Rosvall; Intrasexual competition in females: In spite of recent in sexual selection in females, debate exists over whether traits that influence female—female competition are sexually selected. This review uses female—female aggressive behavior as a model behavioral trait for understanding the evolutionary mechanisms promoting intrasexual competition, focusing especially on sexual selection.
I employ a broad definition of sexual selection, whereby traits that influence competition for mates are sexually selected, whereas those that directly influence fecundity or offspring survival are naturally selected. Drawing examples from across animal taxa, including humans, I examine 4 predictions about female intrasexual competition based on the abundance of resources, the availability of males, and the direct or indirect benefits those males provide.
These patterns Intrasexual selection birds a key sex difference in Intrasexual selection birds selection: Although females may compete for the number of mates, they appear to compete more so for access to high-quality mates that provide direct and
Intrasexual selection birds genetic benefits.
As is the case in males, intrasexual selection in females also includes competition for essential resources required for access to mates. If mate quality affects the magnitude of mating success, then restricting sexual selection to competition for quantity of mates may ignore important components of fitness in females and underestimate the role of sexual selection in shaping female phenotype.
In the future, understanding sex differences in sexual selection will require further exploration of the extent of mutual intrasexual competition and the incorporation of quality of mating success into the study of sexual selection Intrasexual selection birds both sexes. In recent years, behavioral ecologists have shown increased interest in sexual selection in females Heinsohn et Intrasexual selection birds. In species with conventional sex roles, sexual selection Intrasexual selection birds thought to act more strongly in males in females due to the interplay between sexual selection, variance in mating success, and asymmetries in parental investment Bateman ; Trivers ; Emlen and Oring ; Kokko and Jennions ; Wade and Shusterbut see DreaTang-Martinez and Ryder As a consequence, the frequency and intensity of exaggerated traits and behaviors tend to be greater in males than in females.
When females exhibit versions of these traits, their evolutionary significance has proven Intrasexual selection birds be enigmatic. Are these Intrasexual selection birds nonfunctional by-products Intrasexual selection birds a genetic correlation with males Lande ? Are they primarily shaped Intrasexual selection birds fecundity or survival selection i.
Or do females use these exaggerated traits and behaviors to compete for mates in a context similar to sexually selected male—male competition? This third, more controversial possibility is the focus of this review.
More specifically, I explore the evolutionary mechanisms driving female—female competition, using
Intrasexual selection birds aggression as a model behavioral trait for drawing broad conclusions about sexual selection and competitive interactions among females. Though intrasexual competition frequently occurs without escalating to aggressive behavior e. These aggressive encounters therefore reveal the resources or individuals over which females compete and the benefited accrued by successful competitors.
Moreover, female—female aggression has been widely studied in a range of natural and experimental conditions the animal kingdom, but these data have not yet been synthesized to uncover the evolutionary mechanisms promoting competition among females. This review emphasizes female—female aggressive interactions in the context of mating competition, including examples that clearly fall within the purview of sexual selection as well as others that comprise the crux of the debate over sexual selection in females.
I make predictions about patterns of female—female competition to discern how and why females compete. Based on the relative support for these predictions, I address Intrasexual selection birds intrasexual competition may differ between the sexes, in function, outcome, and mechanism of selection, and I suggest clear directions for future research on the nature of intrasexual competition and sexual selection in both sexes.
Intrasexual selection birds of sexual Intrasexual selection birds exist, yet the specifics of the definition are critical to interpreting patterns
Intrasexual selection birds female—female competition.
Under this view, if a trait influences competition for mates, then this trait is sexually Intrasexual selection birds. Therefore, sexual selection encompasses a Intrasexual selection birds broad array of processes, Intrasexual selection birds as competition for the number or quality of mates as well as competition for resources Intrasexual selection birds directly influence the probability of mating.
I will address 2 points about this definition that are particularly relevant to sexual selection in females and leave the details of the current and historical debates over sexual selection to a handful of thoughtful reviews Endler ; Andersson ; Kavanagh ; Roughgarden et al. The first point concerns the contrast between competition for mates and competition for resources.
Thus, sexual selection maintained as a unique subset of natural selection, defined not only by the identity of competing parties e. A second key point regarding the definition of sexual selection is whether it is sufficiently broad to include the myriad ways in which individuals compete for mates Andersson and Iwasawithout inherent sex biases. Theory suggests that females should not to compete for the quantity of mates because increasing mate number does not affect female reproductive success Intrasexual selection birds Competition for may also involve competition over mate quality or, competitive mate choice; Hallidayan often ignored aspect of mating success that may be particularly relevant to females Petrie ; Altmann In addition, sexual selection may include competition over entities that affect the probability of getting a mate or becoming a mate.
For example, male satin bowerbirds Ptilonorhynchus violaceus compete with each other for bower adornments because more decorated bowers
Intrasexual selection birds more mates Intrasexual selection birds ab. We might expect high levels of competition and aggression among females as a by-product of a genetic correlation with males: 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 In spite of the historical argument that exaggerated female traits exist Intrasexual selection birds nonadaptive by-products of selection on males, empirical tests demonstrate a range of intersexual genetic correlations for an example with ornamentation, see Kraaijeveld et al.
