Secondary sex characteristic
Overview
Secondary sex characteristics are features that distinguish the two sex
Sex
In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety . Sexual reproduction involves combining specialized cells to form offspring that inherit traits from both parents...

es of a species, but that are not directly part of the reproductive system
Reproductive system
The reproductive system or genital system is a system of organs within an organism which work together for the purpose of reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system. Unlike most organ systems, the sexes...

. They are believed to be the product of sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...

 for traits which give an individual an advantage over its rivals in courtship
Courtship
Courtship is the period in a couple's relationship which precedes their engagement and marriage, or establishment of an agreed relationship of a more enduring kind. In courtship, a couple get to know each other and decide if there will be an engagement or other such agreement...

 and aggressive
Aggression
In psychology, as well as other social and behavioral sciences, aggression refers to behavior between members of the same species that is intended to cause humiliation, pain, or harm. Ferguson and Beaver defined aggressive behavior as "Behavior which is intended to increase the social dominance of...

 interactions. They are distinguished from the primary sex characteristics: the sex organ
Sex organ
A sex organ, or primary sexual characteristic, as narrowly defined, is any of the anatomical parts of the body which are involved in sexual reproduction and constitute the reproductive system in a complex organism; flowers are the reproductive organs of flowering plants, cones are the reproductive...

s, which are directly necessary for reproduction to occur.

Well known secondary sex characteristics include manes of male lion
Lion
The lion is one of the four big cats in the genus Panthera, and a member of the family Felidae. With some males exceeding 250 kg in weight, it is the second-largest living cat after the tiger...

s and long feathers of male peacocks.
Encyclopedia
Secondary sex characteristics are features that distinguish the two sex
Sex
In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety . Sexual reproduction involves combining specialized cells to form offspring that inherit traits from both parents...

es of a species, but that are not directly part of the reproductive system
Reproductive system
The reproductive system or genital system is a system of organs within an organism which work together for the purpose of reproduction. Many non-living substances such as fluids, hormones, and pheromones are also important accessories to the reproductive system. Unlike most organ systems, the sexes...

. They are believed to be the product of sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...

 for traits which give an individual an advantage over its rivals in courtship
Courtship
Courtship is the period in a couple's relationship which precedes their engagement and marriage, or establishment of an agreed relationship of a more enduring kind. In courtship, a couple get to know each other and decide if there will be an engagement or other such agreement...

 and aggressive
Aggression
In psychology, as well as other social and behavioral sciences, aggression refers to behavior between members of the same species that is intended to cause humiliation, pain, or harm. Ferguson and Beaver defined aggressive behavior as "Behavior which is intended to increase the social dominance of...

 interactions. They are distinguished from the primary sex characteristics: the sex organ
Sex organ
A sex organ, or primary sexual characteristic, as narrowly defined, is any of the anatomical parts of the body which are involved in sexual reproduction and constitute the reproductive system in a complex organism; flowers are the reproductive organs of flowering plants, cones are the reproductive...

s, which are directly necessary for reproduction to occur.

Well known secondary sex characteristics include manes of male lion
Lion
The lion is one of the four big cats in the genus Panthera, and a member of the family Felidae. With some males exceeding 250 kg in weight, it is the second-largest living cat after the tiger...

s and long feathers of male peacocks. Other dramatic examples include the tusks of male narwhals, enlarged proboscis
Proboscis
A proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In simpler terms, a proboscis is the straw-like mouth found in several varieties of species.-Etymology:...

es in male elephant seals and proboscis monkeys, the bright facial and rump coloration of male mandrills, and horns in many goats and antelopes. Male birds and fish of many species have brighter coloration or other external ornaments. Differences in size
Sexual dimorphism
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.-Examples:-Ornamentation / coloration:...

 between sexes are also considered secondary sexual characteristics.

