In addition, after dividing resources between themselves and another man in the decomposed game task, men showed higher levels of intrasexual competition. See Tweets about #intrasexual on Twitter. See what people are saying and join the conversation. Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a “selective response by animals to particular stimuli” which can be observed as behavior.
Intrasexual competition can be defined as the struggle between members of one sex to increase their access to members of the other sex as. Drawing examples from across animal taxa, including humans, I examine 4 predictions about female intrasexual competition based on the. Mate choice is one of the primary mechanisms under which evolution can occur. It is characterized by a “selective response by animals to particular stimuli” which can be observed as behavior.
Intrasexual competition can be defined as the struggle between members of one sex to increase their access to members of the other sex as. A person that only has sexual intercourse with him/herself. They do not mate with members of either gender, rather they keep to themselves, pleasing only. Across most species, sexually selected traits have a dual role in both gaining mates and in intrasexual competition (Hunt et al. ).
Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with intersexual selectionand compete with members of the same sex for access to members of the opposite sex intrasexual selection. These two forms of selection mean that some individuals have better reproductive success than others within a populationeither because they are more attractive or prefer more attractive partners to produce offspring.
The females then arrive and choose the males with the deepest croaks and best territories. In general, males benefit from frequent mating and monopolizing access to a group of fertile females. Females can have a limited number of offspring and maximize the return on the energy they invest in reproduction. The concept was first articulated by Charles Darwin and Alfred Russel Wallace who described it as driving species adaptations and that many organisms had evolved features whose function was deleterious to their individual survival,  and then developed by Ronald Fisher in the early 20th century.
Sexual selection can lead males to extreme efforts to demonstrate their fitness to be chosen by females, producing sexual dimorphism in secondary sexual characteristicssuch as the ornate plumage of birds such as birds of paradise and peafowlor the antlers of deeror the manes of lionscaused by a positive feedback mechanism known as a Fisherian runawaywhere the passing-on of the desire for a trait in one sex is as important as having the trait in the other sex in producing the runaway effect.
Although the sexy son hypothesis indicates that females would prefer male offspring, Fisher's principle explains why the sex ratio is almost without exception. Sexual selection is also found in plants and fungi. Many non-exclusive hypotheses have been proposed,  including the positive impact of an additional form of selection, sexual selection, on the probability of persistence of a species. Sexual selection was first proposed by Charles Darwin in The Origin of Species and developed in The Descent of Man and Selection in Relation to Sexas he felt that natural selection alone was unable to account for certain types of non-survival adaptations.
He once wrote to a colleague that "The sight of a feather in a peacock 's tail, whenever I gaze at it, makes me sick! These views were to some extent opposed by Alfred Russel Wallacemostly after Darwin's death. He accepted that sexual selection could occur, but argued that it was a intrasexual weak form of selection. He argued that male-male competitions were forms of natural selection, but that the "drab" peahen's coloration is itself adaptive as camouflage.
In his opinion, ascribing mate choice to females was attributing the ability to judge standards of beauty to animals such as beetles far too cognitively undeveloped to be capable of aesthetic feeling.
Ronald Fisherthe English statistician and evolutionary biologist developed a number of ideas about sexual selection in his book The Genetical Theory of Natural Selection including the sexy son hypothesis and Fisher's principle. The Fisherian runaway describes how sexual selection accelerates the preference for a specific ornament, causing the preferred trait and female preference for it to increase together in a positive feedback runaway cycle.
In a remark that was not widely understood  for another 50 years he said:. In the total absence of such checks, it is easy to see that the speed of development will be proportional to the development already attained, which will therefore increase with time exponentiallyor in geometric progression.
This causes a dramatic increase in both the male's conspicuous feature and in female preference for it, resulting in marked sexual dimorphismuntil practical physical constraints halt further exaggeration. A positive feedback loop is created, producing extravagant physical structures in the non-limiting sex. A classic example of female choice and potential runaway selection is the long-tailed widowbird. While males have long tails that are selected for by female choice, female tastes in tail length are still more extreme with females being attracted to tails longer than those that naturally occur.
Long-tailed widowbird offspring of both sexes inherit both sets of genes, with females expressing their genetic preference for long tails, and males showing off the coveted long tail itself. Richard Dawkins presents a non-mathematical explanation of the runaway sexual selection process in his book The Blind Watchmaker. As a result, they carry both sets of genes in their bodies. That is, genes for long tails and for preferring long tails become linked.
The taste for long tails and tail length itself may therefore become correlated, tending to increase together. The more tails lengthen, the more long tails are desired. Any slight initial imbalance between taste and tails may set off an explosion in tail lengths. Fisher wrote that:. The exponential element, which is the kernel intrasexual the thing, arises from the rate of change in hen taste being proportional to the absolute average degree of taste. The female widowbird chooses to mate with the most attractive long-tailed male so that her progeny, if male, will themselves be attractive to females of the next generation—thereby fathering many offspring that carry the female's genes.
