Sexual dimorphism is the systematic difference in form between individuals of different
sexGender commonly refers to the set of characteristics that humans perceive as distinguishing between male and female entities, extending from one's biological sex to, in humans, one's social role or gender identity. As a term, "gender" has more than one valid definition...
in the same
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
. Examples include colour (specifically referred to as
sexual dichromatismSexual dichromatism is a form of sexual dimorphism in which the males and the females of the same species have different marking patterns or different color fur or feathering.- External links :****...
), size, and the presence or absence of parts of the body used in courtship displays or fights, such as ornamental feathers, horns, antlers or tusks.
Examples
In many species, including most
mammalMammals are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.Mammals are divided into three main...
s, the
maleMale refers to the 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...
is larger than the
femaleFemale is the sex of an organism, or a part of an organism, which produces 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...
. In others, such as most
insectInsects are arthropods, having a hard exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae. They are the most diverse group of animals on the planet and include more than a million species that are already described. Insects represent more than half of all...
s,
spiderSpiders are air-breathing chelicerate arthropods that have eight legs, and chelicerae modified into fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s, many
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
, and certain mammals such as the
spotted hyenaThe Spotted Hyena also known as Laughing Hyena, is a carnivorous mammal of the family Hyaenidae, of which it is the largest extant member. Though the species' prehistoric range included Eurasia extending from Atlantic Europe to China, it now only occurs in all of Africa south of the Sahara save...
, the female is larger than the male. Other sex-specific differences include differences in colouration (
sexual dichromatismSexual dichromatism is a form of sexual dimorphism in which the males and the females of the same species have different marking patterns or different color fur or feathering.- External links :****...
), presence vs. absence of certain body parts (such as horns, antlers, tusks or display feathers), size of the eyes (some insects), possession of stings (various kinds of
Hymenoptera Hymenoptera is one of the largest orders of insects, comprising the sawflies, wasps, bees, and ants. The name refers to the heavy wings of the insects, and is derived from the Ancient Greek ὑμήν : membrane and πτερόν : wing...
), and different thresholds for certain behaviors (aggression, infant care, etc).
Among vertebrates, sexual dimorphism is particularly apparent in
duckDuck is the common name for a number of species in the Anatidae family of birds. The ducks are divided between several subfamilies listed in full in the Anatidae article; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered ducks...
s and most gamefowl. This is perhaps most dramatic with species of
peafowlThe term peafowl can refer to the two species of bird in the genus Pavo of the pheasant family, Phasianidae. The African Congo Peafowl is placed in its own genus Afropavo and is not dealt with here. Peafowl are best known for the male's extravagant tail, which it displays as part of courtship...
. Male pheasants are notably larger than females and possess bright plumage, whereas females are usually brown irrespective of the particular
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
. In some birds, females have brighter colors than males; most of these cases are
waderWaders, called shorebirds in North America , are members of the order Charadriiformes, excluding the more marine web-footed seabird groups. The latter are the skuas , gulls , terns , skimmers , and auks...
s such as the
phalaropeA phalarope is any of three living species of slender-necked shorebirds in the genus Phalaropus of the bird family Scolopacidae. They are close relatives of the shanks and tattlers, the Actitis and Terek Sandpipers, and also of the turnstones and calidrids...
s and
painted snipePainted snipes are three distinctive wader species in the family Rostratulidae. The family is composed to two genera, Rostratula and Nycticryphes. The Australian Painted Snipe is often treated as a subspecies of the Greater Painted Snipe, but morphological and genetic differences have resulted in...
s. As this is the opposite of the usual sexual dichromatism, it is termed
reverse sexual dimorphism. In many predatory birds, females are larger than males, often by a considerable margin. This seems to reduce competition between members of a pair, as they have different optimal prey sizes. Some cases of sexual dimorphism in birds are so striking that males and females of the same species were originally taken to be members of entirely different species, as in the case of the
Eclectus ParrotThe Eclectus Parrot, Eclectus roratus, is a parrot native to the Solomon Islands, Sumba, New Guinea and nearby islands, northeastern Australia and the Maluku Islands . It is unusual in the parrot family for its extreme sexual dimorphism of the colours of the plumage; the male having a mostly...
(Eclectus roratus), where the male is predominantly green with an orange beak and the female scarlet and deep blue with a black beak.
The
HuiaThe Huia, was a species of New Zealand Wattlebird endemic to the North Island of New Zealand. It became extinct in the early 20
th century, primarily as a result of overhunting and widespread habitat destruction. The last confirmed sighting was on 28 December 1907 when W.W. Smith saw...
(Heteralocha acutirostris), a New Zealand bird species (now extinct), was another striking example of sexual dimorphism. The male's bill was short, sharp and stout, while the female's was long, thin and crescent-shaped. This beak dimorphism allowed mated pairs of Huia to avoid competing for the same food source, with males chiseling into and breaking apart rotting logs, while females were adept at probing into fresher wood for grubs.