In humans, visible secondary sex characteristics include enlarged breast
Breast
The breast is the upper ventral region of the torso of a primate, in left and right sides, which in a female contains the mammary gland that secretes milk used to feed infants.Both men and women develop breasts from the same embryological tissues...

s of females and facial hair
Facial hair
Facial hair is a secondary sex characteristic of human males. Men often start developing facial hair in the later years of puberty or adolescence, approximately between 17–20 years of age, and most do not finish developing a fully adult beard until their early 20s or even later...

 on males.

Evolutionary roots

Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

 hypothesized that sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...

, or competition within a species for mates, can explain observed differences between sexes in many species. Biologists today distinguish between "male to male combat" and "mate choice", usually female choice of male mates. Sexual characteristics due to combat are such things as antlers, horns and greater size. Characteristics due to mate choice, often referred to as ornaments, include brighter plumage, coloration and other features that have no immediate purpose for survival or combat.

Ornamentation might arise because of some arbitrary female preference that is initially amplified by random genetic drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...

, eventually being reinforced by active selection for males with the appropriate ornament. This is known as the sexy son hypothesis
Sexy son hypothesis
The sexy son hypothesis of evolutionary biology was first proposed by Patrick J. Weatherhead and Raleigh J. Robertson of Queen's University in 1979. It proposes that a female animal's optimal choice among potential mates is a male whose genes will produce male offspring with the best chance of...

. An alternative hypothesis is that some of the genes that enable males to develop impressive ornaments or fighting ability may be correlated with fitness markers such as disease resistance or a more efficient metabolism. This idea is known as the good genes hypothesis.

In humans

Sexual differentiation
Sexual differentiation
Sexual differentiation is the process of development of the differences between males and females from an undifferentiated zygote...

 begins during gestation
Gestation
Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....

, when the gonads are formed. General habitus
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

 and shape of body and face, as well as sex hormone levels, are similar in prepubertal boys and girls. As puberty
Puberty
Puberty is the process of physical changes by which a child's body matures into an adult body capable of reproduction, as initiated by hormonal signals from the brain to the gonads; the ovaries in a girl, the testes in a boy...

 progresses and sex hormone levels rise, differences appear, though puberty causes some similar changes in male and female bodies.

Male levels of testosterone
Testosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...

 directly induce growth of the testicles and penis
Penis
The penis is a biological feature of male animals including both vertebrates and invertebrates...

, and indirectly (via dihydrotestosterone
Dihydrotestosterone
Dihydrotestosterone is an androgen or male sex hormone. The enzyme 5α-reductase synthesises DHT in the prostate, testes, hair follicles, and adrenal glands...

 (DHT)) the prostate
Prostate
The prostate is a compound tubuloalveolar exocrine gland of the male reproductive system in most mammals....

. Estradiol
Estradiol
Estradiol is a sex hormone. Estradiol is abbreviated E2 as it has 2 hydroxyl groups in its molecular structure. Estrone has 1 and estriol has 3 . Estradiol is about 10 times as potent as estrone and about 80 times as potent as estriol in its estrogenic effect...

 and other hormones cause breast
Breast
The breast is the upper ventral region of the torso of a primate, in left and right sides, which in a female contains the mammary gland that secretes milk used to feed infants.Both men and women develop breasts from the same embryological tissues...

s to develop in females. However, fetal or neonatal androgen
Androgen
Androgen, also called androgenic hormone or testoid, is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of male characteristics in vertebrates by binding to androgen receptors...

s may modulate later breast development by reducing the capacity of breast tissue to respond to later estrogen
Estrogen
Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...

.

In males, testosterone
Testosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...

 directly increases size and mass of muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

s, vocal cords, and bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

s, deepening the voice
Human voice
The human voice consists of sound made by a human being using the vocal folds for talking, singing, laughing, crying, screaming, etc. Its frequency ranges from about 60 to 7000 Hz. The human voice is specifically that part of human sound production in which the vocal folds are the primary...

, and changing the shape of the face
Face
The face is a central sense organ complex, for those animals that have one, normally on the ventral surface of the head, and can, depending on the definition in the human case, include the hair, forehead, eyebrow, eyelashes, eyes, nose, ears, cheeks, mouth, lips, philtrum, temple, teeth, skin, and...

 and skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...

