The fundamental cause of sexual dimorphism is an asymmetry in the amount of parental investment in a given mating and the care of any resulting offspring. The sex that invests less time and energy in this process tends to be limited merely by access to mates, and hence is under strong selective pressure to attract as many mates as possible. The result is evolution of showy traits that attract the opposite sex, and competitive traits for competition with the same sex. This sex is often (but by no means always) the male, since ejaculates tend to be relatively cheap and males of many species do not provide parental care. The sex that invests more time and energy in this process is usually not limited by access to mates, and hence is not under strong selective pressure to find as many mates as possible. Rather, this "parental" sex is under pressure to select just a few good mates-sometimes just one. This sex is often, but by no means always, the female, since eggs are relatively large and females often provide parental care.
Intersexual selection refers to sexual selection for increased attractiveness to members of the opposite sex. This form of sexual selection tends to lead to "display" or "advertisement" traits, such as showy or colorful body parts, or exaggerated mating displays. Examples include long tails in male red-collared widowbirds and calling in male frogs. Intrasexual selection refers to sexual selection for increased ability to compete directly with members of the same sex for access to the opposite sex. Intrasexual selection tends to lead to weaponry, armor, fighting ability, and threat displays. Examples include large body size in iguanas, infanticide in lions, antlers in deer, and so on. Note that some traits may serve both functions-for example, bird song often functions both as a display that attracts females and also a threat that deters rival males.
In marine iguanas, many males are larger than the "optimum" body size (the size that can be maintained long-term), and experimental data confirms that survival rates are lower for the largest iguanas than for medium-sized iguanas. Similarly, in long-tailed widowbirds, males with intact long tail feathers lost weight at a greater rate than males with experimentally shortened feathers. These results indicate that sexual selection is working contrary to natural selection, and that "attractive" males may ultimately pay a price for attractiveness, in the form of lowered survival, shorter life spans, or reduced health.
Females may gain "good genes" for their offspring by choosing traits that indicate that the male is healthy and fit. An example is Welch's experiment on gray tree frogs, which demonstrated that offspring of long-calling males outperform those of short-calling males in several measures of health and rapid growth. Second, females may select males who provide them with a valuable resource, such as a food gift. An example is male hangingflies, who provide a food gift to the female. Third, females may have pre-existing sensory biases that can be exploited by males. An example is male water mites, which employ a mating display that appears to take advantage of females' tendency to turn toward vibrations-a trait that originally evolved for hunting, not for mating. Fourth, once a trait is preferred by a majority of females, the trait may become self-perpetuating because females that prefer that trait will tend to have "sexy sons". An example is spotted cucumber beetles, in which most females prefer fast-stroking males-a trait that appears to provide no advantage other than the fact that sons of those males will themselves be fast-strokers.
The pollen-producing parts are under more intense selective pressure for showiness because showy flowers attract more pollinators. Pollen-producing plants (or parts of plants) produce large amounts of tiny pollen, and are limited in reproductive success primarily by the number of mates they have access to-which means, the number of pollinators that they can attract. On the other hand, seed-producing plants (or parts of plants) produce just a few large fruits. Even a few pollinator visits is enough for them to receive enough pollen to fertilize all the seeds that they can bear. They do still need flowers, because they obviously do need to attract a few pollinators, but they do not need especially gaudy flowers because they do not need to attract enormous numbers of pollinators. Therefore, pollen-producers are under intense sexual selection for large and showy flowers, while seed-producers are not.
It is a reasonable inference for any species that a morphological difference between males and females is probably a result of sexual selection. It is particularly reasonable for this species, since data show that male reproductive success is strongly affected by access to mates, and males must, therefore, be under selective pressure for traits that increase their likelihood of obtaining mates. However, either male-male competition or female choice could explain the crests. To test the hypothesis that female choice has selected for this trait, a sample of females could be given visual access to males with different crest sizes. If female choice is operating, females should consistently prefer the male with the largest crest. Male-male competition could be responsible for the trait if, for example, having a larger crest allowed those males to dominate access to preferred breeding spots. This could be tested with a combination of laboratory dominance trials and careful observations in the field.
As the ratio of males to females increases, females will be competing to a lesser degree for access to males. Most females should succeed in finding a mate, and the variance in female reproductive success should decrease (that is, the difference between the "winners" and "losers" will be smaller). The frequency distributions of the number of mates and the number of offspring become more even. The relationship between number of mates and number of offspring should be correspondingly shallower.
A change in the ratio of females to males should have little to no effect on male reproductive success as they are already the limiting sex in this species. All males should garner at least some mates and all should have some reproductive success. Therefore, the graphs for male reproductive success should change minimally, if at all.
Extreme variance in male reproductive success implies that a few males within a breeding population are doing most of the mating; put another way, most females are mating with only a few males. A graph of the hypothesis for the relationship between number of mates and reproductive success for males and females would resemble that for rough-skinned newts: the slope of the line will be statistically indistinguishable from 0 for females and steeply positive for males.
Male elephant seals are likely four times larger than females because of sexual selection for winning male-male combat competition events. Male elephant seals fight one another for control over harems of females on nesting beaches; larger males are likely to have a significant advantage in these encounters. Size may be limited by energetic constraints imposed by the ability of males to find enough food to fuel their bulk. In this species, in which the male lies on top of the female during mating, extremely large males may even crush or smother females during mating attempts.