Certain cases of sexual dimorphism have obvious utility beyond mate attraction. An example of this is the
Blue WildebeestThe Blue Wildebeest is a large ungulate mammal of the Bovid family and one of two species of wildebeest. It grows to 1.7 meters shoulder height and attains a body mass of up to 380 kilograms. They range the open plains, bushveld and dry woodlands of Southern and East Africa, realizing a life span...
(and many other biungulates). The
hornsA horn is a pointed projection of the skin on the head of various animals, consisting of a covering of horn surrounding a core of living bone. True horns are found mainly among the ruminant artiodactyls, in the families Antilocapridae and Bovidae...
of the male are much larger, allowing the male to engage in combat more effectively as he competes with other bucks for mating privileges.
An extreme example of sexual dimorphism is found in the genus
OsedaxOsedax is a genus of deep-sea siboglinid polychaetes, commonly called zombie worms or bone-eating worms. Osedax is Latin for "bone-eating", the name alluding to how the worms bore into the bones of whale carcasses to reach enclosed lipids, on which they rely for sustanence.The genus was first...
of
polychaeteThe Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
worms, which lives on
whale fallWhale fall is the term used for a whale carcass that has fallen to the ocean floor. Whale falls were first observed in the 1980s, with the advent of deep-sea robotic exploration....
s. The females feed on the bones of the dead whale; the males live inside the females and do not develop past their larval stage, except to produce large amounts of sperm. In the
echiuraThe Echiura, or spoon worms, are a small group of marine animals. They are often considered to be a group of annelids, although they lack the segmented structure found in other members of that group, and so may also be treated as a separate phylum...
n
Bonellia viridisThe Green Spoonworm is a marine worm noted for displaying exceptional sexual dimorphism and for the biocidal properties of a pigment in its skin....
, exposure to adult females causes larvae to develop into tiny, semi-parasitic males which are swallowed and live inside the female's genital sac. In the parasitic
barnacleA barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...
s
SacculinaSacculina is a genus of barnacles that parasitize crabs. The adults bear no resemblance to the barnacles that cover ships and piers; they are recognised as barnacles because their larval forms are like other members of the barnacle class Cirripedia...
, the males are tiny, free-ranging animals, whereas the females only exist as a web-like tissue inside their hosts. In the majority of
scale insectThe scale insects are small insects of the order Hemiptera, generally classified as the superfamily Coccoidea. There are about 8,000 species of scale insects....
s, females are highly modified (eyeless and wingless, with non-functional appendages and reduced segmentation), attached permanently to their host plants, while males are rather ordinary though delicate insects, smaller and winged.
Some species of
anglerfishAnglerfish are the members of the order Lophiiformes . They are bony fish named for their characteristic mode of predation, wherein a fleshy growth from the fish's head acts as a lure; this is considered analogous to angling.Some anglerfish are pelagic , others are benthic...
also display extreme sexual dimorphism. Females are typical anglerfish, while males are tiny rudimentary creatures with no 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).
Psychological and behavioral differentiation
Sex steroidSex steroids, aka gonadal steroids, are steroid hormones that interact with vertebrate androgen or estrogen receptors. Their effects are mediated by slow genomic mechanisms through nuclear receptors as well as by fast nongenomic mechanisms through membrane-associated receptors and signaling cascades...
-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.,
mountingIn biology, mating is the pairing of opposite-sex or hermaphroditic organisms for copulation and, in social animals, also to raise their offspring. For animals, mating methods include random mating, disassortative mating, assortative mating, or a mating pool....
or
receptive lordosisLordosis behavior, or Mammalian lordosis, is a sexual response in mammals, such as mice and cats, that consists of a ventral arching of the spine. During lordosis, the spine curves so that the apex points in the ventral direction. That is, the spine arches down.Lordosis aids in copulation, as it...
) can be shifted to that of the other sex by supplementation or deprivation of
androgenAndrogen is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of masculine characteristics in vertebrates by binding to androgen receptors. This includes the activity of the accessory male sex organs and...
s in fetal life or early infancy, even if adult levels are normal.
Handicap principle
The handicap principle is the proposed evolutionary force that gives males of some species traits that at first glance seem to place the organism at a
disadvantage. For example, the bright colouration of male game birds makes them highly visible targets for predators, while the drab females are better
camouflageCamouflage is a method of crypsis that allows an otherwise visible organism or object to remain indiscernible from the surrounding environment through deception. Examples include a tiger's stripes and the battledress of a modern soldier...
d. Other examples are bird of paradise and
lyrebirdA Lyrebird is either of two species of ground-dwelling Australian birds, most notable for their superb ability to mimic natural and artificial sounds from their environment. Lyrebirds have unique plumes of neutral coloured tailfeathers....
, whose males have such large plumes that their flight is inhibited. Strong smells, loud cries and singing can also attract predators.
