Editing Sexual selection (section)

== Sex role reversal ==

Sex role reversal (SRR) was first referred to as the phenomenon of females within a given species competing for mate access, rather than males fulfilling that role.<ref name="Fritzsche et al"/> Later, SRR was redefined to include cases where males in a species have higher parental investment. The concept's most notable mention was in Darwin’s The Descent of Man and Selection in Relation to Sex (1871). He brought attention to females having to undergo selection for access to male mating partners instead of the assumed natural order of males undergoing intra/intersexual competition of the time.<ref name="Eens et al."/>

From this point, several key studies were conducted on populations where females sought males, following Darwin’s description of the bird Barred buttonquail(Turnix suscitator).<ref name="Eens et al."/> This included studies of species that consistently subverted expected sex hierarchy norms, such as studies of the Pipefish family(Syngnathinae) or Seahorse family(Hippocampus).<ref name="Vincent et al"/> Females of these species are generally larger, more colorful, and more aggressive than males.<ref name="Eens et al."/> Most studies that succeeded after Darwin’s notes focused on this supposed reversal of animal behavior and attempted to understand what caused this female dominance.<ref name="Gwynne"/> Most studies during the first half of the 20th century believed it to be a result of unbalanced sex ratios, i.e, many more females than males of a population, and for this hierarchy to be unchanging.<ref name="Fritzsche et al"/> Keynote studies on reproduction of fruit flies(Drosophila) in the 1960s, however, illustrated sexual diversity based on environmental factors such as food availability as well as sex ratios.<ref name="Merrell"/> What was also distinct about the studies was that they demonstrated how quickly the sexes' passive and dominant roles could change given the ecological conditions.<ref name="Merrell"/>

The redefinition and wide use of SRR as a tool in animal behavior came from the 1970s. Darwin’s mention of sex role reversal resulting from a much larger ratio of females to males was later picked up by researchers Stephen T. Emlen and Lewis W. Oring, who reworked the initial concept to a concrete definition. As opposed to females, SRR became redefined by males taking on the bulk of parental investment of offspring.<ref name="Fritzsche et al"/> Researchers were attempting to exclude human-biased projections onto animal behavior. SRR showing parental investment as opposed to “masculine” sexual behaviors performed by females was an attempt to exclude human bias from animal observation. The highest consistency of males taking on higher parental investment than females was most noted in fish, bird, or amphibian species.<ref name="Fritzsche et al"/> Biologists’ new understanding of sexual selection came from observing mate selection based upon resource availability. 
Resource and mating trade-offs exist for any sexually reproducing organism. For example, male Sandpipers(Actitis macularia) are generally responsible for caring for the nest and protecting eggs, a task seen in other bird species to be shared with or exclusive to female members.<ref name="Gwynne"/> Thus, other mating opportunities for male sandpipers are a tradeoff for offspring care, making them unavailable for breeding. Generally, the sex that has to produce eggs and care for offspring in the zygote stage will divest resources from itself in the post-zygotic stage. This means that during this period, they would be unable to engage in other mating opportunities, providing a barrier to mating rates. Instances of the zygote carrying sex (i.e, the female) devoting less time to post-zygote parenting do present many questions to population behaviour.<ref name="Gwynne"/>

After Emlen and Oring's publications,<ref name="Emlen et al"/> questions remained on what environmental conditions led to SRR. A smaller ratio of males to females is the most widely accepted reason for SRR. Populations that consistently have a higher number of females to males have been noted for distinct dimorphism, as well as males undertaking higher parental investment in particular species. Animals exhibiting monogamous mating behaviour very rarely have traits of SRR. The most common mating type to take on SSR is polygamous species, in which dominant individuals have access to many mating partners, and conspecifics are driven away. Examples of polygamy coinciding with SRR are consistent with the Gulf pipefish(Syngnathus scovelli).<ref name="Vincent et al"/> Pipefish have been studied for decades due to observations of males being the choosier sex of mating partners, and females selected based on markers of higher quality, such as sex and the presence of secondary traits. Dominant females had numerous male mates, while other females were driven away. The study found that the same line of strong sexual dimorphism in polygamous species was carried to SRR examples, with females being the ones facing sexual selection as opposed to males.<ref name="Gwynne"/>

Other factors, like resource availability, may affect populations switching towards or away from SRR. Recently, however, biologists studying numerous different insect species have found that nest availability could be an enormous factor. The relative availability of nests for females to lay eggs was a determining factor in whether or not parental investment by males and female sexual dominance became present in the Broad-winged katydid(Microcentrum rhombifolium). When nests are less frequent in an environment, females undergo intrasexual competition to gain access to males, and males generally take on more parenting responsibilities. Co-parenting becomes much more common for populations of katydids in a habitat that allows for more nests, and female aggression decreases.<ref name="Gwynne"/>

Researchers now understand that the dynamics between male and female counterparts are more complex than “female dominant” and “male dominant.” The field is studying the adaptation of different sexual roles as a result of fluid factors such as sex ratio or resource investment.