. Converted into DHT in the skin, it accelerates growth of androgen
Androgen
Androgen, also called androgenic hormone or testoid, is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of male characteristics in vertebrates by binding to androgen receptors...

-responsive facial and body hair, but may slow and eventually stop the growth of head hair. Taller stature is largely a result of later puberty
Puberty
Puberty is the process of physical changes by which a child's body matures into an adult body capable of reproduction, as initiated by hormonal signals from the brain to the gonads; the ovaries in a girl, the testes in a boy...

 and slower epiphyseal fusion.

In females, breasts are a manifestation of higher levels of estrogen
Estrogen
Estrogens , oestrogens , or œstrogens, are a group of compounds named for their importance in the estrous cycle of humans and other animals. They are the primary female sex hormones. Natural estrogens are steroid hormones, while some synthetic ones are non-steroidal...

; estrogen also widens the pelvis and increases the amount of body fat in hips, thighs, buttocks, and breasts. Estrogen also induces growth of the uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...

, proliferation of the endometrium
Endometrium
-Function:The endometrium is the innermost glandular layer and functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle or estrous cycle, the endometrium grows to a...

, and menses
Menstrual cycle
The menstrual cycle is the scientific term for the physiological changes that can occur in fertile women for the purpose of sexual reproduction. This article focuses on the human menstrual cycle....

.

In humans, secondary sex characteristics include:
  • Male
    Male
    Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...

    • Growth of body hair, including underarm
      Underarm hair
      Underarm hair is the composition of hair in the underarm area .- Development and function :...

      , abdominal
      Abdominal hair
      The term abdominal hair refers to the hair that grows on the abdomen of humans and non-human mammals, in the region between the pubic area and the thorax . The growth of abdominal hair follows the same pattern on nearly all mammals, vertically from the pubic area upwards and from the thorax...

      , chest
      Chest hair
      The term chest hair is generally used to describe hair that grows on the chest of human males, in the region between the neck and the abdomen. Chest hair, which is a secondary sex characteristic, develops during and after puberty...

      , and pubic hair
      Pubic hair
      Pubic hair is hair in the frontal genital area, the crotch, and sometimes at the top of the inside of the legs; these areas form the pubic region....

      . Loss of scalp hair androgenic alopecia can also occur
    • Greater mass of thigh muscles in front of the femur, rather than behind it as is typical in mature females
    • Growth of facial hair
      Facial hair
      Facial hair is a secondary sex characteristic of human males. Men often start developing facial hair in the later years of puberty or adolescence, approximately between 17–20 years of age, and most do not finish developing a fully adult beard until their early 20s or even later...

    • Enlargement of larynx
      Larynx
      The larynx , commonly called the voice box, is an organ in the neck of amphibians, reptiles and mammals involved in breathing, sound production, and protecting the trachea against food aspiration. It manipulates pitch and volume...

       (Adam's apple) and deepening of voice
      Human voice
      The human voice consists of sound made by a human being using the vocal folds for talking, singing, laughing, crying, screaming, etc. Its frequency ranges from about 60 to 7000 Hz. The human voice is specifically that part of human sound production in which the vocal folds are the primary...

    • Increased stature
      Human height
      Human height is the distance from the bottom of the feet to the top of the head in a human body standing erect.When populations share genetic background and environmental factors, average height is frequently characteristic within the group...

