Sexual selection, as distinguished by Charles Darwin, is known as a variance in the number of mates. In other words as differences in mating success that results in adaptations for obtaining mates, rather than adaptations for survival. Sexual selection acts to refine secondary sexual characters of the phenotype such as morphological differences between males and females, or differences between male types.

Sexual selection has the capacity to evolve maladaptive traits, by producing individuals with such elaborate ornaments that they must be either energetically costly to develop, maintain, or disrupt the direct survival of the individual. This does not adapt the individual for the environment, as in natural selection, but does aid in mate acquisition. This idea of sexual selection gave an aid in answering the question of why some species had acquired such bizarre traits.

Trivers redefined the idea of sexual selection by observing that females are the limiting sex and care more for the offspring than do the males, making them unavailable for fertilization because they are carrying or caring for young. He observed that males tend to be in excess therefore developing ornaments to attract available females for mating, or to engage in contest with other males. It is not always the case that males are competing for females, in the example of phalaropes a polyandrous species, the brightly colored females compete with one another for the plain colored males.

Sexual selection rises in response to either female choice or male competition. Intersexual selection is in which a female chooses a mate based on elaborate ornamentation or behavior. Intersexual selection influences the evolution of secondary sexual characteristics, which determines the relative ≥attractiveness≤ of members of one sex to another, such as courtship displays and male plumage. Intrasexual selection (intra meaning within), described as selections of traits that increase ability of the same sex too compete to obtain members of the opposite sex. Intrasexual selection occurs for territory and access to females, or area on mating grounds in which displays take place. Male-male competition for mates can lead to intense battles that involve the use of horns, hooves, and teeth. Often this affects fighting ability and, more successful fighters receive more copulations than less successful ones. If one male is physically able to prevent another male form mating with a perspective female, then these strong competitors have a selective advantage despite potential female preferences for other types of males. Male-male competition is the driving force for why we see males of a species being much larger than females, as in the Gorilla. Modern genetic techniques have discovered that the actual genetic mating success is less skewed toward successful fighters then the copulations data would suggest, being the choosy sex may choose for traits other than fighting ability. Often, it is difficult to separate the two forms of sexual selection. There are clearly cases that consist of both intersexual and intrasexual selection, being when males compete among themselves for high quality territories, which when then determine the number of females, which mated with each male.

Sperm competition, or competition between multiple malesπ sperm to fertilize a females egg (Birkhead and Hunter 1990). Evidence of sperm competition is found in invertebrates, reptiles, mammals and humans. An example of sperm competition corresponds to the size of the malesπ testicles. Males of a species will have larger testicles if they need to produce a lot of sperm to fertilize a female. Large testicles allow a male to enter more sperm therefore giving him better odds; as in the chimpanzee that live in loosely structured multi-male, multi-female communities. When the female chimpanzee is in estrus she is often attended by a host of males, many will mate with her on the same day. This means that a lot of sperm from many different individuals will be competing to fertilize one egg and the male with the biggest testicles will produce the most sperm, and so is most likely to fertilize the female. As for gorillas that live in cohesive groups usually consisting of one alpha male and two or three females, when the female gorillas are ready to mate, normally only the alpha male is their partner. In a group where there is more than one male the alpha male normally monopolizes the females.

Sperm competition also affects the evolution of male and female traits. The effects of sperm competition are varied ranging from post-copulatory mate guarding and incredibly high sperm counts to penises fashioned with brushes and hooks to remove rival sperm from the female reproductive tract. Another example of the importance of sperm size is in C. elegans and related nematode species. These soil inhabitants are composed in large part of self-fertilizing hermaphrodites, which can only receive sperm and not donate to other individuals. It was found that the male sperm take precedence in the absence of seminal fluid and that the male sperm cells themselves are competitively superior to hermaphrodite sperm. The factor that turns out to determine competitiveness is sperm size: male sperm are larger than hermaphrodite sperm ( ). Males adopt the strategy of mate guarding to ensure being the last male to deposit sperm into a particular female, exhibited in dungflies, water striders, and damselflies (Warren and Gish 1943). Anti-aphrodisiacs, pheromones to reduce the attractiveness of the female, are sprayed on her after copulation to prevent re-mating, an example the male butterfly Crocthemis erythraea has a penis that inflates inside the sperm theca where its hornlike appendages pack rival males sperm into the corners of the sperm theca where they are less likely to fertilize an egg. Male crickets (Truljalia hibionis) remove sperm from previous mating with their penises and then groom it off their penis and eat it (Birkhead and Hunter 1990). Mammals lack sperm storage vessels, so stratification or physical displacement of sperm by rival males is impossible (Gomendio and Roldan 1993). In determining paternity of the offspring the timing of copulation relative to ovulation, and the amount of sperm inseminated by each male are important. When they do compete, it is advantageous to have the most sperm possible inseminated and that the sperm be fast swimmers (Gomendio and Roldan 1993). It is difficult to say whether sperm competition is grouped is part of intersexual selection or intrasexual selection, males compete for access and timing to females but females manipulate their behavior to favor one male or another (Gomendio and Roldan 1993).