In general, the sex with the greatest reproductive investment should experience strong selection against making such mistakes, as any mistake in identifying an appropriate partner can cause a significant fitness cost. In contrast, the sex with minimal reproductive investment can, in effect, afford a few mistakes, and may benefit from a wider range of partners. A male frog that will mate with a large variety of females-females of various sizes, colors, behaviors, etc.-may occasionally make a "mistake" and stray completely outside its species. But the cost is minor (some sperm), and may be offset by the benefit of being willing to mate with a large variety of females that are indeed the correct species. Females, in contrast, may benefit greatly from not only identifying the appropriate species, sex, and level of physiological maturity of potential mates, but also their overall quality. And for females, the cost of making so flawed a choice that an expensive mating attempt is wasted is extremely high.
The sage grouse is most likely a male, displaying to females to attract mates. (We can guess this simply because males are most often the showier sex; but this is not universally true. See Exploring the Literature, below, for a counterexample.) The fact that this species has evolved elaborate plumage and a behavioral display suggests that access to mates limits reproductive success in males. This is most likely to be the case when males provide little or no parental care. A number of different social systems are possible under these conditions; the most likely is polygyny (one male mates with multiple females while females mate with a single, best male), but promiscuity (both males and females mate with more than one individual) is also a good possibility.
This sounds like a classic case of female choice based on resources provided by the male. If so, selection would favor mechanisms that allowed females to reliably choose the males that provided the most sodium for her eggs. Those mechanisms should include, minimally, variation in male phenotype (morphological or behavioral) correlated with the amount of sodium he sequesters and neurological mechanisms in the female that allow her to assess phenotypic variation in the males and select the ones with the most sodium. These mechanisms, in turn, should be associated with a general courtship pattern in which males "advertise" to females and females select among rival males. To test these ideas, a first step would be to measure many aspects of male phenotype and explore them for correlations with the amount of sodium males can provide. If specific phenotypic features are correlated with sodium, then controlled mate choice tests should reveal that females prefer the phenotypes generally associated with high sodium levels. If the phenotypic trait in question is behavioral and flexible (i.e., males behave differently when they've been able to sequester large amounts of sodium than when they haven't), then we could do more sophisticated tests in which the amount of sodium available to a male is controlled and male behavior and female choice are assessed.
Taken at face value, the data in the graph illustrate a positive correlation between parasite prevalence and the importance of physical attractiveness, suggesting that, in populations where parasite infections are most likely, physical appearance is a mechanism of mate choice. This pattern is consistent with the hypothesis that selection favors mate choice mechanisms that enhance fitness. Under this scenario, males and females are selecting attractive mates because they are the least likely to be carrying parasite infections. This could be because the parasites themselves cause disfiguration, or because the features that are associated with attractiveness cannot be maintained if a portion of the body's energy is being used to fight an ongoing infection. By selecting an attractive mate, an individual reaps the benefit of obtaining a strong genotype (one that confers the ability to ward off parasitic infections) for his or her offspring. A possible cultural explanation for this pattern is that, while attractiveness is still associated with lack of parasites (for the reasons explained above), the mechanism protecting individuals from parasites is simply the wealth and education needed to avoid infection in the first place. Under this scenario, resistance to parasites isn't heritable. Rather, the most attractive individuals are the wealthiest and healthiest, and are preferred because of their material resources.
Males were choosy and females competitive under conditions of low-food availability (the control). Males called less (spent less energy trying to attract females) and rejected females a greater percentage of the time, while females tended to seek multiple matings and exhibited more competition with other females. When food is limiting, the male spermatophore represents a significant investment of energy on the part of the male. In this case, female reproductive success is limited by access to mates, whereas male reproductive success is limited by the ability to make spermatophores. When extra food is provided, male spermatophores represent much less of an investment by the males, returning the system to the more usual condition in which male reproductive success will be limited by access to mates and female success by the ability to lay eggs. Under these conditions, males become competitive and females choosy: males spend more time calling (advertising) and reject fewer females; females seek fewer mates, reject more males, and compete less with one another.
The cost of male symbiosis for the male is that he can never change his choice of mate. If his female has reduced fertility or poor health, or cannot produce a large number of eggs, he is simply stuck with her (literally). In fact, if she dies, he will die, too. In addition, he may have to share the female with other symbiotic males, so he is not even guaranteed paternity of the eggs. The cost for the female of is that she bears a physiological and energetic cost of supplying energy and nutrients to the males, and the attached males also cause hydrodynamic drag while she is swimming. However, there are also benefits, primarily in mate-finding. It is very difficult to find mates in the vast and very sparsely populated environment of the deep ocean floor. Permanent attachment spares both sexes from having to find a mate or having to struggle to not lose each other. Overall, the males' symbiotic habit is likely the result of both natural and sexual selection. In this environment, sexual selection would favor traits that allowed a male to stay with one female for life, since mates are so difficult to find. Sexual selection, therefore, may have initially selected for the initial stages of the evolution of parasitism (physical attachment to the female). Subsequently, natural selection may have favored those aspects of male anatomy and physiology that allowed males to usurp the energy and nutrient resources of the females. However, males that usurped too much of the female's resources would bear a reproductive cost in terms of a reduction in the number of eggs the females could produce-so natural selection should also have favored a reduction in body size to limit the amount of energy required from the female.
