Why would a human female have a sexual display despite male biased operational sex ratio?

Why would a human female have a sexual display despite male biased operational sex ratio?

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why would a human female have a sexual display despite male-biased operational sex ratio?

In human society there is a male-biased sex ratio, due to female child rearing, which results from anisogamy. Human females sometimes expend great amounts of energy and resources performing display acts and doing things to attempt to improve their apparent quality to males. Human males tend to be eager, despite a certain level of socially imposed monogamy, and many males will mate with a low quality female. Why do human women do sexual display behaviors, when there are so many eager males to choose from?

Sexual arms race.

There is a fine and internally consistent theory that this strategy by females evolved to oppose the strategy of infanticide by males. Males of many species including primates and possibly humans can increase their own genetic fitness by killing infants that are not their own. This frees up attention and physical resources of the female to instead have the baby of the infanticidal male. Lions are famous for this. But this is in opposition to the genetic fitness interest of the female, who just invested a bunch of efforts gestating and rearing an infant who was then killed.

My favorite writer on this is Sarah Hrdy. From the Wikipedia article on this

This is where the idea of sexual counter-strategies comes into play. Hrdy theorized that by mating with as many males as possible, particularly males who are not part of the colony, mothers are able to successfully protect their young, as males were unlikely to kill an infant if there was the slightest chance that it might be their own.

This explains why women have the sexual displays referred to in the OP. Other aspects of female human biology match. Humans keep big breasts so they always look like they are lactating (compared to other animals) - to keep males guessing. There is no external sign to indicate when a human is ovulating. Females of other species are receptive and mate when they are ovulating - humans can mate any time.

A strategy of keep the guys guessing. If one time he had sex with a given female, probably he shouldn't kill the kid. Maybe it is his?

An interesting (& extremely arguable!) corollary to this is the premise that human culture in its entirety is the male countermeasure to this prehistoric female defense against infanticide: that male dominated culture is a method to ensure paternity.

Based on what is currently known, I would say that while females might find sperm readily available, it is common for human males to not only provide sperm but also help rear the offspring (e.g., provide care, financial support, food, transportation). So, they might provide sperm to a woman they're not impressed by (by "impressed," I mean convinced the woman has whatever characteristic the sexual display is supposed to exhibit), but they only offer rearing investment to offspring of a woman they are impressed by. So, basically, the woman is making a demonstration in order to garner investment from the male. I think there's more to it than that, but this answer is based on what is currently known and robustly supported in the literature.

Evolution exam #2 Ch. 11

A female's reproductive success is often limited by the number of eggs that she can produce and provision. Females with access to the most resources generally achieve the highest egg (and offspring) numbers and have the highest fitness relative to other females.

Male reproductive success is often limited by the number of eggs that he can fertilize. Males who mate with the most females generally sire the greatest number of offspring and have the highest fitness relative to other males.

Biased operational sex ratios can generate strong sexual selection because the abundant sex (typically males) must compete over access to the limiting sex (typically females).

Males often fight with each other over access to females, and this behavior can generate strong
sexual selection for large body size, weapons, and aggression.

Males compete over harems (groups of females), resources required by females, or display arenas
(leks) visited by females, depending on the species.

Sexual selection affects the sex with the greater variance in reproductive success (usually males).

Intersexual selection (often ca lled female choice) occurs when members of the limiting sex (generally
females) actively discriminate among suitors of the less limited sex (generally males).


We thank Monique Borgerhoff Mulder, Caroline Uggla, Rebecca Sear and the Early Life Conditions Research group (Michael Hollingshaus, Alla Chernenko, Kelli Rasmussen and Heidi Hanson), for their helpful comments and suggestions. We also thank Zhe (David) Yu, Alison Fraser and Diana Lane Reed for invaluable assistance in managing and preparing the data. We are also grateful to Wissenschaftskolleg zu Berlin for funding a platform for us to present and receive valuable feedback on our work. Lastly, we wish to thank Douglas Tharp for his SAS guidance.

Myths of Sex Determination

Myth 1: Sex is typically determined by X and Y chromosomes

Many biologists are habituated to thinking about sex determination through the familiar examples of mammals and D. melanogaster, and assume that sex determination by sex chromosomes is the norm, that males are XY and females are XX, and that sex chromosomes are a stable component of the genome. While biologists are generally aware of other modes of sex determination (such as female heterogamety in birds, temperature-dependent sex determination in reptiles, or development of males from unfertilized eggs in bees), these alternatives are often viewed as strange and aberrant [8].