Since the rate of change in preference is proportional to the average taste amongst females, and as females desire to secure the services of the most sexually attractive males, an additive effect is created that, if unchecked, can yield exponential increases in a given taste and in the corresponding desired sexual attribute. It is important to notice that the conditions of relative stability brought about by these or other means, will be far longer duration than the process in which the ornaments are evolved.
In most existing species the runaway process must have been already checked, and we should expect that the more extraordinary developments of sexual plumage are not due like most characters to a long and even course of evolutionary progress, but to sudden spurts of change. Since Fisher's initial conceptual model of the 'runaway' process, Russell Lande  and Peter O'Donald  have provided detailed mathematical proofs that define the circumstances under which runaway sexual selection can take place.
The reproductive success of an organism is measured by the number of offspring left behind, and their quality or probable fitness. Sexual preference creates a tendency towards assortative mating or homogamy. The general conditions of sexual discrimination appear to be 1 the acceptance of one mate precludes the effective acceptance of alternative mates, and 2 the rejection of an offer is followed by other offers, either certainly or at such high chance that the risk of non-occurrence is smaller than the chance advantage to be gained by selecting a mate.
The conditions determining which sex becomes the more limited resource in intersexual selection have been hypothesized with Bateman's principlewhich states that the sex which invests the most in producing offspring becomes a limiting resource for which the other sex competes, illustrated by the greater nutritional investment of an egg in a zygoteand the limited capacity of females to reproduce; for example, in humans, a woman can only give birth every ten months, whereas a male can become a father numerous times in the same period.
Hubbell and Johnson suggested that variance in reproductive success can be influenced by the time and allocations of mating. InGowaty and Hubbell suggested that mating tendencies depend on the choice of strategy; in some cases, males can be more selective than females, whereas Bateman suggested that his paradigm would be "almost universal" among sexually reproducing species.
Critics proposed that females might be more subject to sexual selection than males, but not in all circumstances. Darwin's ideas on sexual selection were met with scepticism by his contemporaries and not considered of great importance until in the s biologists decided to include sexual selection as a mode of natural selection. A ten-year study, experimentally varying sexual selection intrasexual flour beetles with other factors held constant, showed that sexual selection protected even an inbred population against extinction.
Hamiltonholds that the fact that the male is able to survive until and through the age of reproduction with such a seemingly maladaptive trait is taken by the female to be a testament to his overall fitness. Such handicaps might prove he is either free of or resistant to diseaseor that he possesses more speed or a greater physical strength that is used to combat the troubles brought on intrasexual the exaggerated trait.
InHamilton and Marlene Zuk introduced the "Bright Male" hypothesis, suggesting that male elaborations might serve as a marker of health, by exaggerating the effects of disease and deficiency. Rice have been added.
In the late s, Janzen and Mary Willson, noting that male flowers are often larger than female flowers, expanded the field of sexual selection into plants. In the past few years, the field has exploded to include other areas of study, not all of which fit Darwin's definition of sexual selection.
These include cuckoldrynuptial gifts, sperm competitioninfanticide especially in primatesphysical beautymating intrasexual subterfuge, species isolation mechanisms, male parental care, ambiparental care, mate location, polygamy, and homosexual rape in certain male animals. Sexual conflict leads to an antagonistic co-evolution in which one sex tends to control the other, resulting in a tug of war.
Besides, the sexual propaganda theory only argued that mates were opportunistically led, on the basis of various factors determining the choice such as phenotypic characteristics, apparent vigour of individuals, strength of mate signals, trophic resources, territoriality, etc. Several workers have brought attention to the fact that elaborated characters that ought to be costly in one way or intrasexual for their bearers e.
One possible explanation for the apparent lack of costs is that "compensatory traits" have evolved in concert with the sexually selected traits. Sexual selection may explain how certain characteristics such as feathers had distinct survival value at an early stage in their evolution. Geoffrey Miller proposes that sexual selection might have contributed by creating evolutionary modules such as Archaeopteryx feathers as sexual ornaments, at first. Some have suggested that the feathers served as insulation, helping females incubate their eggs.
But perhaps the feathers served as the kinds of sexual ornaments still common in most bird species, and especially in birds such as peacocks and birds-of-paradise today. If proto-bird courtship displays combined displays of forelimb feathers with energetic jumps, then the transition from display to aerodynamic functions could have been relatively smooth.
Sexual selection sometimes generates features that may help cause a species' extinction, as has been suggested  for the giant antlers of the Irish elk Megaloceros giganteus that became extinct in Pleistocene Europe. Sex differences directly related to reproduction and serving no direct purpose in courtship are called primary sexual characteristics.