Rape, or forced copulation without the females consent, occurs in the animal kingdom explained in an evolutionary context as a natural behavior (Shield and Shield 1983). The individuals able to adapt most effectively to their environment by possessing characteristics that will benefit the survival of their genes will pass their traits onto their ancestors (Alcock 1984). In certain situations forced copulation, may had been a beneficial behavior, and as a result, had become more pronounced in the gene pool. A male increases his fitness by mating with more females (Alcock 1984). Thornhill and Thornhill (1983a) cite three reproductive strategies that are characteristic of the male Panorpa. Males may present food items or salivary masses to the female to feed on during copulation, they may feed the female a dead arthropod and they may force copulation. In the first two strategies, the male provides resources (food) for the female and then mates with her. In the third instance, the male does not bring anything for the female, and yet still copulates with her. Because the female Panorpa risks raising her offspring with no help from the father, and she gains nothing directly from the mating, she has been selected to resist copulating with males lacking resources. It would be more beneficial for her to receive a nuptial gift and copulate than copulate and receive nothing. Males lacking resources should be more likely to rape, as they cannot offer the female anything before mating. The male would have had to force copulation in order to fertilize any eggs with his sperm and pass along his genes to the next generation. Natural selection would have favored those males who were able to reproduce sexually with desirable mates, even if this was accomplished through forced copulation (Thornhill and Thornhill 1983b).

Sexual competition occurs for every species whether intersexual selection, intrasexual selection, or sperm competition. Competition between males of a species can either be by physical contact (male-male competition) or physiological (sperm competition), these can act alone or can combine and several types of competition can occur at the same time.

There are four mating systems expressed by Darwin. Monogamy, mating occurs with only one partner. Ninety percent of birds are monogamous, due to the high level of parental care their young require. Polygamy, mating occurring with many partners. Polygyny, a male mating with several female, occurring especially when the males have control over the resources or females. Polygyny creates strong selection for larger males, as seen in the gorillas previously and elephant seals. Polyandry, a female mates with several males, a very rare mating system that occurs when suitable breeding sites are scarce and nests are under heavy predation as in the American Jacana. A mating system is determined by several factors. If an environment is rich in resources with a patchy distribution and synchronous breeding among females, these features can either, accompany one another or exist solo and polygyny will occur. Monogamy exists in a impoverished environment and an area of high predation; the couple would have a higher survival rate if they were together rather than separate. When there is a high requirement for the parents by the offspring than the father is either monogamous or polygynous. In many species, females mate with more than one male as an insurance in case the first male is infertile. Multiple paternity also increases the genetic diversity of the offspring and the proportion of their survival. A general rule is that the smaller the proportion of males that mate, the more intense will be the selection for large male size (ten percent in elephant seals).

There are benefits for females of a species to be choosy. It is hypothesized that when males transfer resources to females it effects how they produce eggs, choose partners, and select sperm to fertilize the egg. Males will sometimes present nuptial gifts usually in the form of a nutritious medium found either in the ejaculate or spermatophore, which the female can then incorporate into their eggs. Nuptial gifts occur in various forms and are offered by a number of species. In some species such as the praying mantis the females are much larger and aggressive and often devourers the males head while mating or shortly there after. The females release pheromones that initiate copulation and gain nutrition from the cannibalistic act. The male spider has benefits to being cannibalized, he can inseminate more eggs posthumously then he would if he had survived and the female is less likely to mate if she has mated and consumed a male. The male redback spider persuades his mate to eat him during copulation by doing a somersault bringing his body right next to the femalesπ jaws.

Females, in some species, will choose a male based upon appearance or courtship displays more than for material considerations. Generally, females choose based upon more intense and varied courtship attempts, and larger more vivid bright displayed ornaments, more extreme rates of acoustical and tactical displays or a combination of all of the above (Kodric-Brown). Females that take these cues are thought to believe that their mate can offer them something in return for copulation. Symmetry can also play a role in female selection of a mate. Females tend to prefer symmetrical to asymmetrical males, observed in zebra finches, barn swallows, Japanese scorpion flies, and in the facial and body structure of humans. Asymmetry, believed by females to be a sign of a disruption of homeostasis and equal development of both body halves. Thus, body asymmetry is associated with poor health or bad genes.

There are three theories associated with a mate choice by a female. The first being the healthy mate theory, that states that the appearance of the males courtships inform females of potential sex partnersπ health or parasite load. Matting with these individuals will less likely pass on parasites and micro bacterial pathogens. The next, being the good genes theory, argues that male courtship and appearance provide information to females about the health and condition of a would be mate but states that the benefit from mating with a robust and healthy partner lies in the acquisition of genes conducive to offspring survival. Runaway selection (a.k.a. Fisherian sexual selection) proposes a different sort of genetic benefit. Postulated that females prefer traits in mates that are sexually attractive, so as to pass these traits to their sons, making it more likely for them to reproduce and pass on their genetic information.

Bibliography

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3. Kodric-Brown, A. 1993. Female choice of multiple male criteria in guppies: Interacting effects of dominance, coloration, and courtship. Behavioral Ecology and Sociobiology. 32: 415-420.

4. Gomendio, M. and Roldan, E.R.S. 1993. Mechanisms of sperm competition: linking physiology and behavioral ecology. Trends in Ecology and Evolution. 8:95-99.

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