Myth 2: Sex is controlled by one master-switch gene

Sex determination in model species suggests that a master-switch gene (e.g. Sry in mammals, Sxl in D. melanogaster, and xol-1 in C. elegans) acts as the main control element to trigger either male or female sexual development. Changes in the sex determination pathways across taxa are assumed to involve adding a new master-switch gene to this molecular pathway (as in some fly taxa [9]), with little change to downstream elements of the sex determination pathway [10]. A few genes are thought to have the capacity to take on the role of sex determination genes, and these have been co-opted as master-switch genes independently in different lineages (for example, dmrt1 in several vertebrates [11]–[14] and tra in insects [15]–[17]).

Myth 3: Sex chromosome differentiation and degeneration is inevitable

Sex chromosomes originate from identical autosomes by acquiring a sex determination gene (for example, the origin of the Sry gene in mammals approximately 180 million years ago or Sxl in the Drosophila genus >60 million years ago). They are then thought to differentiate through an inevitable and irreversible process in which recombination between X and Y chromosomes is shut down and the Y degenerates (see Figure 1). Ultimately, Y chromosomes are fated to disappear entirely (“born to be destroyed,” [18]). Thus, sex chromosomes that are morphologically similar (homomorphic) must be evolutionarily young, and in time they too will degenerate.

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Study species

The common lizard is a ground-dwelling ovoviviparous lizard (adult snout–vent length [SVL]: 50–70 mm, SVL at hatching: 15–25 mm) that inhabits moist habitats across Eurasia. Individuals start hibernating in late September. Males become active from February/March and mating may occur one month later when females emerge. In our study area, individuals can reproduce once per year from the age of one year. Maximum female and male life span in natural populations are 11 and 7 years, respectively. Clutch size ranges from 1 to 12 eggs, depending partly on body size ( Boudjemadi et al. 1999). Hatchlings are independent at birth with no parental care after birth ( Massot et al. 1992). No nuptial gifts are provided ( Heulin 1988), and sperm has little effect on the nutrition of the young ( Depeiges et al. 1987).

Age affects individual performance by influencing current reproductive value and survival ( Ronce et al. 1998 Richard et al. 2005). Hereafter, we will use the term “adults” for individuals of at least 2 years of age and the term “yearlings” for 1-year-old individuals. Females display a period of senescence for annual survival, which decreases from about 50% to 30% at the age of 4 years. The fecundity of both sexes and the probability of survival for their offspring also increase until the age of 4 years and decrease thereafter. In females, annual fecundity is highest in 3- and 4-year-old individuals, whereas the 2- and 3-year-old individuals have the highest survival rates ( Ronce et al. 1998 Richard et al. 2005). Middle-aged individuals (between the ages of 2 and 4 years) thus have the highest level of performance, whereas yearlings and old individuals (5 years and older) perform less well.

Populations studied

The lizards used in this study originated from natural populations from the Cévennes area (1400–1600 m in altitude, lat 44°30′N, long 3°45′E) and had been kept for 3 years in seminatural conditions at the Ecological Research Station at Foljuif (60 m in altitude, lat 48°17′N, long 2°4′E). As these individuals were part of a long-term study, all were individually marked by toe clipping and the year of birth was known for most individuals. Previous studies have shown that confined populations have similar life-history traits and mating patterns to natural populations in terms of age at first reproduction, clutch size, and proportion of multiply sired clutches ( Boudjemadi et al. 1999 Laloi et al. 2004 Lecomte et al. 2004).

In June 2004, we captured individuals in holding enclosures. These individuals were then released to create 16 seminatural populations containing a mean of 50.6 ± 14.6 (mean ±standard deviation [SD]) individuals, including 25.2 ± 7.8 adults. Each seminatural population created was housed in an outdoor enclosure, 10 × 10 m in size. Enclosure size corresponded to adult individuals’ home range under natural conditions. As home ranges overlap to a great extent in this species, 30 adult individuals can share an area of similar size in natural conditions ( Massot et al. 1992 Boudjemadi et al. 1999 Lecomte et al. 2004). Thus, the densities of adults created in our experiment were similar to those observed in natural populations ( Lecomte and Clobert 1996). The sex ratio of the created populations (adult sex ratio [ASR], the proportion of adults that were male) was biased toward females (mean: 0.38 ± 0.04SD). This is generally the case in natural populations, although substantial spatial and temporal variations are observed (from 0.15 to 0.65, Le Galliard, Fitze, Cote, et al. 2005). The age structures of experimental populations were similar to those of natural populations, with a mean of 46% yearlings (range 0.38–0.57), a proportion similar to that found in several natural populations from the Cévennes area (43–65%, Massot et al. 1992 Meylan et al. 2007).