Traits amenable to sexual selection, which give an organism an advantage over its rivals such as in courtship without being directly involved in reproductionare called secondary sex characteristics.
In most sexual species the males and females have different equilibrium strategies, due to a difference in relative investment in producing offspring. As formulated in Bateman's principle, females have a greater initial investment in producing offspring pregnancy in mammals or the production of the egg in birds and reptilesand this difference in initial investment creates differences in variance in expected reproductive success and bootstraps the sexual intrasexual processes.
Classic examples of reversed sex-role species include the pipefishand Wilson's phalarope. As a result of these factors, males can be expected to be more willing to mate than females, while females are expected to be the ones doing the choosing except in cases of forced copulationswhich can occur for example in certain primates and ducks.
The effects of sexual selection are thus often more pronounced in males than in females. Differences in secondary sexual characteristics between males and females of a species are referred to as sexual dimorphisms. These can be as subtle as a size difference sexual size dimorphism, often abbreviated as SSD or as extreme as horns and colour patterns. Sexual dimorphisms abound in nature. Examples include the possession of antlers by only male deerthe brighter coloration of many male birds in comparison with females of the same species, or even more distinct differences in basic morphology, such as the drastically increased eye-span of the male stalk-eyed fly.
The peacockwith its elaborate and colourful tail feathers, which the peahen lacks, is often referred to as perhaps the most extraordinary example of a dimorphism. Male and female black-throated blue warblers and Guianan cock-of-the-rocks also differ radically in their plumage.
Early naturalists even believed the females to be a separate species. The largest sexual size dimorphism in vertebrates is the shell dwelling cichlid fish Neolamprologus callipterus in which males are intrasexual to 30 times the size of females.
Extreme sexual size dimorphism, with females larger than males, is quite common in spiders and birds of prey. Male-male competition occurs when two males of the same species compete for the opportunity to mate with a female. Sexually dimorphic traits, size, sex ratio,  and the social situation  may all play a role in the effects male-male competition has on the reproductive success of a male and the mate choice of a female. Larger males tend to win male-male conflicts due to their sheer strength and ability to ward off other males from taking over their females.
For instance, in the fly Dryomyza anilissize shows the strongest correlation to the outcome of male-male conflicts over resources like territory and females. There are multiple types of male-male competition that may occur in a population at different times depending on the conditions. Competition variation occurs based on the frequency of various mating behaviours present in the population.
However, all techniques are not equally successful when in competition for reproductive success. Disruption results in a shorter copulation period and can therefore disrupt the fertilization of the eggs by the sperm, which frequently results in lower rates of fertilization and smaller clutch size. Another factor that can influence male-male competition is the value of the resource to competitors.
Male-male competition can pose many risks to a male's fitness, such as high energy expenditure, physical injury, lower sperm quality and lost paternity. A male is more likely to engage in competition for a resource that improves their reproductive success if the resource value is higher. While male-male competition can occur in the presence or absence of a female, competition occurs more frequently in the presence of a female.
A third factor that can impact the success of a male in competition is winner-loser effects. Male-male competition can both positively and negatively affect female fitness. When there is a high density of males in a population and a large number of males attempting to mate with the female, she is more likely to resist mating attempts, resulting in lower fertilization rates. However, group mating in Japanese medaka has been shown to positively affect the fitness of females due to an increase in genetic variation, a higher likelihood of paternal care and a higher likelihood of successful fertilization.
Sexual selection has been observed to occur in plants, animals and fungi. Today, biologists say that certain evolutionary traits intrasexual be explained by intraspecific competition —competition between members of the same species—distinguishing between competition before or after sexual intercourse.
Before copulationintrasexual selection —usually between males—may take the form of male-to-male combat.
A When males are subject to stronger sexual selection than females, males will evolve secondary sexual characters that result in marked differences between the sexes. Peacocks do not provide any parental care, and some males are more successful than others who may never reproduce, leading to marked dimorphism. B When males contribute to offspring care, the selection gradient is lower and the sexes will be monomorphic. Many seabirds are monogamous and raise offspring together and the sexes are indistinguishable.
C When males provide all the parental care, the selection gradient can be reversed and females may have to compete for access to males, leading to reverse sexual dimorphism. Red-necked phalaropes compete for access to males who provide all the parental care. Females are larger and more aggressive than males. Courtesty of Arthur Grosset. Evidence of female choice for good genes remains scarce despite decades of studies of female mate choice in many taxa.
This apparent lack of success continues to create debate as to the importance of the good genes model in the field. Sexual selection can affect reproductive success at multiple reproductive stages. First, it acts during all the processes that lead to acquiring mating opportunities i.
Darwin referred exclusively to pre-copulatory sexual selection in his discussions, erroneously assuming that mating would inevitably result in reproductive success. In recent years, evidence that copulatory and post-copulatory events play an important role in determining the outcome of fertilization and reproduction has been increasing.
