Female Choice and Sexual Conflict Theory
When females are the choosy sex and retain primary control over which males they will mate with. Thus, members of the lower investment sex, usually males, compete for features than are attractive to members of the high investing sex, usually females. In this sense, females choose on the basis of male displays or traits.
The outcome of sexual selection is the evolution of secondary sexual characteristics that are designed to attract and compete over mates. Traits that evolve via sexual selection do so as a result of competition for mates (intrasexual competition) or mate attraction (epigamic selection). Intrasexual competition is competition between members of the low investment sex (usually males) for copulations with the high investment sex (usually females). Since males are usually the low investment sex, they are the most intrasexually selected. This results in traits beneficial for competing with other males such as organs of threat (antlers, large size, aggressiveness, etc.). In some cases, these traits will be used in direct physical combat with other male; in other cases, they may be used to establish dominance hierarchies where the males at the top monopolize mating opportunities. An example of intrasexual selection for size is found in elephant seals where males are substantially larger than females, spend much of their time competing for females, and the most dominant males father the majority of pups in any given season.
Epigamic selection is competition between members of the low investment sex for features that are attractive to the high investment sex. As with intrasexual selection, the sex that invests less per offspring is usually the male. Thus, this type of selection is often referred to as female choice; the females choose on the basis of male displays or attributes. The result of female choice is the elaboration of displays, coloration, and courtship rituals as the males try to “impress” the females by competing with other males via display mechanisms. Under female choice, highly elaborated traits can evolve, such as the peacock’s tail (Petrie 1994). Such traits exist because of the female preference for them. Males with those traits, or the most impressive examples of such, were the most successful at reproducing and passed those traits on to their offspring. However, this raises the question of what benefit those traits are to the reproductive success of females? Why do they choose them?
Runaway Selection or the Sexy Sons Hypothesis
The effects of female choice can lead to what Fisher (1915, 1930) referred to as runaway selection. In this process, females are initially predisposed to favor a particular male trait (reason unspecified) and those males possessing the favored trait attract females and experience greater reproductive success, which in turn passes on the male trait as well as the female preference for that trait. The joint selection for that trait creates a positive feedback loop that continuously favors both these traits (the trait in males and preference for it in females). The result can be “runaway” selection of the male trait, leading to the evolution of more elaborate or extreme versions of the original trait. This has been used to explain the evolution of elaborate male ornamentation or other secondary sexual traits that would seem to reduce the survival of those who possess them, though presumably the runaway process would cease when the survival costs outweigh the benefits of attracting mates (Pomiankowski and Iwasa 1993). Where does the original preference for the trait on the part of females come from? The trait might have had some adaptive value, as a health indicator, for example, bright coloration revealing the male to be free of parasitic infection. Or the trait could have been preferred by females for purely arbitrary reasons unrelated to fitness. For example, a bird could have a color preference or sensory bias due to a food item of that color being a major component of their diet. As a consequence, anything with that color, including a male of their species, may be perceived as attractive. It has been suggested that such sensory bias has influenced female choice in guppies. Rodd et al. (2002) suggested that male color patterns are the result of female choice of males with orange spots due to the attraction of both sexes to inanimate orange objects. Guppies frequently eat orange food items; thus, the suggestion is that selection for easy detection of orange food items results in selection for preferences for orange males. However, Houde (1997) and Kennedy et al. (1987) suggest that female choice for males with orange spots is adaptive because males must ingest carotenoids to produce these colors and thus the bright color could be an honest indicator of male quality.
Related to runaway selection is the sexy sons hypothesis which states that females choose males with attractive traits, such as large size or bright colors, because these traits will be passed onto their sons who will also be able to attract many mates. From this perspective, females that choose a male whose genes will give her sexy sons will have a good chance of having their own genes passed on by her sexy sons. Evidence in support of the sexy sons hypothesis of female choice has been found in several bird species including European starlings (Gwinner and Schwabl 2005) and beetles (Pai and Yan 2002) though others suggest that good genes are what females are choosing for as will be discussed next.
The Handicap Principle and Honest Advertisements
Why do males evolve extravagant costly traits like peacock tails or nest displays like satin bowerbirds? While these traits help them attract mates, they also attract predators, can make it difficult to escape from them, and are metabolically costly.
Zahavi and Zahavi (1997) suggested that a trait would be selected for if it provides a cost to its carrier that indicates something about the quality of the carrier. The peacock is often highlighted as an example of Zahavi’s handicap principle with the male tail serving as a costly signal. Peahens select from a pool of possible mates. Assuming that genes are all that the males provide and that males vary in genetic quality, peahens should aim for the male with the best quality genes available.
As genetic quality cannot be directly assessed, peahens attend to cues that males provide. Peacocks advertise genetic quality via large, colorful, symmetric tails. Such an ornament is a handicap in that it is energetically costly to build and difficult to hide from predators. A poor quality male would not be able to produce an impressive tail as his energies are required just for basic somatic maintenance as well as predator avoidance. A high quality healthy male, however, can afford the cost of such an advertisement. Since only high quality males can afford impressive tails, peahens prefer males with large, bright, symmetrical tails to mate with as they serve as honest indicators of male quality (Loyau et al. 2005; Petrie 1994). Studies have documented similar impacts of female choice on advertisements in long-tailed widowbirds (Andersson 1994), the vocalizations of male American Bison (Wyman et al. 2008). Studies in humans have also suggested a role for honest advertisements in the mating arena including face shape preferences, such as male face shape as an indicator of immunocompetance (Scott et al. 2014), and humor on the part of males as an indicator of intelligence and good genes (Miller 2000; Greengross and Miller 2011).
Is Female Resistance a Form of Female Choice?
Female resistance is defined as the process by which females evolved to resist male control of mate choice, allowing females control over their own mate choice decisions. Thus, female resistance can be seen as a form of sexual conflict through which females use resistance to select mates and perhaps shape the evolution of male structures and behaviors. Female resistance to males might be designed to gain indirect fitness benefits by allowing the female to select for males based on their ability to manipulate females or their physical strength in cases where male defense of offspring and mate might be required, and to select for “better genes” or material resources (Cordero and Eberhard 2003; West-Eberhard 2014).
Female selection of antagonistic traits in males via resistance has been documented in gladiator frogs (Kluge 1981). However, the evidence with regard to the role of female resistance in female choice in other species is rather limited. In the case of human females, it has been suggested that the commitment skepticism bias in Haselton and Buss (2000) is a form of filter resistance that deters females from being deceived about the costs of mating that a male may impose due to a lack of commitment. Such a bias on the part of females requires males to clearly demonstrate their willingness to commit and ability to provide benefits rather than impose costs.
While mate choice was once viewed as something males engaged in, males mainly competing for access to females, it is clear that females also engage in mate selection and their choices influence the evolution of male traits. Females may exert choice through inciting male display of honest advertisements of quality in addition to antagonistic displays, including female resistance. Females may also shape male traits via selection for the production of sexy sons.
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