It is clear that in sexually reproducing species, we see both physiological and behavioral differences between the sexes. For example, in many bird species, the male is more brightly colored than the female, and in mammals, we see males possessing very large canine teeth, manes, or horns/antlers/tusks that females do not have.
Some of these differences between the sexes cannot be explained from the simple physiological necessities of successful reproduction (which could be explained by natural selection). For example, a male bird of paradise could father a successful clutch if his elaborate plume was plucked and a male red deer could sire many offspring if his antlers were sawn off. Why do males of so many species have elaborate characteristics?
These traits are adaptations in that they allow for gaining mating access to females; that is the plumes of a bird attract the female and the antlers of a deer allow him to fight for access to a harem of females. Over time, evolution has selected and exaggerated traits that already existed in a species, and made them more and more noticeable; however since the purpose of these exaggerations is to either attract or compete for a mate, even very differing sets of characteristics can be compared in terms of selective processes that produced the traits between species (the assumption that similar selective forces will result in similar adaptations). For example, the antlers of a red deer and the claws of a fiddler crab are essentially the same thing; that is, they are a weapon that allows access to females because of male-male competition.
In order to talk about the selective forces that are implicated in sex differences, we need to differentiate between natural selection and sexual selection. Natural selection has to do with traits that organisms clearly need to survive and to reproduce as males or females. Testes, ovaries and associated plumbing are accepted as having evolved, at least initially, through natural selection. However, sex differences cannot be explained so easily and we must be careful to make a distinction between differences that have evolved through natural selection and those that have evolved through selection in relation to sex. The distinction, although somewhat artificial is valuable. For example, a male crab may need claws large enough to hold a female during copulation. If some crabs copulate in rougher waters than others, their claws may need to be larger to hold on to the female. Those claws would be organs necessary for reproduction and would be the result of natural selection. However, some crabs, like the fiddler crab have claws which are far larger than necessary to clasp the female, and which may hamper their survival. These sex differences are of interest because they do not have a ready explanation. For example, what causes variation among species in the degree of enlargement of male claws? Or, when only one sperm is necessary for an egg to be fertilized, why do the males of some species have a much larger sperm-producing capacity than their close relatives? We might suspect that these are the males of species where the females need to fertilize more than one egg or where the male gets the opportunity to fertilize the eggs of more than one female. We would be wrong. If we were right, the differences between related species would follow as a result of what Darwin called natural selection, rather than what we call sexual selection.
Historically, sex differences arising from sexual selection have been considered to be of two types.
First, differences can result from mating preferences: if females will mate only with males that have a particular characteristic such as bright colors, then males may evolve bright colors.
Second, differences can result from selection for success at combat among members of one sex to gain mating access to members of the other sex; if females are a limited resource, then males that can win fights for access to them will be the ones that leave offspring.
More recently, a number of sex differences have been ascribed to a more subtle form of combat involving sperm competition. Whenever females mate with several males, then those males might have evolved characteristics to help them win at fertilizing the eggs.
Which sex will diverge farthest from the natural selection optimum?
The answer depends on which sex is a more limited resource for reproduction by the other. Often, males provide very little for their young, and their rate of reproduction is limited only by the number of eggs they can fertilize. The potential reproductive rate of females, on the other hand, tends to be limited by the number of eggs they can produce or by the number of young they can raise. If females are a limited resource for males, Darwin argued that the consequent 'law of battle' among males was bound to favor the elaboration of behavioral displays and ornaments to attract females, as well as weapons and greater body size to fight with other males. The evolution of sexually selected traits will then depend on genetic variation for precursors in the lineage and proceed to the point where the cost in terms of reduced viability is offset by the gains in reproductive success (adaptation). Where females contribute little or no parental care relative to the male, and male care cannot be shared easily among the offspring of many females, males become a limiting resource. Females are thus more competitive, and typically evolve more elaborate courtship displays and weaponry. Well-known examples include phalaropes and some species of pipefishes, dendrobatid frogs, midwife toads, giant water bugs and bush crickets.
Sexual selection can be important in sexually reproducing species, including plants, and it can have profound effects extending beyond reproduction. For example, if males evolve a larger size than females, we should expect them to have a different diet and generally to behave in ways that accord with their larger size. In other words, they will come to occupy a different ecological niche from females.
More in-depth exploration of Mate choice, combat and sperm competition:
Do females prefer ornamentation simply for the fact of ornamentation?