Post-copulatory selection refers to the events that occur during and after mating. Post-copulatory male-male competition is known as sperm competition a term coined by Parker who recognized that when females mate with multiple males, their ejaculates compete inside the female reproductive tract for access to eggs.
Post-copulatory female choice refers to the ability of females to affect the likelihood that sperm from a particular male fertilizes their eggs, and their decision to invest in offspring based on the identity of the male with whom they mate.
Females exert this choice via morphological, chemical and behavioral adaptations. This type of selection is called cryptic choice because it occurs inside the female reproductive tract and cannot be detected from behavioral studies alone Eberhard Although both sexes are seeking to optimize their reproductive success, their genetic interests are not aligned, resulting in sexual conflict Parker Traits that allow a male to increase his reproductive success at the expense of the female will be positively selected if the female mates with multiple males.
These traits will be genetically transmitted and spread in the population, despite their negative effects on female reproductive success, if the reproductive success of these males is higher than that of males lacking such traits Parker Sexual conflict can often result in an evolutionary arms race, whereby the evolution of a trait that imposes harm on one sex will result in evolution of a counter-trait to mitigate the harm on the affected sex, with subsequent escalation in both Chapman et al.
Examples of sexual conflict include traumatic insemination in bed bugs, copulatory grasping and anti-grasping structures in waterstriders, and genital coevolution in waterfowl. Birkhead, T. Sperm Competition and Sexual Selection. Calhim, S.
Testes size in birds: quality versus quantity — assumptions, errors and estimates. Behavioral Ecology 18 , Chapman, T.
Sexual conflict. Trends in Ecology and Evolution 3 , Clutton-Brock, T. Sexual coercion in animal societies. Animal Behavior 49 , Darwin, C. London, UK: Murray, Eberhard, W. Emlen, D. The Evolution of Animal Weapons. Annual Review of Ecology , Systematics, and Evolution 39 , Fisher, R. The Genetical Theory of Natural Selection. Oxford, UK: Clarendon Press, Hamilton, W. Heritable true fitness and bright birds: a role for parasites? Science , Keller, L.
Why do females mate with multiple males? The sexually selected sperm hypothesis. Advanced Studies in Behavior , 24 , Kirkpatrick, M. Sexual selection and the evolution of female choice. Evolution 82, Lande, R. Models of speciation by sexual selection on polygenic traits.
LeBoeuf, B. Male-male competition and reproductive success in Elephant seals. American Zoologist 14 , Parker, G. Sperm competition and its evolutionary consequences in the insects.
Biological Reviews 45, Sexual selection and sexual conflict. Blum, M. New York: Academic Press, : Prum, R. Phylogenetic analysis of the evolution of display behavior in the neotropical manakins Aves: Pipridae. Ethology 84 , Stutt, A. Traumatic insemination and sexual conflict in the bed bug Cimex lectularius. Proceedings of the National Academy of Sciences , U. Trivers, R. Parental investment and sexual selection.
In Sexual Selection and the Descent of Man Campbell, B. London: Heinemann : Zahavi, A. Mate selection: a selection for a handicap. Journal of Theoretical Biology 53, The Diversity of Behavior. How Does Social Behavior Evolve? An Introduction to Animal Communication. Animal Behavior Introduction. Mating Systems in Sexual Animals. Measuring Animal Preferences and Choice Behavior. Perceptual Worlds and Sensory Ecology. An Introduction to Eusociality.
The Ecology of Avian Brood Parasitism. Social Parasitism in Ants. Causes and Consequences of Biodiversity Declines. Disease Ecology. Animal Migration. Sexual Selection. Territoriality and Aggression. The Development of Birdsong. Why does the peacock have such an elaborate tail? This cumbersome trait significantly decreases the male's chances of survival. It only exists because it confers an advantage to its bearer in the form of increased reproductive success.
Aa Aa Aa. Which Sex is Under Stronger Selection? Males and females in a population have the same average reproductive success R. The Role of Parental Care. Not surprisingly, much of the support for this hypothesis comes from mammalian species, with female aggression peaking during pregnancy and lactation Boness et al.
Female—female competition and associated aggression also tend to be more frequent at higher densities Cassini ; Klatt et al. Aggressive interactions that determine long-term rank relationships may likewise affect competition for food.
For example, higher ranking female chimpanzees Pan troglodytes , gain access to higher quality foraging areas Murray et al. Although some evidence points to food as a source of female—female competition, other work advocates an offspring protection hypothesis Maestripieri ; Wolff and Peterson Females often defend eggs or offspring, especially those with a protracted dependent phase during which they are at risk of injury or death from infanticidal rivals Hrdy ; Ebensperger For example, female northern elephant seals Mirounga angustirostris that initiate more frequent aggressive interactions are more effective at preventing other females from biting their offspring, and these offspring are more likely to survive to weaning Christenson and Leboeuf Similarly, nonmammalian females respond aggressively in defense of eggs from infanticidal females, as may be the case in some frogs Summers , reptiles Sinn et al.
