural selection.     

Different impacts of resources on opposite sex ratings of physical attractiveness by males and females

Parental investment hypotheses regarding mate selection suggest that human males should seek partners featured by youth and high fertility. However, females should be more sensitive to resources that can be invested on themselves and their offspring. Previous studies indicate that economic status is indeed important in male attractiveness. However, no previous study has quantified and compared the impact of equivalent resources on male and female attractiveness. Annual salary is a direct way to evaluate economic status. Here, we combined images of male and female body shape with information on annual salary to elucidate the influence of economic status on the attractiveness ratings by opposite sex raters in American, Chinese and European populations. We found that ratings of attractiveness were around 1000 times more sensitive to salary for females rating males, compared to males rating females. These results indicate that higher economic status can offset lower physical attractiveness in men much more easily than in women. Neither raters' BMI nor age influenced this effect for females rating male attractiveness. This difference explains many features of human mating behavior and may pose a barrier for male engagement in low-consumption lifestyles.     

Sexual selection and magic traits in speciation with gene flow

The extent to which sexual selection is involved in speciation with gene flow remains an open question and the subject of much research.Here,we propose that some insight can be gained from considering the concept of magic traits (i.e.,traits involved in both reproductive isolation and ecological divergence).Both magic traits and other,“non-magic”,traits can contribute to speciation via a number of specific mechanisms.We argue that many of these mechanisms are likely to differ widely in the extent to which they involve sexual selection.Furthermore,in some cases where sexual selection is present,it may be prone to inhibit rather than drive speciation.Finally,there are a priori reasons to believe that certain categories of traits are much more effective than others in driving speciation.The combination of these points suggests a classification of traits that may shed light on the broader role of sexual selection in speciation with gene flow.In particular,we suggest that sexual selection can act as a driver of speciation in some scenarios,but may play a negligible role in potentially common categories of magic traits,and may be likely to inhibit speeiation in common categories of non-magic traits.     

Patterns of subcutaneous fat deposition and the relationship between body mass index and waist-to-hip ratio: Implications for models of physical attractiveness

Body mass index (BMI) and waist-to-hip ratio (WHR) are two widely used anthropometric indices of body shape argued to convey different information about health and fertility. Both indices have also been shown to affect attractiveness ratings of female bodies. However, BMI and WHR are naturally positively correlated, complicating studies designed to identify their relative importance in predicting health and attractiveness outcomes. We show that the correlation between BMI and WHR depends on the assumed model of subcutaneous fat deposition. An additive model, whereby fat is added to the waist and hips at a constant rate, predicts a correlation between BMI and WHR because with increasing fat, the difference between the waist and hips becomes smaller relative to total width. This model is supported by longitudinal and cross-sectional data. We parameterised the function relating WHR to BMI for white UK females of reproductive age, and used this function to statistically decompose body shape into two independent components. We show that judgements of the attractiveness of female bodies are well explained by the component of curvaceousness related to BMI but not by residual curvaceousness. Our findings resolve a long-standing dispute in the attractiveness literature by confirming that although WHR appears to be an important predictor of attractiveness, this is largely explained by the direct effect of total body fat on WHR, thus reinforcing the conclusion that total body fat is the primary determinant of female body shape attractiveness.     

Pelvic dimorphism in relation to body size and body size dimorphism in humans

Many mammalian species display sexual dimorphism in the pelvis, where females possess larger dimensions of the obstetric (pelvic) canal than males. This is contrary to the general pattern of body size dimorphism, where males are larger than females. Pelvic dimorphism is often attributed to selection relating to parturition, or as a developmental consequence of secondary sexual differentiation (different allometric growth trajectories of each sex). Among anthropoid primates, species with higher body size dimorphism have higher pelvic dimorphism (in converse directions), which is consistent with an explanation of differential growth trajectories for pelvic dimorphism. This study investigates whether the pattern holds intraspecifically in humans by asking: Do human populations with high body size dimorphism also display high pelvic dimorphism? Previous research demonstrated that in some small-bodied populations, relative pelvic canal size can be larger than in large-bodied populations, while others have suggested that larger-bodied human populations display greater body size dimorphism. Eleven human skeletal samples (total N: male = 229, female = 208) were utilized, representing a range of body sizes and geographical regions. Skeletal measurements of the pelvis and femur were collected and indices of sexual dimorphism for the pelvis and femur were calculated for each sample [ln(M/F)]. Linear regression was used to examine the relationships between indices of pelvic and femoral size dimorphism, and between pelvic dimorphism and female femoral size. Contrary to expectations, the results suggest that pelvic dimorphism in humans is generally not correlated with body size dimorphism or female body size. These results indicate that divergent patterns of dimorphism exist for the pelvis and body size in humans. Implications for the evaluation of the evolution of pelvic dimorphism and rotational childbirth in Homo are considered.     

The relative importance of lethal and non-lethal effects of fish on insect colonisation of ponds

1. We hypothesised that adult insects actively monitor potential habitats for the presence of fish by means of chemical cues and avoid sites that pose significant risks. This was examined by quantifying colonisation of insects in outdoor pools with no fish (controls), fish (direct predation effect) or caged fish (chemical predator cues).
2. A significant direct effect of predation was found, but no indirect effect (avoidance of chemical cue pools), on the total biomass of colonising insects. However, predatory insects avoided fish-cue pools, thus releasing non-predatory insects from predation. This resulted in significantly greater biomass of non-predatory insects in fish-cue pools than control pools.
3. Fish reduced the number of species of colonising insects in pools through predation. This negative influence of fish implies that caution is necessary when stocking wetlands and ponds with fish if the goal is to maximise biodiversity.
4. Our data suggest that although predatory aquatic insects may use chemical signals to assess the quality of potential habitats with respect to predation risk, direct predation is the main method by which fish affect insect assemblages in ponds. Because fish and invertebrate predators may both have strong effects on prey mortality, behavioural adjustment by insects to the actual predator regime within a habitat should be more important than avoiding colonisation of habitats with fish.     

