Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.
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A common type of dimorphism is ornamentation. Dichromatism is where a sex of a given species is more than one color including that of saturated, vibrant hues.[1]
Exaggerated dimorphic traits are used predominantly in the competition over mates.[2] Ornaments are costly to produce or maintain.[3][4][5] but differ depending on the type of color mechanism involved.
One excellent example of this is in the peacock. The peacock has a very ornate plumage which it uses to attract peahens. The peacock will use its ornate plumage to court a mate. At first look, one may mistake a peacock for a completely different species from the peahen because of its vibrant colors and the sheer size of the male's plumage. The peahen is more of a dullish brown color and lacks the ornate plumage. Although the plumage of the peacock is helpful when courting a mate, it is actually detrimental to the survival of the peacock. The large plumage makes it nearly impossible for the peacock to fly. The peacock is limited to short bursts of flight when they attempt to escape predators by taking refuge in trees. The feathers' bright colors make it harder for the bird to remain unseen, not only by predators but also by humans, who see the feathers as a desired item.
Another example of sexual dichromatism is that of the nestling blue tits. Males are chromatically more yellow than females. It is believed that this is obtained by the ingestion of green lepidopteran larvae, which contain large amounts of the carotenoids lutein and zeaxanthin.[6] This diet also affects the sexually dimorphic colors in the human-invisible UV spectrum.[7][8] In other words, the male birds, although appearing yellow to humans actually have a violet-tinted plumage that is seen by females. This plumage is thought to be an indicator of male parental abilities.[9] Perhaps this is a good indicator for females because it shows that they are good at obtaining a food supply which the carotenoid is obtained from. There is a positive correlation between the chromas of the tail and breast feathers and body condition.[1] Carotenoids play an important role in portraying immune function for many animals. More carotenoids mean a higher immune resistance. Those with more yellow and blue pigmentation were also found to be heavier.
In many instances, females show preference for exaggerated male secondary sexual characteristics when choosing a mate (Ryan and Rand, 1993). Females tend to show direction preferences for more elaborate males.[2] Females have been shown to discriminate against males which are dull in color.[10] There have also been species such as estrildid finch where premating isolation was seen due to lack of vibrant colors by the males. This female preference for ornamentation may affect the evolution of discriminatory mating preferences.[10] This is known as the ornamentation hypothesis.
In some species such as insects, spiders, many fish, birds of prey and certain mammals such as the spotted hyena and the blue whale, the female is larger than the male. As an example, in some species females are sedentary and sparsely distributed, and so males must search for them. Vollrath and Parker argue that this difference in behaviour leads to radically different selection pressures on the two sexes, evidently favouring smaller males.[11] Cases where the male is larger than the female have been studied as well,[11] and require alternate explanations.
One example of sexual size dimorphism is the bat Myotis nigricans. In this species, females are substantially larger than males. They differ in body weight, skull measurement, and forearm length.[12] The difference in size is believed to be caused by natural selection for a large female size due to a fecundity advantage. The interaction between the sexes and energetic needs such as time and energy required to produce viable offspring make it favorable for females to be larger in this species. Females bear the energetic cost of producing eggs which is much greater than that of the male who only bears the cost of making sperm. The fecundity advantage hypothesis states, that a big mother is able to produce more offspring and give those offspring more favorable conditions to ensure their survival. This is true for most ectotherms. Another reason why females are believed to be larger is due to the fact that they provide parental care for a substantial amount of time while the offspring matures. The time of gestation and lactation is fairly long in the M. nigricans, where females suckle their offspring until nearly adult size.[13] They would not be able to fly and catch prey if they did not compensate for the additional mass of the offspring during this time.In addition to the hypothesis that explains an advantage of large female size, it is hypothesized that smaller male size is an adaptation for males to increase maneuverability and agility. This selection for agility in flying is a helpful adaptation which allows males to better compete with females for food and other resources.