Finally, females may compete to maximize their own survival, with more aggressive or dominant females obtain safer, more central positions in groups and reducing their own risk of predation Ron et al. Within the broad context of competition for mates, I review the evidence for several possible functions of female—female competition, emphasizing taxonomic patterns and differences between the sexes whenever possible. I make several predictions about the nature of intrasexual competition in females based on the availability of mates or mating resources and the potential direct and indirect benefits to be obtained via competition for mates.
I describe select examples in support of or opposition to each prediction. To determine whether sexual selection shapes the patterns described under each prediction, the issue at hand is which component of fitness is affected by this competition: If the selection differential or, covariance between competitive ability and reproductive success is directly affected by variance in fecundity or survival, then, intrasexual competition is shaped by natural selection.
Thus, for each prediction, I distinguish between natural and sexual selection whenever possible, although this task may be difficult in some cases, especially when natural and sexual selection work in the same direction Darwin ; Clutton-Brock ; Carranza Each prediction therefore serves as a starting point for exploring patterns of female competition, with the aims of clarifying recent debates and highlighting key next steps in the study of intrasexual selection.
If the operational sex ratio OSR determines which sex will compete for access to the other sex Emlen and Oring ; Kvarnemo and Ahnesjo , we should expect frequent female—female competition in populations where males are limiting, such as sex-role reversed systems where the OSR is female biased Eens and Pinxten, In the sex-role reversed tidewater goby Eucyclogobius newberryi , for example, females compete for access to territorial males and their associated burrows Swenson Strong same-sex competition in role-reversed species has long been seen as supporting classical sexual selection theory Darwin ; Trivers , but whether systems with conventional sex roles show similar patterns is less clear.
In populations with dynamic OSRs, we should expect increased female—female competition as the OSR becomes more female biased. Both experimental and observational evidence supports this prediction, with more frequent competitive interactions among females when there are fewer available males or more ready-to-mate females e.
In one fish species, the sand goby Pomatoschistus minutus , OSR, not density, predicts the frequency of female—female competitive interactions, just as it does with males Kvarnemo et al. The observation that higher densities do not increase female competition also suggests that female sand gobies do not compete primarily for density-dependent resources, such as food.
Instead, they appear to compete for access to males themselves or for male-held nest sites. Similar patterns of female aggression are also found in species where males do not care for offspring Weckerly et al. Pairs of captive female house mice Mus musculus , for example, are more aggressive toward each other when presented with one male than when presented with 3 males Rusu and Krackow , again suggesting that females vie for access to males themselves.
Humans are also thought to show increased levels of female—female competition in populations with a scarcity of available males typically associated with political or war-time demographic shifts, Schuster ; Campbell , although more rigorous cross-cultural tests are warranted. In the sand goby case described above, female competition changed as the OSR became more female biased, but all females were able to find a mate regardless of OSR Kvarnemo et al.
When the OSR was male biased, however, some males were excluded from breeding altogether. This sexual disparity in the impact of OSR on mating success draws into focus a key question: If the outcome of female—female competition does not predict the number of mates, why should females compete?
In other words, if males are not limiting, why do females bother to compete at all? One solution to this question is that females do not compete for the number of mates, but instead, they compete for the direct and indirect benefits those males provide.
In many systems, females receive a variety of direct benefits from their mates e. Patterns of competition and aggression suggest that females may compete for these direct benefits or the high-quality males that are best able to provide these benefits.
For example, female—female aggressive interactions are common in many insect species where males provide nutrient rich spermatophores to their mates e. If the outcome of these competitive interactions predicts the quality of mates females obtain, then this competition may be the target of sexual selection as well as natural selection for increased fecundity. Access to defended space i. The question of whether females compete for access to territories can be difficult to answer empirically because females often acquire a mate at the same time as a territory.
The primary difference between these 2 scenarios is whether the female directly assesses territory quality or mate quality. For convenience, I will instead consider evidence for this particular direct benefit in the context of mating resources over which females compete Prediction 4 , with full understanding that competition for high-quality territories may well represent competition for male direct benefits as well. Parental care is among the best studied direct benefit that a female might obtain from her mate, and it thus provides an excellent case study with which to examine possible female—female competition for this aspect of mate quality.
In polygynous and monogamous species in which males provide parental care, females may compete over a monogamous pair-bond, using overt aggressive behaviors to ward off additional females, thus ensuring exclusive social access to a particular male reviewed in Wittenberger and Tilson ; Slagsvold and Lifjeld If secondary females receive less paternal care than primary or monogamously mated females Breiehagen and Slagsvold ; Kokita and Nakazono and this reduction in care leads to decreased nesting success Clutton-Brock ; Kokita and Nakazono , but see Dunn and Hannon , selection should act on females to repel rival females that may diminish the direct benefits received from males.