Males of many species spend a large amount of time displaying rather ornate characteristics, such as bright colors or long tails. Why have these evolved? Darwin suspected that such traits might appeal to an esthetic sense of females. Darwin, however, lacked experimental evidence for this supposition, although he did talk a pigeon fancier he knew into dyeing the feathers of male birds magenta. However, the female pigeons were not impressed, nor were other theorists in general impressed with the idea that females should prefer non-adaptive ornaments and displays.
However, Semler and Andersson noticed that male three-spined sticklebacks in a Washington lake varied greatly in the extent of coloration on their throats; females were silver-brownish, and preferred to mate with red throated males. They then painted dull-colored males with red lipstick and others with transparent lip gloss (controls). In lab preference tests, females preferred the red males, suggesting that female choice may be at least partly responsible for the evolution of red colors in male sticklebacks (although the red color may indicate something else and thus not simply be a preference for red by the female).
Andersson then used the African long-tailed willowbird to test for female choice. The females are dull with short tails, while the males are black with red wing epaulets and tails measured about 50 cm. Anderssson cut the tails of some males and used the feathers to extend the tails of other males. The result was that the females nested in the territories of males with elongated tails at the expense of males with normal or shortened tails.
Similar experiments have been performed on a variety of species. For example, in barn swallows, the females prefer the males with elongated tails and if the male they are paired with does not have a long tail, they will engage in extra- pair copulations with the long-tailed males.
Although female choice may be at least partly responsible for the evolution of elaborate male display traits, it is still not obvious why females prefer mating with such males.
Several possibilities have been discussed in the literature, but at the moment, there is no consensus.
R. A Fisher suggested that if females develop a slight preference for males of a particular sort, say with tails longer than average, then those males will father a disproportionate number of offspring and at the same time, the characteristic will become exaggerated. Fisher envisaged that females were selected to prefer males with longer tails because, originally, longer tails had a natural selection advantage, but that tail size would increase due to the mating advantage it conferred on sons until it reached beyond its optimum and had a natural selection disadvantage (this would occur only if there were no cost associated to the female with the choice of a longer tailed male) (runaway selection). Thus, females that mated with preferred males would produce offspring that inherited the genes for the preference from their mothers AND the genes for the attractive male characteristic from their fathers. Sons that expressed the preferred trait would have better fitness and daughters that respond to the trait would gain by producing sons who are preferred. Thus, female mate choice genes and genes for the preferred male trait can be inherited together. This could begin a runaway process in which ever more extreme female preferences and male traits spread together as new mutations occur. The runaway process would end only when natural selection against costly or risky displays balanced sexual selection in favor of traits that are preferred by females. Models of sexual selection have also demonstrated that there not need be any underlying selection advantage to the trait, but that females would simply have to demonstrate some kind of underlying sensory process that favored a particular sensory stimulus. Thus, the trait might be a disadvantage to the sex that has it, and the only advantage is that females prefer it.
Others have argued that the displays indicated that their bearers possess 'good genes' of some kind. Zahavi suggested that if a male can survive with a viability handicap, such as a long tail, females should choose to mate with him, since he has demonstrated his vigor in the face of adversity. Although this idea was treated with a great deal of scorn for many years, it has gained increasing acceptance, and has produced some innovative spinoffs. For example, Hamilton and Zuk argued that parasitized birds cannot develop extravagant secondary sexual characteristics, thus if resistance to parasites is heritable, by choosing showy males as mates, females will produce offspring that are resistant to parasites. More recently, Moller and Thornhill have argued that the degree of bilateral asymmetry in sexually selected traits might indicate quality. During development of a fetus, it is physiologically difficult for animals to produce symmetry, and any genetic abnormality or environmental insult such as toxin exposure, malnutrition, stress, etc. might be reflected in differences in the lengths of the left and right elongated tail feathers of barn swallows or in the size and shape of patches of color on each side of a fish's body. Early tests have been encouraging in support of this idea.
A third possibility is that extravagant male characteristics could reliably indicate direct benefits to females or their offspring. In species in which males contribute parental care or territorial resources, perhaps some male sexual traits reliably reflect such direct benefit in addition to any good genetic quality they might indicate. Again, bilateral asymmetry of ornaments is a potential cue whereby animals could assess either genetic or environmentally caused variation in quality of prospective mates, and hence, the resources they will provide for offspring. Ornaments might also indicate more subtle direct benefits, to mates, even in species where members of the opposite sex appear to contribute little besides genes to the offspring. For example, if long tails demonstrate that males are healthy and dominant over rivals, females might choose such males because they run a lower risk of catching a sexually transmitted disease or of being harassed by other males when mating. If the costs of choice are low, then these slight benefits could augment genetic benefits and explain why females are often choosy even in species where males have little to offer.