In early June 2005, all the surviving lizards were captured. Due to demographic stochasticity, survival rates from release to capture differed between populations, creating a continuous distribution of population density and sex ratio ( Table 1). Density (total number of individuals within an enclosure) and sex ratio in the 16 populations were indeed continuous and not correlated (Kendall correlation, tau-b = −0.02, P > 0.8 Table 1). This distribution provided us with an opportunity to analyze the correlations between both population density and sex ratio with reproductive output. Population density and sex ratio in May 2005 were thus used as independent continuous covariates in the analyses. During the breeding period studied, in May 2005, populations contained 198 females and 164 males. At the first capture of June 2005, we recorded body length (SVL) and body mass for all individuals. An index of body condition was calculated for males and females as the residual of the regression between body mass and body length. Individual body condition in June 2005 was not significantly related to age, population density, sex ratio, or any other second-order interaction, including age squared, density squared, or ASR squared (linear mixed models—see statistical analyses section for methods—with male and female body conditions as dependent variables and population as a random effect, all P values > 0.05). Age structure was similar between populations, with the proportion of each age class unrelated to density, sex ratio, or second-order interactions (linear mixed models with the proportions of each age class treated as dependent variables, all P values > 0.05).


The spatial distribution of individuals has been proposed as a means to quantify sexual selection ( Emlen and Oring 1977 Shuster and Wade 2003). Here we investigated whether 2 methods of measuring spatial distribution reflect mating competition, a major force in the evolution of mating systems. Our study shows that the spatial distribution only partly reflects sexual selection in the form of mating competition. We found that mean crowding behaved according to predictions in relation to increased mating competition (more female-biased OSR) for the most competitive sex. However, mean crowding did not reflect the predicted level of mating competition in males. Our measure of spatial association between the sexes (X), behaved in a predicted fashion to the manipulations of density and sex ratio, and even if this measure has not (to the best of our knowledge) been used in studies of animal behavior, it might be a useful measure of between-sex interactions on the population level. In general, we conclude that extensive knowledge about the behavioral ecology of the species is critical to interpret the mechanisms behind spatial patterns. Based on our findings, we recommend caution with the application of measures of spatial distribution in studies of sexual selection because we demonstrate that the spatial distribution of males and females does not necessarily reflect the predicted direction and strength of mating competition in all situations.

Essential Reads

Sexual Selection and Mate Choice

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Herbert Spencer first coined the phrase survival of the fittest in 1864, to describe Darwin's observations of natural selection. One way to quantify the fitness of an individual is in the number of offspring that it can contribute to the next generation - so an organism can improve its fitness by producing more offspring that are then able to successfully reproduce themselves. Reproduction is a critical element of fitness, and sexually-reproducing organisms must mate in order to pass their genes on to the next generation. This means that for sexually-reproducing organisms, competing for mates becomes a key part of fitness.

This pressure to find a mate leads to a phenomenon known as sexual selection - a type of selection that acts upon traits associated with mating and courtship. Typically, sexual selection is more intense for the sex that invests the least amount of energy in reproduction. Since sperm is energetically inexpensive to produce, males generally bear the brunt of sexual selection pressures. Conversely, eggs are energetically expensive to produce and then gestate. This means that females are expected to be more selective when choosing mates, and males must invest substantial resources in attracting females. This observation is related to the operational sex ratio, or the OSR - which is the ratio of sexually-mature males to fertile females. Often the OSR is male-biased, due to their longer reproductive lifespans.

Due to the often-intense sexual selection pressures males face, many species of animals display extravagant ornamentation that distinguishes males from females. When males and females of the same species appear phenotypically different, we call this sexual dimorphism. Sexual selection, however, isn't limited to only male ornamentation. There are two types of sexual selection, intersexual and intrasexual. In intersexual selection, one sex, typically the males, will display a certain trait or behavior with the goal of attracting and mating with the opposite sex. Intrasexual selection, on the other hand, occurs between members of the same sex. For example, male sea lions compete for dominance over rookeries of females. Here, intrasexual selection acts on the physiology of the sea lion, to make it large enough to compete with other males for a group of females. But with attributes like vibrant colors and larger size come costs. Like increased visibility to predators, or decreased immunity due to the overproduction of androgens. This means that natural selection and sexual selection are often at odds with each other, pulling traits in opposite directions. Because of this, individuals with exceptionally high- or low-quality scores are rare. Instead, individuals of average quality are expected to make up the majority of a population.

In this lab, you will perform simulations of mating scenarios in which females and males can select their mates based on varying amounts of information on their potential mate's quality.

Choosing Mates

Evolutionary fitness is largely determined by the ability of an organism to survive and successfully produce offspring. Critical to this process is sexual selection, which plays a large role in the determination of mating pairs, and thus which genes are passed on to the next generation. Often, intense competition for mates within a population places selective pressure on traits related to courtship and copulation. Natural selection that results from these pressures is called sexual selection. Sexually selected traits include characteristics like ornamentation or coloration for the sole purpose of attracting mates. These traits can also serve to enhance the distinction between males and females within a species, termed sexual dimorphism. Dimorphism and other sexually selected traits help individuals of a species determine the fitness of potential mates and select an appropriate breeding partner. This type of selection occurs in most species, from insects to humans and countless others.