Several lines of evidence support the prediction that females compete over access to male parental care i.
Because females that are more aggressive are more likely to be monogamously mated Sandell and primary females are often more aggressive than secondary females Yasukawa and Searcy ; Hobson and Sealy ; Williams , but see Breiehagen and Slagsvold , chasing, fighting, and other forms of aggression may deter secondary females from settling see also Ratti et al. Likewise, the experimental addition of a nearby nest-box for a secondary female increases the prevalence of female—female aggression in the facultatively polygynous starling Sturnus vulgaris Sandell and Smith If females compete for high-quality males that provide parental care, female—female aggression should also be more intense when vying for a male whose phenotype suggests that he will be a high-quality male that will provide more care than other males.
In the sharknose goby Elacatinus evelynae , more aggressive females are more likely to mate with a larger male, and larger males typically provide more parental care Whiteman and Cote Thus, females do appear to compete for exclusive access to a high-quality male that will provide direct benefits. A natural continuation of this logic, of course, is that in the absence of paternal care, female competition should be rare or should occur for different reasons e.
Indeed, temporal patterns of female aggression in birds and fish i. In nearly, all species of birds studied to date, female aggression peaks prior to egg laying but decreases during offspring rearing Gowaty ; Breiehagen and Slagsvold ; Slagsvold ; Cristol and Johnsen ; Sandell and Smith ; Elekonich ; Garcia and Arroyo ; Gill et al. In most mammal species, females are most aggressive during the offspring-rearing period Boness et al. These interspecific temporal patterns of female competition suggest that sexual selection via female competition for mate quality may be more common in species with male direct benefits i.
Determining the generality of this suggestion will ultimately require phylogenetically controlled tests within taxa that vary in the extent of male care.
In one such comparison of burying beetles, females of the biparental species Nicrophorus quadripunctatus frequently compete for access to carcasses on which they rear their young, whereas female—female competition is nearly nonexistent in Ptomascopus morio , a species without parental care Suzuki et al. If sperm is limited Dewsbury ; Wedell et al.
Evidence suggests that preferred males are more likely to be sperm limited Jones , especially later in the mating season Preston et al. Therefore, a female that is more adept at monopolizing preferred males may maximize the probability of fertilization by the best male if she can mate with him earlier than other females. If females compete for genes from the best males, we should expect female—female competition on leks, where genes are essentially the only contribution from the male e.
In fact, several examples from lekking species indicate that female aggressive encounters are more intense or frequent when vying for the best male on the lek Petrie et al. For example, female—female fights among topi antelope Damaliscus lunatus are more common at the center of the lek, where the most preferred male is located. Females in nonlekking species may also compete for access to good genes e.
Although not yet tested directly, these results support the prediction that more aggressive females are better able to access the high-quality males that are most desirable for extrapair matings. Much as females may compete for access to mates themselves when mates are limited Prediction 1 , females may also compete for mating opportunities or mating resources i.
As detailed above, these competitive interactions lie at the heart of one current debate regarding the scope of sexual selection in females. It is therefore especially important to identify the component of fitness affected by female—female competition for mating opportunities or mating resources. The fact that competition occurs between females in relation to reproduction is not sufficient to demonstrate competition for mates i.
Nesting sites can be a limiting resource for females of many species, independent of the OSR. However, it can be difficult to determine whether females compete for access to a mate or to a critical mating resource e. In spite of this difficulty, several studies suggest that female aggression provides a competitive advantage when breeding or mating opportunities are rare. For example, in the common goby Po. Similarly, females are known compete with one another in close proximity to oviposition or nesting sites.
In the parasitoid Pachycrepoideus vindemmiae , winners of aggressive interactions have at least temporarily exclusive access to an oviposition site, with losers being evicted Goubault et al. If more aggressive females are more proficient at delaying or preventing superparasitism i. Females burying beetles compete for access to a vertebrate carcass on which they rear their larvae.
Looking closer at the components of fitness in these last 2 examples, competitive interactions appear to directly influence food availability for offspring without affecting any component of competition for mates. Accordingly, some aspects of female—female competition for breeding resources do not fall within the purview of sexual selection via competition for mates.
If sexual selection is to shape female—female competition over mating opportunities, one key question is whether particular females are able to competitively exclude others from mating altogether, as is often the case in males Brown Direct experimental support for this prediction comes from work on tree swallows Tachycineta bicolor , a secondary cavity—nesting bird with limited nesting sites Holroyd Female aggressiveness predicts the likelihood of obtaining a nesting cavity after an experimental reduction in cavity availability Rosvall In this species, females that do not obtain a nesting cavity do not have alternative routes to mating or reproductive success because intraspecific brood parasitism is virtually nonexistent Robertson et al.