Some species of anglerfish also display extreme sexual dimorphism. Females are more typical in appearance to other fish, whereas the males are tiny rudimentary creatures with stunted digestive systems. A male must find a female and fuse with her: he then lives parasitically, becoming little more than a sperm-producing body. A similar situation is found in the Zeus water bug Phoreticovelia disparata where the female has a glandular area on her back that can serve to feed a male that clings to her (note that although males can survive away from females, they generally are not free-living).[14]
Sex steroid-induced differentiation of adult reproductive and other behavior has been demonstrated experimentally in many animals. In some mammals, adult sex-dimorphic reproductive behavior (e.g., mounting or receptive lordosis) can be shifted to that of the other sex by supplementation or deprivation of androgens in fetal life or early infancy, even if adult levels are normal.
In 1871 Charles Darwin advanced the theory of sexual selection, which related sexual dimorphism with sexual selection.
In many non-monogamous species, the benefit to a male's reproductive fitness of mating with multiple females is large, whereas the benefit to a female's reproductive fitness of mating with multiple males is small or non-existent.[15] In these species, there is a selection pressure for whatever traits enable a male to have more matings. The male may therefore come to have different traits from the female.
These traits could be ones that allow him to fight off other males for control of territory or a harem, such as large size or weapons;[16] or they could be traits that females, for whatever reason, prefer in mates.[17] Male-male competition poses no deep theoretical questions[18] but female choice does.
Females may choose males that appear strong and healthy, thus likely to possess "good alleles" and give rise to healthy offspring.[19] However, in some species females seem to choose males with traits that do not improve offspring survival rates, and even traits that reduce it (potentially leading to traits like the peacock's tail).[18] Two hypotheses for explaining this fact are the sexy son hypothesis and the handicap principle.
The sexy son hypothesis states that females may initially choose a trait because it improves the survival of their young, but once this preference has become widespread, females must continue to choose the trait, even if it becomes harmful. Those that do not will have sons that are unattractive to most females (since the preference is widespread) and so receive few matings.[20]
The handicap principle states that a male who survives despite possessing some sort of handicap thus proves that the rest of his genes are "good alleles". If males with "bad alleles" could not survive the handicap, females may evolve to choose males with this sort of handicap; the trait is acting as a hard-to-fake signal of fitness.[21]
The brains of many birds and mammals, including humans, are significantly different for males and females of the species.[22] Both genes and hormones affect the formation of many animal brains before "birth" (or hatching), and also behaviour of adult individuals. Hormones significantly affect human brain formation, and also brain development at puberty. A 2004 review in Nature Reviews Neuroscience observed that "because it is easier to manipulate hormone levels than the expression of sex chromosome genes, the effects of hormones have been studied much more extensively, and are much better understood, than the direct actions in the brain of sex chromosome genes." It concluded that while "the differentiating effects of gonadal secretions seem to be dominant," the existing body of research "support the idea that sex differences in neural expression of X and Y genes significantly contribute to sex differences in brain functions and disease."[23]
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Top: Stylised illustration of humans on the Pioneer plaque, showing both male (left) and female (right). |
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Sexual dimorphism in humans is the subject of much controversy, especially when extended beyond physical differences to mental ability and psychological gender. (For discussion, see sex and psychology, gender, and transgender.) Obvious differences between males and females include all the features related to reproductive role, notably the endocrine (hormonal) systems and their physiological and behavioural effects. Such undisputed sexual dimorphism includes gonadal differentiation, internal genital differentiation, external genital differentiation, breast differentiation, muscle mass differentiation, height differentiation, and hair differentiation.
Externally, the most sexually dimorphic portions of the body are the chest, the lower half of the face, and the area between the waist and the knees.
The basal metabolic rate is about 6 percent higher in adolescent males than females and increases to about 10 percent higher after puberty. Females tend to convert more food into fat, while men convert more into muscle and expendable circulating energy reserves. According to one small study of 8 men and 8 women, females (on average) are about 52 percent as strong as males in the upper body, and about 66 percent as strong in the lower.[24] In general, studies have indicated that women have 40-60% the upper body strength of men, and 70-75% the lower body strength.[25] Males, on average, have denser, stronger bones, tendons, and ligaments.