Currently there is a compelling body of experimental evidence that shows the importance of mate choice in the evolution of sexually dimorphic traits. But, so far, few solid predictions have been upheld by comparisons among species of display characteristics, and correlations of selective forces and extravagance are weak.
Could displays be selected for because of reasons other than mate choice?
Sex differences in colors may not depend solely on mate choice, but on differences in the importance of signaling to predators. For example, some traits may signal to predators that this will be 'unprofitable prey' because of the animal's ability to run or hide, or they signal poison or a bad taste.
Perhaps ornamentation develops in species unless it is prevented. Thus, those species for whom a color would be costly in terms of attracting predators may not develop them, while those for whom the cost is low do develop them and they have nothing to do with mate choice at all. The corollary to this is that costly ornaments will not develop even if there is sexual selection; ornaments only develop when the benefits outweigh the costs.
It could be that a species has a preexisting bias in a sensory system due to some other pressure. For example, if in the past, a preferred food was highly colored, a preference that has nothing to do with mate choice might have developed. Some studies of frogs and swordtail fish have confirmed that females may have pre-existing preferences for traits which males of their own species may not possess. The implication is that if such traits to arise, they may evolve rapidly by female choice, perhaps because initially their bearers are easy for females to detect and later, as the traits become costly, they indicate male quality.
The finding that many species considered to be monogamous actually engage in extra-pair matings has also made determining whether or not a trait is due to sexual selection more difficult. We cannot assume that males sire every offspring born to their mates, since it is clear that many offspring are born from matings with other males.
In many species, the struggle to obtain mates can be rather violent, and Darwin suggested that fights between members of the same sex for dominance or resources which allow them to attract members of the opposite sex may be responsible for the evolution of many costly traits, such as antlers, tusks, or large body size. The potential gains in reproductive success are likely to be particularly high whenever there is the opportunity to mate with numerous members of the opposite sex. Since these are two sided contests, they constitute the essential components of an arms race, which encourages an escalation in the development of weapons. Selection, in the long run, will favor the evolution of larger and more effective weapons, as long as the functional benefits outweigh the overall costs of developing and carrying such an armory.
A brief survey across mammals reveals many familiar examples of specialized weaponry: tusks (walrus, elephant, bush pig and hippopotamus) spiral or curved horns (antelope, sheep, goats, cattle), skin covered pedicles (giraffe) antlers (deer, elk, reindeer). In most, if not all of these cases, it appears to be competition between individuals within the species (intraspecific selection) that accounts for the evolution of the weaponry. In particular, it is the competition between males for social dominance, harems or territory that favors the development of weapons. Thus, males have bigger teeth, horns and antlers than females, and males of polygynous (one male mating with many females) species, where competition for mates is most intense, have the biggest weaponry of all.
In the case of females who develop large weaponry, the functional significance of such equipment usually relates to its value in defense from predators, particularly defense of young and in intraspecific competition when females compete with other females (and/or maybe males) for food, space and shelter. In most social species there is a clearly defined dominance hierarchy, with dominance allowing access to the best feeding and resting places. The best example of females with well-developed weaponry are in species that live in large mixed sex groups where competition for food is particularly intense (gemsbok, reindeer, both species in which food supply can be very patchy and seasonal).
In a comparative study, Craig Packer attempted to distinguish between the effects of sexual and natural selection on the horns of African antelopes. Male antelopes use their horns in vigorous head to head clashes when competing for access to females. Both sexes use their horns for defense against predators and females use their horns to defend their young. Packer compared three components of horn morphology: total length, basal area and shape. Among those species where both sexes possess horns, males and females have horns of equal length, but male horns have double the basal area and tend to be more curved with tips pointing back to the base rather than forward as in females.
Packer interpreted his results, as follows: The increased basal
area of male's horns allows them to withstand twice the force without breaking during a butting or shoving match. Packer goes on to argue that the more complex horn shape in the male serves to catch the blows of an opponent. Since females use their horns to ward off attacks by predators, they do not need such thick horns. A study of the structure and function of male antlers revealed that the antler is strengthened mechanically by the deposition of extra bone around the main beam and tines and in all species studied, the actual strength was many times greater than that necessary to support the weight of the antlers. The conclusion is that antlers are designed to fulfill their role as fighting weapons.