Thus, the outcome of female—female competition predicts mating success and significant reproductive skew among females i. A more extreme version of competitive exclusion is the physiological or behavioral suppression of subordinate females in group living or communally breeding animals Wasser and Barash ; Reeve and Sherman ; Solomon and French ; Clutton-Brock et al.
In communal breeders in particular, skew among females is typically higher than skew among males, with one dominant female breeding and several subordinates assisting in parental care.
Overt aggression among females is thought to play a role in reproductive suppression by preventing ovulation or increasing stress in subordinates McLeod et al. Even without complete reproductive suppression, this active competition may successfully delay reproduction in rivals by interrupting mating, delaying ovulation, or increasing stress Wasser and Starling ; Hohmann and Fruth While these examples of intense female—female competition clearly predict important reproductive variance, females do not apparently compete for mates in any direct way.
Competitive interactions that establish dominance and thus, the likelihood of mating at all may be a form of indirect mate competition, by which more aggressive females are allowed to mate. However, similar to female—female competition for territories, this example may be difficult to parse into natural and sexual selection, with 2 mechanisms targeting the same outcome.
Female—female competitive interactions lead to a number of possible fitness benefits for the winning female, suggesting that traits conferring a competitive edge among females are unlikely to exist merely as nonadaptive by-products of selection on males. Furthermore, patterns of overt aggression among females map onto the availability of resources and mates, and the direct or indirect benefits provided by those mates.
These patterns reveal specific currencies of differential mating success and the evolutionary mechanisms shaping competitive interactions among females. At one end of the spectrum of relative parental investment, with extensive female-only care, sexual selection appears to play a lesser role, with fecundity and mortality selection shaping female competition to protect offspring or acquire the food necessary for prolonged maternal care.
In the absence of male direct benefits, females appear to compete for access to genetic benefits from high-quality males. When males provide parental care, females also may compete for exclusive social access to those high-quality males. In either case, if all females cannot mate with the best male and if males vary in the direct or indirect benefits they provide, then sexual selection should favor females who are more adept at gaining access to these high-quality mates.
In short, both natural and sexual selection favor female—female competition in a number of contexts. Whether the selective advantage of overt aggression outweighs potential costs is an empirical question for future research, one that is a key step in addressing whether the overall selection differential for female—female competition is positive.
Aspects of female—female competition appear very similar to their male counterparts, with some notable differences. The availability of mates influences female—female competitive interactions in species with standard and reversed sex roles Darwin ; Trivers ; Eens and Pinxten When males are limited, the frequency and intensity of female—female competitive interactions increases Prediction 1.
In addition to this competition over access to mates, there are additional ways in which female competition affects mating success without influencing the number of mates, per se. Overt aggressive behaviors also increase when females compete for high-quality mates that provide either direct benefits Prediction 2 or indirect benefits Prediction 3.
While the number of males may not be limiting to females outside of role-reversed species, the number of high-quality mates may be limiting Petrie ; Altmann , and competition for these males constitutes a form of mating competition. Accordingly, sexual selection may favor females that outcompete other females for male parental care, indirect genetic benefits, and nesting sites held by preferred males. As a consequence, it is clear that if we restrict sexual selection to competition for the quantity of mates without including competition for high-quality mates, we ignore a potentially important component of mating success, particularly in females.
If mating success is strictly limited to mate number, then 2 individuals with one mate each have equal mating success. If, however, one of these mates is high quality and the other low quality, should we not also consider the former individual to have higher mating success than the latter? And, if competitive interactions between these 2 individuals determined this outcome, should we not also consider this process part of sexual selection? The answer to these questions essentially boils down to differences in the quality and quantity of mates.
In light of empirical evidence that mate quality impacts fitness in many systems Andersson , it may be overly simplistic to ignore mate quality in measuring mating success and sexual selection. The gray area of sexual selection centers around female—female competition for territories or other mating resources that qualify an individual as a mate Prediction 4 because many of these examples affect female survival and fecundity as well as mating success.
Competition among females for nesting sites required for both mating and reproduction, for example, provides a nearly identical parallel to sexual selection via male—male competition for females and territories, although some instead consider this process social selection in both sexes West-Eberhard Females that are more aggressive may obtain access to resources that allow them to mate or mate more, or mate with better mates , thus couching this process in the overarching theme of sexual selection i.
Females also may compete over resources that directly affect fertility, fecundity, or offspring survival without affecting competition for mate quantity or quality, and thus, this sort of intrasexual competition does not fit within sexual selection. Selection should favor competition among females if variance in female competitive ability maps onto even slight variance in any component of fitness, whether sexually or naturally selected. For females, variance in mating success may be less dependent on mate number than on mate quality, which in turn may affect quality or even lifetime quantity of offspring.
