Sexual dimorphism in human physiology refers to the distinct physiological characteristics associated with either male or female humans. These differences are caused by the effects of the different sex chromosome complement in males and females, and differential exposure to gonadal sex hormones during development. Sexual dimorphism is a term for the phenotypic difference between males and females of the same species.
The process of meiosis and fertilization (with rare exceptions) results in a zygote with either two X chromosomes (an XX female) or one X and one Y chromosome (an XY male), which then develops the typical female or male phenotype. Physiological sex differences include discrete features such as the respective male and female reproductive systems, as well as average differences between males and females including size and strength, bodily proportions, hair distribution, breast differentiation, voice pitch, and brain size and structure.
Other than external genitals, there are few physical differences between male and female children before puberty. Small differences in height and start of physical maturity are seen. The gradual growth in sex difference throughout a person's life is a product of various hormones. Testosterone is the major active hormone in male development, while estrogen is the dominant female hormone. These hormones are not, however, limited to each sex. Both males and females have both testosterone and estrogen.
Sex determination and differentiation
The human genome consists of two copies of each of 23 chromosomes (a total of 46). One set of 23 comes from the mother and one set comes from the father. Of these 23 pairs of chromosomes, 22 are autosomes, and one is a sex chromosome. There are two kinds of sex chromosomes–X and Y. In humans and in almost all other mammals, females carry two X chromosomes, designated XX, and males carry one X and one Y, designated XY.
A human egg contains only one set of chromosomes (23) and is a haploid. Sperm also have only one set of 23 chromosomes and are therefore haploid. When an egg and sperm fuse at fertilization, the two sets of chromosomes come together to form a unique diploid individual with 46 chromosomes.
The sex chromosome in a human egg is always an X chromosome since a female only has X sex chromosomes. In sperm, about half the sperm have an X chromosome and half have a Y chromosome. If an egg fuses with sperm with a Y chromosome, the resulting individual is male. If an egg fuses with sperm with an X chromosome, the resulting individual is female. There are rare exceptions to this rule in which, for example, XX individuals develop as males or XY individuals develop as females. There are other variations of sex chromosomes that lead to a variety of different physical expressions.
The X-chromosome carries a larger number of genes in comparison to the Y-chromosome. In humans, X-chromosome inactivation enables males and females to have an equal expression of the genes on the X-chromosome since females have two X-chromosomes while males have a single X and a Y chromosome. X-chromosome inactivation is random in the somatic cells of the body as either the maternal or paternal X-chromosome can become inactivated in each cell. Thus, females are genetic mosaics.
Sexual organs and reproductive systems
As with all other Therian mammals, human males and females have different sex organs, which constitute the primary sex characteristics. Embryos begin undifferentiated, with a system of paramesonephric and mesonephric ducts and a urogenital sinus. As embryos develop, the secretion of sex hormones causes these organs to develop differently, eventually turning into fully-developed sex organs. Females have an external vulva, a uterus which is connected to a vagina, and two ovaries that store the eggs. Males have testicles that produce sperm inside the scrotum behind the penis.
Both males and females have the ability to produce ejaculate. Female ejaculate does not contain sperm, but is otherwise of a reasonably similar composition to male ejaculate. They are respectively produced in the prostate in males and Skene's gland in females, which despite being different organs arise from the same set of embryonic tissue. Female ejaculate is distinct from the release of urine during sex, or 'squirting', which is relatively common in females and with which it is often confused.
As sperm must reach the uterus in order to fuse with an egg, the male ejaculation is a necessary part of the regular reproductive process, which in turn necessitates a male orgasm as under normal circumstances males must orgasm to ejaculate. The female orgasm is by contrast not necessary for reproduction; it is believed to have evolved among basal mammals as part of induced ovulation. As humans are spontaneous ovulators, the female orgasm being maintained suggests it may play a role in sexual selection, though this has not been confirmed.
Reproductive cost The differing male and female reproductive systems lead to a large difference between the sexes in the biological cost of children. Males have a relatively low reproductive cost; they on average produce about two hundred million sperm a day, and a single ejaculation is enough to achieve fertilization with no further involvement on the male's part. Females, by contrast, typically produce one ovum a month that can be fertilized into an embryo, which must then be incubated for nine months before birth. This is very energy-intensive process, consuming about 80,000 kilocalories of energy on average. These increased energy requirements make frequent parenting more risky for females than males, exacerbated by the risk of death or serious injury in childbirth.