This and other studies suggest that to understand differences among species in weapons and body size, we should pay attention to the details of how animals fight with one another. Shoving, biting, grappling or stabbing can each select for different combative and defensive traits. Whichever traits are favored:
- they tend to be more pronounced in males, compared with females
- occur in mammal species where females live in groups (usually, combinations of the need for defense against predators and the clumping of food resources in space and time select for differences in female group size).
- occur in species in which males have potential mating access to more than one female.
Since the 1970s, many studies have demonstrated clear relationships between the extent of male weaponry and the degree of polygyny, as measured by female group size. For example, species of deer with a highly polygynous mating system have bigger antlers relative to body size than deer that form small mating groups. This is consistent with the idea that increased competition between males, which occurs with increasing polygyny, favors the evolution of larger and more complex weapons, The selective pressure associated with polygyny also results in increased body size, musculature and aggressive behavior, all attributes that contribute to fighting success. Species such as red deer, wapiti (elk), fallow deer and reindeer are typical of large, social, polygynous species that fight overtly during a brief rutting season for access to females.
Studies of deer have indicated that during the rut, stags fight frequently. Once study showed that stags on the Isle of Rhum (off west coast of Scotland) are involved in a serious fight once every five days on average during the rutting season and injuries are common. The animals use roaring behavior to advertise their presence and to intimidate other stags. A challenge between two rivals involves a ritualized display. The two animals parade side by side a few yards apart. If neither animal withdraws, a fight will develop as the opponents turn to face each other. The antlers are interlocked and animals viciously attempt to force each other backwards and off balance. The sharp points of the antlers act as dangerous weapons, while the curved tines form a shield to block the opponent. The outcome of a fight is usually decisive and the vanquished animal is chased from the scene. Virtually all matings are achieved by the stags that can exclude rivals and can monopolize a harem during the main period of rut when the hinds are coming into estrus and ovulating.
A number of studies in deer have experimentally manipulated the size of antlers. In one study, the main portion of the antler was removed from the dominant stag shortly before the rutting season. He was immediately challenged by the number two stag in the bachelor group. There was a brief and vicious fight, which the dominant stag won by virtue of his greater body weight, although he clearly had to compensate for the missing antlers (he was fighting for a while with imaginary antlers and making judgment mistakes about when contact would be made). Although he won that fight, a series of other challenges ensued by other subordinate stags and after a series of defeats, the experimental animal was relegated to near the bottom of the social hierarchy. During the ensuing rutting season, he was excluded from the company of the hinds by the other stags and never formed a stable harem. His lowly status continued throughout the winter until the following year when he redeveloped his normal set of antlers and regained his dominance and regained a harem during the rut.
In an influential series of papers in the 1970s, Roger Short suggested that differences in testes size among great apes (humans, gorillas, chimps, bonobos) might result from sperm competition: gorillas are four times the weight of chimpanzees, but chimpanzees have testes that are four times the weight of gorilla testes. Since female gorillas mate with just one male per estrus (a female mates only every 3 - 4 years and a harem contains 2 - 4 females, so a male gorilla may only be mating once a year), that male only needs to produce enough sperm to ensure fertilization of the egg. In contrast, female chimpanzees mate with several males during their estrus, thereby causing sperm competition. One way of increasing the chance of fathering offspring is to 'have more tickets in the lottery', that is to produce more sperm (hence the need for larger testes).
Similar patterns have been found in other mammals and birds. For example, in birds, the largest testes occur in polyandrous (females mate with many males) species where females attempt to form pair bonds with more than one male per season, and hence the potential for sperm competition is particularly high (in contrast, in species in which females mate with only one male, even though males mate many times with many females, testes are small; e.g. lek mating systems in birds).
It is important to realize that the comparative patterns expected from sperm competition, a form of post-mating sexual selection, are different from those expected from pre-mating sexual selection. For example, there is more potential for male combat to be advantageous in polygynous primates than in monogamous ones, but among the former, only those species where females mate with multiple partners will be selected to have large testes. As a consequence, we find the following patterns:
Monogamy (some species of birds, prairie voles): In some long-lived birds and a few mammals, the male is mated monogamously to the same female for life. In this kind of breeding system, a number of things will be true:
First of all, the kind of breeding system a species has is assumed to be related to the energy investment that each parent must make in raising the young. Thus, bird species are more often monogamous than mammals, since bird parents feed their young equally, while mammalian mothers must lactate and invest an inordinate amount of energy (in comparison to the male) in raising the young (thus, monogamy = equal parental investment).