While quality is a more subtle measure of mating success than quantity, the critical condition for selection to favor female—female competition is a positive selection differential, even if the magnitude of the payoff in females is smaller than in males. It would be a mistake to confuse this potentially smaller fitness payoff of intrasexual competition in females with a lack of sexual selection in females.
Behavioral ecologists should continue to address the many questions about the nature of intrasexual competition in females and its role in the framework of sexual selection.
Based on the above discussion, I suggest 4 specific avenues for research:. Do females compete for breeding resources and mate quality more so than males? A central tenet of mating systems and sexual selection theory is that the sexes differ fundamentally in their route to reproductive success, with male reproductive skew based on competition for mates, and female reproductive skew based on access to resources that affect fecundity Bateman ; Trivers ; Wade and Shuster With growing interest in sexual selection in females and the entities over which females compete, and fewer biases against the study of sexual selection in females Berglund et al.
They also must extend the predictions laid out here to include other ways in which both sexes might compete for high-quality mates or mating resources, looking at competitive interactions that are more subtle than overt aggression e. As was common with the burst of research on sexual selection in males beginning about 30 years ago, comparative studies are needed to explore patterns of female—female competition while controlling for differences in mating system, parental investment, and phylogenetics.
How easily can both sexes compete? In the last decade or so, both theoretical and empirical works have explored conditions under which mutual mate choice is likely Bergstrom and Real ; Kokko and Johnstone ; Kraaijeveld et al. In future work on sexual selection, it will be essential to identify the extent of simultaneous intrasexual competition in both sexes as well as the spatiotemporal dynamism of competition in one sex versus the other.
Do the same rules govern male and female contests? Because males and females may compete over different entities and receive different benefits from winning a competitive interaction, they may be expected to follow different rules of engagement as well.
Consistent with this view, resource-holding power e. Instead, resource value e. Along similar lines, the dear enemy hypothesis predicts a more aggressive response to strangers than neighbors and has been largely supported in males Temeles In female New Zealand bellbirds Anthornis melanura , however, the opposite appears to be true, perhaps because of sex differences in the relative threat posed by neighbors versus strangers Brunton et al. In humans as well, there is some indication that male—male competition and female—female competition follow different rules, with the former being characterized by overt violence and aggression and the latter by more subtle expressions of aggression, such as gossiping and ostracizing Campbell ; Geary Although it is premature to draw any definitive conclusions, these studies leave open the possibility that sex differences in intrasexual competition extend beyond the function of aggression and into the nature of contest rules.
How does mate quality affect theoretical models of sexual selection? With mating success typically focused on the number of mates, measures of mate quality are generally not incorporated into quantitative genetic models of sexual selection Wade and Arnold ; Kokko et al.
Although there are difficulties with assigning quality to fitness measures Moore et al. Many thanks to K. Cain and E.
Derryberry for extensive comments and discussion, to R. Brooks and 2 anonymous reviewers for constructive criticism, and to S. Nowicki for encouraging me to write this review. National Center for Biotechnology Information , U. Behav Ecol. Published online Sep 8.
Kimberly A. Author information Article notes Copyright and License information Disclaimer. Corresponding author. Address correspondence to K. E-mail: ude. All rights reserved. For permissions, please e-mail: journals. This article has been cited by other articles in PMC. Abstract In spite of recent interest in sexual selection in females, debate exists over whether traits that influence female—female competition are sexually selected.
Keywords: aggression, female competition, intrasexual selection, mating success, sexual selection. What is sexual selection? Nonadaptive hypotheses We might expect high levels of competition and aggression among females as a by-product of a genetic correlation with males: The sexes share the vast majority of their genome, and so, a behavior that is favored in one sex may exist in the other sex via correlational selection Wallace ; Lande Female—female competition for mates Within the broad context of competition for mates, I review the evidence for several possible functions of female—female competition, emphasizing taxonomic patterns and differences between the sexes whenever possible.
Competition for access to mates Prediction 1: as the operational sex ratio becomes more female biased, females should compete for access to males. Competition for high quality mates Prediction 2: females should compete for access to males that provide direct benefits.
Competition for male parental care Parental care is among the best studied direct benefit that a female might obtain from her mate, and it thus provides an excellent case study with which to examine possible female—female competition for this aspect of mate quality. Prediction 3: females should compete for males that will provide indirect genetic benefits. Competition over mating opportunities Much as females may compete for access to mates themselves when mates are limited Prediction 1 , females may also compete for mating opportunities or mating resources i.
Prediction 4: female—female competition should increase when mating opportunities or resources are limited. Based on the above discussion, I suggest 4 specific avenues for research: Do females compete for breeding resources and mate quality more so than males? Acknowledgments Many thanks to K. References Adkins-Regan E. Hormones and animal social behavior. Using potential reproductive rates to predict mating competition among individuals qualified to mate.
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