Males dissipate heat faster than females through their sweat glands. Females have a greater insulation and energy reserves stored in subcutaneous fat, absorbing exothermic heat less and retaining endothermic heat to a greater degree.
Males typically have larger tracheae and branching bronchi, with about 30 percent greater lung volume per body mass. They have larger hearts, 10 percent higher red blood cell count, higher haemoglobin, hence greater oxygen-carrying capacity. They also have higher circulating clotting factors (vitamin K, prothrombin and platelets). These differences lead to faster healing of wounds and higher peripheral pain tolerance.[26]
Females typically have more white blood cells (stored and circulating), more granulocytes and B and T lymphocytes. Additionally, they produce more antibodies at a faster rate than males. Hence they develop fewer infectious diseases and succumb for shorter periods.[26] Ethologists argue that females, interacting with other females and multiple offspring in social groups, have experienced such traits as a selective advantage.[27][28][29][30][31]
Some biologists theorise that a species' degree of sexual dimorphism is inversely related to the degree of paternal investment in parenting. Species with the highest sexual dimorphism, such as the pheasant, tend to be those species in which the care and raising of offspring is done only by the mother, with no involvement of the father (low degree of paternal investment).
Considerable discussion in academic literature concerns potential evolutionary advantages associated with sexual competition (both intrasexual and intersexual) and short- and long-term sexual strategies.[32]
According to Daly and Wilson, "The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals."[33] One proposed explanation is that human sexuality has developed more in common with its close relative the bonobo, who have similar sexual dimorphism and which are polygynandrous and use recreational sex to reinforce social bonds and reduce aggression.[34]
In the human brain, a difference between sexes was observed in the transcription of the PCDH11X/Y gene pair unique to Homo sapiens.[35] The relationship between sex differences in the brain and human behavior is a subject of controversy in psychology and society at large.[36][37] Women on average have a higher percentage of gray matter in comparison to men.[38][39] However, men have larger brains on average than women, and when adjusted for total brain volume the gray matter differences between sexes is small or nonexistent. Thus, the percentage of gray matter appears to be more related to brain size than it is to gender.[40][41] Differences in brain physiology between sexes do not necessarily relate to differences in intellect. Haier et al. found in a 2004 study that "men and women apparently achieve similar IQ results with different brain regions, suggesting that there is no singular underlying neuroanatomical structure to general intelligence and that different types of brain designs may manifest equivalent intellectual performance".[42] (See the sex and intelligence article for more on this subject.)
Sexual dimorphism in birds can be manifested in size or plumage differences between the sexes. Sexual size dimorphism varies among taxa with males typically being larger, though this is not always the case i.e. birds of prey and some species of flightless birds.[43] Plumage dimorphism, in the form of ornamentation or coloration, also varies, though males are typically the more ornamented or brightly colored sex.[44] Such differences have been attributed to the unequal reproductive contributions of the sexes.[45] In some species, the male's contribution to reproduction ends at copulation, while in other species the male becomes the main caregiver. Plumage polymorphisms have evolved to reflect these differences and other measures of reproductive fitness, such as body condition[46] or survival.[47] The male phenotype sends signals to females who then choose the 'fittest' available male.
Sexual dimorphism is a product of both genetics and environmental factors. An example of sexual polymorphism determined by environmental conditions exists in the house finch. House finch males can be classified into three categories during breeding season: black breeders, brown breeders, and brown auxiliaries.[46] These differences arise in response to the bird's body condition: if they are healthy they will produce more androgens thus becoming black breeders, while less healthy birds produce less androgens and become brown auxiliaries.[46] The reproductive success of the male is thus determined by his success during each year's non-breeding season, causing reproductive success to vary with each year's environmental conditions.