In a monogamous system, it is assumed that the number of adult females and males is approximately equal and that both sexes have to compete for mates to approximately the same extent. Thus, there is little intraspecific competition for mates, leading to # 3:
no male-male or female-female combat to obtain mates, leading to #4:
Males and females are likely to be the same size (little sexual size dimorphism) since males don't have to compete with other males for access to females and engage in combat; leading to # 5:
male mating should not be delayed since males do not have to attain a certain body size or competence to fight for access to females.
Since males are mated permanently to a particular female, a male's breeding success depends largely on the breeding success of his mate and lifetime reproductive success (the closest measurement of fitness we have) is usually equal in the two sexes (although it may be greater in the male if he engages in extra-pair copulations).
since males and females are copulating within a pair bonded couple, females should not need to advertise when they are in estrus or ovulating, so ovulation should be hidden.
Since males and females are monogamous, there should be no sperm competition, since males are not copulating with many females that many males have access to, leading to #9:
there shouldn't be sexually selected characteristics, there is no sperm competition and no combat, leading to #10:
(Note: there are 3 suggested ultimate causes for sexually selected for characteristics, mate attraction, male-male combat and sperm competition. Here we have eliminated two, what about mate attraction. Could a monogamous species have sexually selected for characteristics because of attraction to colors or long tails (for example)? Yes, although in those species that have such sexually selected for characteristics, whom we formerly thought were monogamous, we are now finding that there is actually sperm competition occurring, with females engaging in extra pair copulations. Exp: barn swallows).
testes size should be small, since there is no sperm competition, and leading to #11:
high numbers of abnormal sperm, because there is no sperm competition.
Polygyny: (one male, many females; females only mate with one male; gorillas, for example)
parental investment is greater in females than males, females do almost all of the child care.
the ratio of receptive females to sexually active males (the operational sex-ratio) typically shows a strong bias towards males:
male-male combat for access to a female or a group of females, and one male does all or most of the mating with a group of females.
Where large body size has a greater effect on the breeding success of males than of females, males are likely to evolve to be larger than females, and this is presumably why a pronounced sexual size dimorphism is a common feature of polygynous vertebrates. In contrast, where body size has little effect on male success, little or no sexual dimorphism is seen. So, for example, in two closely related species, the monogamous prairie vole has little sexual size dimorphism; while the meadow vole, which is polygynous, has a pronounced sexual size dimorphism.
male mating is delayed until the male is large enough and experienced enough to challenge other males to combat, thus, mating may be delayed for quite some time after sexual maturity is reached. For example, in red deer, males rarely mate successfully before reaching adult weight at 7 or 8 years of age, while bull elephant seals rarely breed before the age of 9 years.
male's fitness can be much greater than that of females, since they are mating with many females. For example, in red deer, the most successful stags produce about 30 offspring in a lifetime, while the most successful does produce about 9 young. However, males in polygynous species rarely have a prolonged prime; red deer males rarely breed past the age of 12 and bull elephant seals past the age of 13; in contrast the effective breeding life-span of females of these species is about twice as long as the males.
This compression of breeding life-span varies with the form of inter-male competition which exists in a species. For example, in some primate species and in dolphins, males commonly assist each other in competition for females and, at least for primates, a male's status and breeding success is heavily influenced by the rank of the assisting males. Thus breeding life-span is affected less by age than in a more intensively competitive species like deer. In some polygynous species, such as some human societies, breeding success is actually increased by age, since as a male accumulates wealth and power he becomes more attractive as a mate and may acquire multiple wives and have a very long reproductive life-span.
there should be hidden estrus, since females are not advertising to attract males to mate with them.
since once a male has access to the female, she is not mating with other males, there should be no sperm competition.
there should be sexually selected characteristics related to combat , such as plumage, antlers or horns, teeth and hair.
testes size should be small, since males are not competing with other males in sperm competition (gorillas = 10 g).
high numbers of abnormal sperm, because there is no sperm competition.