These differences theoretically mean that during a lifetime males are able to father a significantly greater number of children than females can give birth to. However in practice this rarely happens, as the extreme K-selection of human beings forces a high decree of paternal social care for a male's offspring that makes a large number of children undesirable for fathers. Extremes such as Moroccan Emperor Ismail Ibn Sharif, who reportedly fathered more than 800 children from a harem of 500 women, are not considered reflective of the wider male population.
Fertility Fertility for both males and females decreases with age, with female fertility starting to decline in the early 30s and male fertility in the mid-30s. The pattern of this decrease differs substantially in that females go through menopause, after which fertility ends, whereas male fertility tapers without necessarily becoming zero. Declining fertility in males is marked by reduced sperm count, semen volume, and a degradation of sperm DNA that causes more of the remaining sperm to be non-viable.
Both advanced maternal age and paternal age represent an increased risk of non-viable embryos, miscarriages and stillbirths. Advanced paternal age specifically is correlated with a higher risk of conditions caused by a single DNA error, such as craniosynostosis and achondroplasia, alongside increased prevalence of psychiatric disorders such as schizophrenia, bipolar disorder and autism. Advanced maternal age is in turn potentially associated with an increased risk of cardiovascular disease among offspring.
Size and body shape
Human males are on average about 13 cm taller than females. The exact causes of this difference are not currently known; analysis of the heights of intersex people with XXY chromosomes and women with XY chromosomes and complete androgen insensitivity syndrome suggests that about a quarter of the difference can be traced to the Y chromosome itself and another quarter to androgens produced in the testicles, but studies have differed as to the precise amount these factors impact height.
Human body shapes are also sexually dimorphic, with males on average weighing slightly more than females even after adjusting for height difference. Females have larger hips in proportion to their waist and a larger percentage of body fat than males do, caused by the release of estrogen during puberty that guides patterns of fat distribution. Once females reach menopause, a reduction in levels of estrogen causes their fat to shift to a more male-like distribution.
Breasts One of the most notable examples of sexual dimorphism between human males and females is that the latter, unique among primates, have permanent breasts. A number of theories have been suggested for why permanent breasts evolved in humans, such as that they developed as a courtship display during the human transition to bipedalism, that they are a side-effect of increased fat levels caused by increased meat consumption in Homo ergaster and its descendants, including modern humans, or some combination of these factors.
In spite of the fact that males lack permanent breasts, the internal biology of the male and female breast is similar; both sexes have nipples and milk ducts, and galactorrhea, or the production of breast milk absent childbirth, can affect both sexes. During early childhood male and female breasts are largely the same, but they diverge during puberty as increased levels of estrogen, progesterone and prolactin cause female breasts to develop into their adult form. As it is these hormones that cause permanent breasts to develop in humans, males may also develop them as a result of conditions such as gynecomastia, and transgender females may develop them through hormone replacement therapy.
Skin and hair
Skin Males' skin is thicker and oilier than females' skin. Females have more subcutaneous fat than males. This helps keep them warmer and enables them to survive lower temperatures than males during the cold. As a result of greater vasoconstriction, while the surface of female skin is colder than male skin, the deep-skin temperature in females is higher than in males.
Males generally have darker skin than females, due to having higher amounts of melanin. The lighter skin in females helps their bodies synthesize more Vitamin D from sunlight and absorb more calcium, which is needed during pregnancy and lactation. In lighter-complected humans male skin is visibly redder; this is due to greater blood volume rather than melanin.
Women typically show signs of skin aging earlier than men, with fine lines often appearing in their 30s. They also tend to experience a faster loss of collagen during menopause. In contrast, men’s skin typically ages more gradually until later in life.
Hair On average, males have more body hair than females. Males have relatively more of a type of hair called terminal hair, especially on the face, chest, abdomen and back. Females have more vellus hair, which is thinner, shorter, and lighter, and therefore less visible.
Although males grow hair faster than females, baldness is more prevalent in males than in females. The main cause for this is male pattern baldness. Male pattern baldness is a condition where hair loss occurs in a typical pattern of a receding hairline and hair thinning on the crown. It is caused by hormones and genetic predisposition.
In one study, almost twice as many females as males had red or auburn hair. A higher proportion of females were also found to have blond hair, whereas males were more likely to have black or dark brown hair. A different study found that while females tend to have a lower frequency of black hair, males have a higher frequency of platinum blond hair, blue eyes and lighter skin. According to this one theory the cause for this is a higher frequency of genetic recombination in females than in males, possibly due to sex-linked genes, and as a result females tend to show less phenotypical variation in any given population.
Skeleton and muscular system Skeleton