      ; adult males are taller than adult females, on average
    • Heavier skull
      Human skull
      The human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...

       and bone
      Bone
      Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

       structure
    • Increased muscle
      Muscle
      Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

       mass and strength
    • Larger hands and feet than women, prepubescent boys and girls
    • Square face
    • Small waist, but wider than females
    • Broadening of shoulder
      Shoulder
      The human shoulder is made up of three bones: the clavicle , the scapula , and the humerus as well as associated muscles, ligaments and tendons. The articulations between the bones of the shoulder make up the shoulder joints. The major joint of the shoulder is the glenohumeral joint, which...

      s and chest; shoulders wider than hips
    • Increased secretions of oil and sweat glands, often causing acne and body odor
    • Coarsening or rigidity of skin texture, due to less subcutaneous fat
    • Higher waist-to-hip ratio than prepubescent or adult females or prepubescent males, on average
    • Lower bodyfat percentage
      Body fat percentage
      A person's body mass percentage is the total weight of the person's fat divided by the person's weight and consists of essential body fat and storage body fat. Essential body fat is necessary to maintain life and reproductive functions. The percentage of essential body fat for women is greater than...

       than prepubescent or adult females or prepubescent males, on average
    • Enlargement growth
      Growth
      Growth refers to an increase in some quantity over time.The quantity can be:*Physical *Abstract ....

       of the penis

  • Female
    Female
    Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...

    • Enlargement of breasts and erection of nipples.
    • Growth of body hair, most prominently underarm
      Underarm hair
      Underarm hair is the composition of hair in the underarm area .- Development and function :...

       and pubic hair
      Pubic hair
      Pubic hair is hair in the frontal genital area, the crotch, and sometimes at the top of the inside of the legs; these areas form the pubic region....

    • Greater development of thigh muscles behind the femur
      Femur
      The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

      , rather than in front of it
    • Widening of hip
      Hip (anatomy)
      In vertebrate anatomy, hip refer to either an anatomical region or a joint.The hip region is located lateral to the gluteal region , inferior to the iliac crest, and overlying the greater trochanter of the femur, or "thigh bone"...

      s; lower waist to hip ratio than adult males, on average
    • Smaller hands and feet than men
    • Rounder face
    • Smaller waist than men
    • Upper arms approximately 2 cm longer, on average, for a given height
    • Changed distribution in weight and fat; more subcutaneous fat and fat
      Fat
      Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...

       deposits mainly around the buttocks
      Buttocks
      The buttocks are two rounded portions of the anatomy, located on the posterior of the pelvic region of apes and humans, and many other bipeds or quadrupeds, and comprise a layer of fat superimposed on the gluteus maximus and gluteus medius muscles. Physiologically, the buttocks enable weight to...

      , thigh
      Thigh
      In humans the thigh is the area between the pelvis and the knee. Anatomically, it is part of the lower limb.The single bone in the thigh is called the femur...

      s and hip
      Hip (anatomy)
      In vertebrate anatomy, hip refer to either an anatomical region or a joint.The hip region is located lateral to the gluteal region , inferior to the iliac crest, and overlying the greater trochanter of the femur, or "thigh bone"...

      s

See also

  • Body shape
    Body shape
    Human body shape is a complex phenomenon with sophisticated detail and function. The general shape or figure of a person is defined mainly by skeletal structure, muscles and fat...

  • Estrogenic fat
    Estrogenic fat
    Estrogenic fat is a form of adipose tissue which develops under the influence of estrogen, and particularly estradiol, in women.-Natural physiology:...

  • Female body shape
    Female body shape
    Female body shape or figure is the cumulative product of a woman's skeletal structure and the quantity and distribution of muscle and fat on the body. There are, and have been, wide differences on what should be considered an ideal or preferred body shape, both for attractiveness and for health...

  • Sex differences in humans
    Sex differences in humans
    A sex difference is a distinction of biological and/or physiological characteristics associated with either males or females of a species. These can be of several types, including direct and indirect. Direct being the direct result of differences prescribed by the Y-chromosome, and indirect being...

  • Sexual dimorphism
    Sexual dimorphism
    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.-Examples:-Ornamentation / coloration:...

  • Menstrual cycle
    Menstrual cycle
    The menstrual cycle is the scientific term for the physiological changes that can occur in fertile women for the purpose of sexual reproduction. This article focuses on the human menstrual cycle....

  • Virilization
    Virilization
    In biology and medicine, virilization refers to the biological development of sex differences, changes that make a male body different from a female body. Most of the changes of virilization are produced by androgens...

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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