Polygyny + Polyandry (Polygamy) (Chimpanzees):
parental investment; females do all the child care
roughly equal, since no combat
no male-male combat
body size is equal since there is no combat
male mating is not delayed since males do not have to delay due to attaining a certain body size or competence to fight.
fitness should be roughly equal, since males are competing with sperm competition
females advertise estrus so that many males will mate with them when they are ovulating
yes, sperm competition
sexual selection, yes, due to sperm competition
large testes because of sperm competition
low numbers of abnormal sperm because of sperm competition
Compare gorillas and chimpanzees. Gorillas are characterized by intense male-male competition for access to females and their breeding system is polygynous. As predicted by this competition, male gorillas are considerably larger than females (250 kg male body weight, 10 g testes; body size is twice that of females). However, copulatory frequency is low (few females - a harem of 3 - 4 females, long gestation [almost 9 mo; females breed on average every 3-4 years] and lactation), thus the testes, whose size is determined almost exclusively by the volume of the seminiferous tubules (which produce the sperm; the testosterone-producing Leydig cells account only for a small proportion of overall testicular size) are small and there is a high percentage of abnormal sperm; since an adult gorilla does not mate until age 15 or so and then may mate only once a year.
In contrast, chimpanzees compete less intensively for access to females and are cooperative maters and thus there is little sexual dimorphism in body size (50 kg body weight) are only slightly bigger than females, but copulatory activity is frequent, so they have 60 g testes, because they have sperm competition (instead of inter-male competition). Thus males are not competing for access to mate with females, because many males get to mate with an estrous females, rather they must produce lots of viable sperm, since the competition occurs in which sperm fertilizes the egg.
Thus you find this interaction of breeding system, body size, and testicular size. Selection for body size is concerned with successful inter-male aggression and competition for mates, and you find little or no somatic dimorphism in monogamous species that pair for life, and since copulatory frequencies are also low in monogamous species, the testis is relatively small.
In polygynous species, however, where one male has exclusive access to several females, enhanced inter-male competition will result in the males becoming much larger than the females (like the gorilla). But, if copulatory frequency is still low, as in the gorilla, the testes may remain relatively small. In the chimpanzee, you have a less dimorphism in body size because the males don't compete for access for females, but large testes, because mating opportunities are frequent.
What about female advertisement of estrus? In the monogamous breeding system, the female does not have to advertise her sexual state to all-comers by developing pronounced sexual swellings because the male is with her at all times. In the polygynous gorilla situation, where there is one male in constant attendance on the females, they also do not need to advertise, since the male is constantly with them. In the polygynous, polyandrous chimpanzee situation, the females advertise their sexual readiness since she is going to mate with many males and it pays for her to advertise that she is about to ovulate.
What about humans? Men are 15 - 20% larger than females, suggesting that we are not inherently monogamous. The relatively small size of the male's testes (about 20 g), the high proportion of abnormal spermatozoa, and the lack of any cyclical sexual swellings in females suggests that neither are we adapted to a multi-male promiscuous mating system such as the chimpanzee's. A woman's ability to initiate copulation at any stage of her reproductive cycle (not tied to hormones), rather than just around the time of ovulation, has greatly increased the frequency of copulation. But, since the vast majority of human acts of intercourse are unrelated to reproduction and are probably for social bonding, the size of a man's testes has not increased accordingly. The best guess would be that we are basically a polygynous (polygamous?) primate in which the polygyny usually takes the form of serial monogamy. Since there is no male menopause, men have a longer fertile life than women, so a man is likely to produce more offspring in his lifetime than a woman. This is accentuated by the fact that if older men remarry, almost invariably they marry a younger woman.
Humans have developed a variety of secondary sexual characteristics like a large penis and well-developed breasts that further enhance inter-sexual bonding and woman are unique amongst mammals in being covert ovulators, concealing the event not only from males, but also often from themselves. These physical characteristics would have first developed due to natural selection (related to reproduction), but then been enhanced due to sexual selection.
Humans, as a matter of fact (according to Short, but see Bonobos and de Waal's rebuttal), have the largest penises (per body size) and largest breasts (per body size) of any mammal on earth. Women don't need large breasts to adequately feed their young, nor do men need large penises to impregnate females. These sexual characteristics have evolved purely as sexual attractants and thus all the jokes about men being attracted to breasts and women to large penises are true.
Questions to consider:
1. Do humans have sexual selection in both sexes? If so, why?
2. Do humans have = parental care?
3. Who is the limited sex in humans? Is there one?
4. Are there costs of mating for both sexes, not just one?
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