Quanta Magazine

The idea of sexual selection is a concept that Charles Darwin first wrote about in The Origin of Species. He then expanded on it in The Descent of Man, where he described reproductive preferences between the sexes and how they could drive evolutionary change. It makes sense that people prefer mates who are fit. However, sexual selection has a crucial point. Attractiveness to potential partners can be a criterion of selection by itself. One sex member can have traits or behaviors that are appealing to the other, which could directly contradict survival-driven natural selection. This can lead to the unusually long display feathers of male birds that are too heavy and bulky for mating contests.The Victorian View of FemalesHowever, science has always focused on males from the beginning. Darwin saw females as choosing mates reluctantly from a group of desperate male suitors. Darwin was open to the possibility of sexual selection in any direction. However, the intense competition for male mates fed the notion that sexual selection occurred primarily to males. Females were prized to be won. Although the females may have set the rules for the mating competitions they were not the ones who were being changed by evolution.Darwin's view was typical for his time. The Victorian era was when theories about sexual selection emerged. There were certain sexual stereotypes about women's behavior, according to Rebecca Boulton, an evolutionary biologist from the University of Exeter in the U.K. This is why the field of sexual selection essentially sprung up in that period. They are either coy or choosy.Researchers behind the new study claim that this viewpoint has led to an omnipresent bias in sexual selection research over the past century. According to them, studies on male-male competition and female choice are 10x more common than studies focusing on the reverse.Salom Fromonteil is a graduate student studying evolutionary biology at Uppsala University, Sweden, and Ludwig Maximilian University, Munich. He was also the lead author of the study. It is influenced by what you read. They read that sexual selection is primarily a male phenomenon.There are, however, exceptions. Researchers have noticed some species that have sex roles other than the traditional arrangement. For example, the dance flies. The American tropical wading birds, wattled jacanas (Jacana Jacana), have females that protect territories full of male mates. Males can even incubate their young inside a pouch, which is used to incubate seahorses or other pipefish.Scientists studying sexual selection continue to rely on Darwin's original observations from the 19th century. It was widely accepted that males who were more inclined to courtship displays and ornaments experienced higher sexual selection pressures.That's not the way research should be done, according to Tim Janicke, an evolutionary biologist from the Center for Functional and Evolutionary Ecology, University of Montpellier, France. He is also the senior author of this new study. Data-driven syntheses are needed to help us understand general patterns found in nature.Janicke and his colleagues looked into the literature on the strength of sexual selectivity acting on different animal species. They then compared the values between the sexes. Science Advances published a study that confirmed that males are subject to greater sexual selection than their female counterparts.Janicke and his colleagues began to notice more examples of female sexual selection in species. Janicke stated that they began to wonder if it was a rare phenomenon or if it is a common pattern.Janicke, Fromonteil and others planned to begin an empirical study of sexual selection in beetles in 2020. Laboratory experiments became impossible after the COVID-19 pandemic, which caused institutional shutdowns. Janicke says that a meta-analysis could be performed even in confinement using data from the 2016 study.The scientists needed a way to quantify the intensity of sexual selection in order to compare the strength of the different species. The Bateman gradient was a measurement that was named after Angus John Bateman, a British geneticist.More Mates, more Sexual SelectionBateman realized that although males can produce a lot of sperm at a low metabolic cost, females must make higher investments in eggs and have to invest more. Bateman's 1940s research on fruit fly breeding led him to conclude that the fundamental difference in gamete investments drives the differences between male and female mating strategies. Males might seek out multiple partners to maximize their reproductive potential, while females may develop a preference for a few.This idea was further developed by other researchers who created the Bateman gradient, which describes the health benefits of multiple mating partners. This is the slope of a line that compares reproductive output and the number of mating events. It shows how much an organism's number of offspring rises with increasing mating. The slope of the line comparing reproductive output to the number of mating events is steeper, meaning that more mating events will have a greater fitness benefit. In most cases, this means more mates. A female can hedge her bets by mating with several males, rather than one.Janicke stated that if there is positive selection for having more mating partner, it should translate into competition. This competition is the essence of Darwiniansexual selection. Janicke said that Bateman gradients can be used to indirectly quantify sexual selection.The team calculated 111 Bateman gradients for females from 72 species of animal species using scientific literature. They used this information to compile the gradients. Although the gradients varied, they tended to be in the positive range. As expected, the team found that species with polyandrous males had Bateman gradients significantly higher than species with monoandrous females, who mainly mated with one male.These findings indicate that, as it has been assumed for many years that males have a greater fitness advantage than females due to multiple matings. This opens up the possibility of widespread sexual selection. Janicke stated that the positive female Bateman gradients are not as strong as the ones for males. However, their widespread presence suggests the importance of sexual selection in the evolution and development of females in all species.Janicke acknowledged that the Bateman gradient can be used to measure sexual selection. However, Janicke pointed out that it only represents selection that is directly related to mating. There are also opportunities for selection in species that produce a lot of eggs, like fish and corals, as eggs compete for sperm or have choice capabilities that influence fertilization. Janickes' study did not include this type of post-mating sexual selectivity. Although it is possible that more female sexual selection occurs that what the current study suggests, future research will be needed to confirm that possibility.Pizzari believes that the study confirms what we as a community have been beginning to understand for a while now: that sexual selection can be quite important for females of a variety of species as well as males.However, alternative explanations are still needed to be tested. Janickes team found that some of these gradients may be due to females' inherent attraction to males because of their reproductive output. Pizzari suggested that it could be possible that females with more eggs to produce attract more males because they have higher reproductive values. It wouldn't make sense that more matings would be more beneficial for females. Instead, it could be that males are more keen to mat with fecund womens.Jonathan Henshaw, an evolutionary biologist from the University of Freiburg, Germany, believes empirical tests may be able to isolate the effect if that is true. He said that if you want to determine the cause of mating and reproductive success, it would be a good idea to follow the example of others and manipulate the number of mates available to each female.Experiments may also reveal how sexual selection plays out in wild females, and how selection may be influencing traits that females use in competition. Boulton stated that there is potential for other mechanisms of sexual selection. It is difficult to discern if this is what is actually happening.Confrontations and DecorationsScientists are now more aware of the importance of looking for evidence that female sexual selection is occurring. This may make it easier to identify the traits and behaviors that females develop. Black grouse (Tetrao Tetrix) are a classic example male sexual selection. The males of this species have distinct and vivid colors, and compete for females in elaborate courtship displays called leks. Recent evidence shows that the males may be putting on a show but the females are jockeying for their picks. The female Mediterranean fruit flies Ceratitis capitata (Ceratitis capitata), have a similar approach. Some female dung beetles have even developed horns that can be used to fight with other females for access to males.Boulton says that intrasexual fighting is unlikely to occur in most females. Because fighting involves bodily harm, it is something males can afford to take on. They just have to live until they get pregnant. It takes time for female reproductive success: They must live long enough to conceive all their eggs and care for the babies. She said that the males have less to lose than they do.Sometimes, the ornaments and competitions used by females to find mates are overlooked as they are subtler and appear like a reduced version of what males do. Female Malurus fairy-wrens, for example, are subject to sexual selection on the color of their plumage and bill. This is independent from similar pressures placed on males. Male and female chickens (Gallus Gallus) both have fleshy ornamental hairs. Hens will choose roosters that have larger combs for mates. However, hens will also prefer roosters that have bigger combs.