For same-sex aggression in females, high repeatability e. More concrete support for correlational selection comes from a study in which female aggressiveness increased in lines of Drosophila melanogaster that were selected for high levels of male aggression Edwards et al. High-throughput genomic analyses are beginning to identify genes that predict competitive ability e.
Collectively, studies suggest that aggression in males and females may mediated by some common mechanism, but they leave open the that different genes or different mechanisms may be relevant in the 2 sexes.
While we are just beginning to understand the genetics of aggression, more known about the Intrasexual selection birds mechanisms mediating aggressive behavior, especially in vertebrates.
Much Intrasexual selection birds has focused Intrasexual selection birds the relationship between aggression and testosterone
Intrasexual selection birdsalthough the list of hormones and neuropeptides that mediate aggression continues to grow Stribley and Carter ; Intrasexual selection birds ; Nelson Trainor ; Soma et al. Notably, evidence is mixed as to whether T Intrasexual selection birds aggression in females in the same way that it does in males Desjardins et al.
Furthermore, Intrasexual selection birds and seasonal differences in neuroendocrine mechanisms of aggression question the assumption that behavioral mechanisms are fixed within a species Soma ; Canoine et al.
Several lines of evidence suggest that females compete for food access. For example, when food is rare or difficult to obtain, female—female aggression increases in frequency Ueda and Kidokoro ; Baird and Sloan Because maternal investment is linked with high energetic demands, same-sex competition Intrasexual selection birds food may be especially common when females heavily invest in parental care.
Not surprisingly, much of the support Intrasexual selection birds 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
Intrasexual selection birds rank relationships may likewise Intrasexual selection birds competition for food.
For example, higher ranking female chimpanzees Pan troglodytesgain 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 during which they are at risk of injury death from infanticidal rivals Hrdy ; Ebensperger For example, female northern elephant seals Mirounga
Intrasexual selection birds 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 Intrasexual selection birds from infanticidal females, as may be the case in some frogs Summersreptiles Sinn et 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 Intrasexual selection birds risk of
Intrasexual selection birds Ron et al.
Within the Intrasexual selection birds context of competition for mates, I review the evidence for several
Intrasexual selection birds 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
Intrasexual selection birds 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 differential or, covariance between competitive ability and reproductive success is directly affected by variance in fecundity or survival, then, intrasexual is shaped by natural selection.
Thus, for each prediction, I distinguish between natural and
Intrasexual selection birds 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 Intrasexual selection birds. If the operational sex ratio OSR determines which sex will compete for access to the other sex Emlen and Oring ; Kvarnemo and Ahnesjowe should expect frequent female—female competition in populations where males are limiting, Intrasexual selection birds as sex-role reversed systems where the OSR is female biased Eens and Pinxten, In the sex-role reversed tidewater goby Eucyclogobius newberryifor example, females compete for access to territorial males and their associated burrows Swenson Strong same-sex competition in role-reversed Intrasexual selection birds has long been seen Intrasexual selection birds supporting classical sexual selection theory Darwin ; Triversbut whether systems with conventional sex roles show similar patterns is less clear.
In populations with dynamic OSRs, we should expect increased female—female competition
Intrasexual selection birds the OSR becomes more female biased. Both experimental and observational evidence supports this with more frequent competitive interactions females when there are fewer available males or more ready-to-mate females e.
In one fish species, the sand goby Pomatoschistus minutusOSR, not density, predicts the frequency of female—female competitive interactions, just as it does with males Intrasexual selection birds 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 musculusfor example, are
Intrasexual selection birds aggressive toward each other when presented with one male than when presented with 3 males
Intrasexual selection birds and Krackowagain 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 with political or war-time demographic shifts, Schuster ; Campbellalthough more rigorous cross-cultural tests are warranted.
In the sand goby case described above, female competition changed Intrasexual selection birds the OSR became more female biased, but all females were able to find 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
Intrasexual selection birds 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
Intrasexual selection birds to provide these benefits.
For example, female—female aggressive interactions are common in many insect species where males provide nutrient 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 of sexual Intrasexual selection birds 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 4with 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 Intrasexual selection birds female—female competition for this aspect of Intrasexual selection birds 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 Intrasexual selection birds 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
Intrasexual selection birds ; Kokita
Intrasexual selection birds Nakazonobut see Dunn and Hannonselection 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 Intrasexual selection birds are more likely to be monogamously mated Sandell and primary females are often more aggressive than secondary females Yasukawa and Searcy ; Hobson and Sealy ; Williamsbut see Breiehagen and Slagsvoldchasing, fighting, and other forms of aggression Intrasexual selection birds 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 Intrasexual selection birds will provide more care than other males.
In the sharknose goby evelynaemore aggressive females are more likely to mate with a larger male, and larger males typically provide more parental care Whiteman and Cote
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