Inter- and Intra-sexual Selection

The two basic types of sexual selection are intersexual (between-sex) and intrasexual (within-sex) selection. Intersexual selection occurs as a result of interactions between males and females of a species. One sex, typically males, will develop and display traits or behavior patterns to attract the opposite sex. Examples of such traits include plumage on birds, the mating calls of frogs, and courtship displays in fish. In contrast, intrasexual selection occurs between members of the same sex. Competition between males is common, as with deer or horned beetles, which fight for dominance and the ability mate with nearby females. In such cases, sexual selection acts on traits that facilitate competition among individuals of the same sex. For example, the strength and size of the “horn” of horned beetles or antler size in deer. Both intersexual and intrasexual selection influence the development of sexually selected traits in numerous species.

Signaling Fitness and the Operational Sex Ratio (OSR)

The display of sexually-selected traits can be important signals of individual fitness to potential mates. Such signals are often energetically costly, and thus indicate the health, genetics, and nutrition status of an individual. Often, though not always, these traits are displayed by males to attract females. This is the result of the difference in energetic investment into offspring production and rearing. Female eggs are produced in fewer numbers and at a higher energetic cost than male sperm, prompting females to be more selective in mate choice. The level of competition for mates can be quantified by the operational sex ratio (OSR). The OSR is based on the ratio of sexually mature males to females in a population, and is often male-biased in groups that contain more males or for populations in which males mate with multiple females. However, the OSR can be skewed by several factors, including the relative investment by each sex into parental care, mate bonding processes, and the overall rate of reproduction 1-2 .

Because ORS is often male skewed, males of a species tend to experience stronger inter- and intra-sexual selective pressures. In this context, natural selection has favored the development of traits to attract females or compete with other males, even when these traits come at a significant energetic or fitness cost. Traits to attract females, for instance, are often colorful or flashy, making individuals more easily seen by predators. These traits may require significant energy to produce and can reduce immune function in individuals. Furthermore, males put themselves at risk when competing with other males. Battles between males, mating dances, vocalizations, and displays are often not only physically demanding, but may also be loud, flashy and distracting, reducing awareness to predators and potentially injuring individuals. Because of this, natural selection and sexual selection are often in conflict, seemingly pulling such traits in opposite directions. The influence of natural selection prevents sexually selected traits from becoming too extravagant. As a result, sexually associated traits that significantly reduce an individual’s ability to survive long enough to mate will be selected against. This balance between natural and sexual selection causes most individuals within a population to exhibit fitness and sexual traits of average or intermediate quality to maximize reproductive success and survival.

Trading Off: Sexual Selection Versus Natural Selection

However, under unique circumstances, sexually selected traits may not be constrained by survival pressures. For instance, birds within the “bird-of-paradise” family include a variety of species that occupy remote areas lacking natural predators. As a result, males have developed extravagant colorations and mating displays that would normally be selected against due to predation. Such “runaway selection” can produce beautiful and highly ornamented species. Unfortunately, due to human activity, the introduction of new predators like cats threatens the survival of these endangered species.

Understanding sexual selection influences the way we view animal traits, behaviors, and mate choice, including humans 3 . There is often more to sexual selection than visual cues, with many species relying solely on smell or sound to find mates. In humans, the influence of smell was demonstrated in an experiment by Wedekind et al, which showed a female preference for the smell of males who were more different, genetically 4 . Enhanced genetic diversity, especially as it relates to immune function, may provide offspring with greater fitness. In this and other ways, sexual selection has influenced the way humans and organisms of all types evolve, behave, select mates, and reproduce.

Further Reading:

  1. Verdade, L. M. (1996). 'The influence of hunting pressure on the social behavior of vertebrates.' Rev Bras Biol 56(1): 1-13.
  2. Lifshitz, N. and C. C. St Clair (2016). 'Coloured ornamental traits could be effective and non-invasive indicators of pollution exposure for wildlife.' Conserv Physiol 4(1): cow028.
  3. Stanyon, R. and F. Bigoni (2014). 'Sexual selection and the evolution of behavior, morphology, neuroanatomy and genes in humans and other primates.' Neurosci Biobehav Rev 46P4: 579-590.
  4. Wedekind, C. and S. Furi (1997). 'Body odour preferences in men and women: do they aim for specific MHC combinations or simply heterozygosity?' Proc Biol Sci 264(1387): 1471-1479.