Sexual dimorphism is maintained by the counteracting pressures of natural selection and sexual selection. For example, sexual dimorphism in coloration increases the vulnerability of bird species to predation by European sparrowhawks in Denmark.[48] Presumably, increased sexual dimorphism means males are brighter and more conspicuous, leading to increased predation.[48] Moreover, the production of more exaggerated ornaments in males may come at the cost of suppressed immune function.[46] So long as the reproductive benefits of the trait due to sexual selection are greater than the costs imposed by natural selection, then the trait will propagate throughout the population. Reproductive benefits arise in the form of a larger number of offspring, while natural selection imposes costs in the form of reduced survival. This means that even if the trait causes males to die earlier, the trait is still beneficial so long as males with the trait produce more offspring than males lacking the trait.
Such differences in form and reproductive roles often cause differences in behavior. As previously stated, males and females often have different roles in reproduction. The courtship and mating behavior of males and females are regulated largely by hormones throughout a bird's lifetime.[49] Activational hormones occur during puberty and adulthood and serve to 'activate' certain behaviors when appropriate, such as territoriality during breeding season.[49] Organizational hormones occur only during a critical period early in development, either just before or just after hatching in most birds, and determine patterns of behavior for the rest of the bird's life.[49] Such behavioral differences can cause disproportionate sensitivities to anthropogenic pressures.[50] Females of the whinchat in Switzerland breed in intensely managed grasslands.[50] Earlier harvesting of the grasses during the breeding season lead to more female deaths.[50] Populations of many birds are often male-skewed and when sexual differences in behavior increase this ratio, populations decline at a more rapid rate.[50]
Consequently, sexual dimorphism has important ramifications for conservation. However, sexual dimorphism is not only found in birds and is thus important to the conservation of many animals. Such differences in form and behavior can lead to sexual segregation, defined as sex differences in space and resource use.[51] Most sexual segregation research has been done on ungulates,[51] but such research extends to bats,[52] kangaroos,[53] and birds.[54] Sex-specific conservation plans have even been suggested for species with pronounced sexual segregation.[52]
There are also cases where males are substantially larger than that of females. An example is Lamprologus callipterus, a type of cichlid fish. In this fish, the males are characterized as being up to 60 times larger than that of the females. The males increased size is believed to be advantageous because males collect and defend empty snail shells in each of which a female breeds.[55] Males must be larger and more powerful in order to collect the largest shells. The female's body size must remain small because in order for her to breed, she must lay her eggs inside the empty shells. If she grows too large, she will not fit in the shells and will be unable to breed.
The female's small body size is also likely beneficial to her chances of finding an unoccupied shell. Larger shells, although preferred by females, are often limited in availability.[56] Hence, the female is limited to the growth of the size of the shell and may actually change her growth rate according to shell size availability.[57] In other words, the male's ability to collect large shells depends on his size. The larger the male, the larger the shells he is able to collect. This then allows for females to be larger in his brooding nest which makes the difference between the sizes of the sexes less substantial. Male-male competition in this fish species also selects for large size in males. There is aggressive competition by males over territory and access to larger shells. Large males win fights and steal shells from competitors. Sexual dimorphism also occurs in hermaphroditic fish. These species are known as sequential hermaphrodites. In fish, reproductive histories often include the sex-change from female to male where there is a strong connection between growth, the sex of an individual, and the mating system it operates within.[58] In protogynous mating systems where males dominate mating with many females, size plays a significant role in male reproductive success.[59] Males have a propensity to be larger than females of a comparable age but it is unclear whether the size increase is due to a growth spurt at the time of the sexual transition or due to the history of faster growth in sex changing individuals.[60] Larger males are able to stifle the growth of females and control environmental resources.
Social organization plays a large role in the changing of sex by the fish. It is often seen that a fish will change its sex when there is a lack of dominant male within the social hierarchy. The females that change sex are often those who attain and preserve an initial size advantage early in life. In either case, females which change sex to males are larger and prove to be a good example of dimorphism.
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