The relative body weights of Bornean and Sumatran orangutans.

Although original field data suggested that the average Bornean orangutan weighed twice as much as the Sumatran ones, this is shown to be an error. The only statistically significant difference in weight is between males and females, regardless of geographical area or data source.     

The role of amplifiers in sexual selection: An integration of the amplifying and the Fisherian mechanisms

Summary In a non-Fisherian genetic model I have shown that sexual displays can evolve even if displays are not directly and unconditionally preferred by females (a basic requirement in any Fisherian model), provided that they amplify previously recognized differences in male quality. Here I show how this amplifying mechanism interacts with the traditional Fisherian mechanism of sexual selection. The theory that integrates these two mechanisms provides a more robust, entirely selective scenario of the evolution of mating preferences and sexual displays.     

The estimation of body mass index and physical attractiveness is dependent on the observer's own body mass index

A disturbance in the evaluation of personal body mass and shape is a key feature of both anorexia and bulimia nervosa. However, it is uncertain whether overestimation is a causal factor in the development of these eating disorders or is merely a secondary effect of having a low body mass. Moreover, does this overestimation extend to the perception of other people''s bodies? Since body mass is an important factor in the perception of physical attractiveness, we wanted to determine whether this putative overestimation of self body mass extended to include the perceived attractiveness of others. We asked 204 female observers (31 anorexic, 30 bulimic and 143 control) to estimate the body mass and rate the attractiveness of a set of 25 photographic images showing people of varying body mass index (BMI). BMI is a measure of weight scaled for height (kg m(- 2)). The observers also estimated their own BMI. Anorexic and bulimic observers systematically overestimated the body mass of both their own and other people''s bodies, relative to controls, and they rated a significantly lower body mass to be optimally attractive. When the degree of overestimation is plotted against the BMI of the observer there is a strong correlation. Taken across all our observers, as the BMI of the observer declines, the overestimation of body mass increases. One possible explanation for this result is that the overestimation is a secondary effect caused by weight loss. Moreover, if the degree of body mass overestimation is taken into account, then there are no significant differences in the perceptions of attractiveness between anorexic and bulimic observers and control observers. Our results suggest a significant perceptual overestimation of BMI that is based on the observer''s own BMI and not correlated with cognitive factors, and suggests that this overestimation in eating-disordered patients must be addressed directly in treatment regimes.     

Some implications of relative biomechanical neck length in hominid femora

Demonstration of a size-based influence on relative biomechanical neck length of the femur predicts relatively longer necks for smaller femurs. Fossil hominids through the middle Pleistocene appear to have relatively longer femur necks than expected from this relation, excepting the two small australopithecine females. It is suggested that this variation results from smaller crania at birth in the fossils, and the possibility is raised that australopithecine populations were characterized by marked brain size differences at birth.     

Sexual selection and natural selection in body size differentiation of subterranean mole rats

We exlored indirectly, the operation of sexual selection in subterranean mole rats of the Spalax ebrenbergi superspecies in Israel comprising four chromosomal species, 2n = 52, 54, 58 and 60. We reanalzed two previously available data sets of 1. body size differentiation (Nevo et al. 1986a) and 2. the intensity of “Total Aggression” in mole rats (Nevo et al. 1986b). We correlated the mean size difference between the two sexes, in each of the 12 populations of the chromosomal species, with the mean level of agression, and with climatic factors, both displaying significant correlations. The results indicated that for 2n = 52, 54, 58 and 60, the population averae difference in body weight between the sexes decreases southward as follows: 37.7g (30.8 % of females body weight), 39.3g(29.0%) 26.3g(22.8%) and 20.3g (19.3%), respectively. We interpret the higher body size diherential ketween the sexes in the north as due to sexual selection.     

Contest competition and men's facial hair: beards may not provide advantages in combat

In contemporary human societies, where direct male-male competition is reduced compared to ancestral societies, sporting competitions remain an avenue for status acquisition via intra-sexual competition. Beards are the most visually salient and sexually dimorphic of men's secondary sexual traits and play a strong role in communicating masculinity, dominance and aggressiveness intra-sexually. Hypotheses have been advanced that beards provide advantages in intra-sexual combat, as protective organs and honest signals of fighting ability. Here we provide the first test of these hypotheses using data from professional mixed martial arts fighters competing in the Ultimate Fighting Championship. We explored whether secondary sexual traits (height, weight, beardedness), fighting stance (southpaw, orthodox), arm reach and past contest experiences impact on contest outcomes. If beards function as protective organs, bearded fighters should succumb to fewer knock-outs, and hence protection to injuries to the jaw, fewer abrasions and lacerations to the face and brain damage than clean-shaven fighters. Alternatively, if beards signal fighting ability then bearded fighters should win more fights. We found no evidence that beardedness was associated with fewer losses by knock-out or greater fighter ability. While fighters with longer reaches won more fights, neither stance nor past experience influenced fight outcomes. We suggest that beards represent dishonest signals of formidability that may serve to curtail the escalation of intra-sexual conflict through intimidation rather than providing advantages in direct combat.