The answer that has been suggested to this apparent paradox is that, at a biological level, the reproductive success of an organism is often more important than duration of life. This is particularly apparent in the case of game birds: a male
Common PheasantThe Common Pheasant , is a bird in the pheasant family . It is native to Asia and has been widely introduced elsewhere as a game bird. In parts of its range, namely in places where none of its relatives occur such as in Europe , it is simply known as the "pheasant"...
in the wild often lives no more than 10 months, with females living twice as long. However, a male pheasant's ability to reproduce depends not on how long he lives but whether females will select him to be their mate. A brightly coloured and heavily plumed male presumably demonstrates to the female that he is
fitFitness is a central concept in evolutionary theory. It describes the capability of an individual of certain genotype to reproduce, and usually is equal to the proportion of the individual's genes in all the genes of the next generation...
in evolutionary terms—he has been able to survive in spite of impediments and must therefore be a good choice to father her chicks.
Development of such characters could not at first be explained in terms of simple
natural selectionNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
. In 1871
Charles DarwinCharles Robert Darwin FRS was an English naturalist who realised and presented compelling evidence that all species of life have evolved over time from common ancestors, through the process he called natural selection...
advanced the theory of sexual selection, which related sexual dimorphism with
sexual selectionSexual selection is the theory proposed by Charles Darwin that states that certain evolutionary traits can be explained by intraspecific competition. Darwin defined sexual selection as the effects of the "struggle between the individuals of one sex, generally the males, for the possession of the...
.
Polygamy
Comparison of sexual dimorphism in birds and their mating habits shows that the time spent in search for mates, staking territories and mating competes with the demands of taking care of young. For birds and in general, it can be stated that the stronger the dimorphism in a species, the more likely is it to be
polygamousThe term polygamy is used in related ways in social anthropology, sociobiology, sociology, as well as in popular speech. Polygamy can be defined as any "form of marriage in which a person [has] more than one spouse."In social anthropology, polygamy is the practice of marriage to more than one...
and the less is the task of caring for offspring shared among the sexes. This theory is developed by
R. L. Trivers'Robert L. Trivers is an American evolutionary biologist and sociobiologist, most noted for proposing the theories of reciprocal altruism , parental investment , and parent-offspring conflict...
in the
parental investment theoryIn evolutionary biology, parental investment is any parental expenditure that benefits one offspring at a cost to parents' ability to invest in other components of fitness...
. It applies to all ecology.
Sexual dimorphism in humans
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Top: Stylised illustration of humans on the Pioneer plaqueThe Pioneer plaques are a pair of gold anodized aluminum plaques which were placed on board the 1972 Pioneer 10 and 1973 Pioneer 11 spacecraft, featuring a pictorial message, in case either Pioneer 10 or 11 are intercepted by extraterrestrial beings...
, showing both male (left) and female (right).
Above: Comparison between male (left) and female (right) pelvisIn human anatomy, the pelvis is the part of the trunk inferioposterior to the abdomen in the transition area between the trunk and the lower limbs...
es. |
Sexual dimorphism in humans is the subject of much controversy, especially when extended beyond physical differences to mental ability and psychological gender. (For a discussion, see
biology of genderThe biology of gender is scientific analysis of the physical basis for behavioural differences between men and women. It is more specific than sexual dimorphism, which covers physical and behavioural differences between males and females of any sexually reproducing species, or sexual...
,
sex and intelligenceSex and intelligence research investigates differences in the distributions of cognitive skills between men and women. This research employs experimental tests of cognition, which take a variety of forms. Research focuses on differences in individual skills as well as overall differences in general...
,
genderGender commonly refers to the set of characteristics that humans perceive as distinguishing between male and female entities, extending from one's biological sex to, in humans, one's social role or gender identity. As a term, "gender" has more than one valid definition...
, and
transgenderTransgender is a general term applied to a variety of individuals, behaviors, and groups involving tendencies to diverge from the normative gender roles....
.) Obvious differences between men and women include all the features related to reproductive role, notably the endocrine (hormonal) systems and their physiological and behavioural effects. Such undisputed sexual dimorphism include gonadal differentiation, internal genital differentiation, external genital differentiation, breast differentiation, muscle mass differentiation, and hair differentiation.
Externally, the most sexually dimorphic portions of the body are the chest, the lower half of the face, and the entire area between the waist and the knees.
The
basal metabolic rateBasal metabolic rate , and the closely related resting metabolic rate , is the amount of energy expended while at rest in a [neutrally temperate environment, in the post-absorptive state...
is about 6 percent higher in adolescent boys than girls and increases to about 10 percent higher after puberty. Women tend to convert more food into
fatFats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water. Chemically, fats are generally triesters of glycerol and fatty acids. Fats may be either solid or liquid at normal room temperature, depending on their structure and composition...
, while men convert more into
muscleMuscle is the contractile tissue of the body 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...
and expendable circulating energy reserves. Women (on average) are about 52 percent as strong as men in the upper body, and about 66 percent as strong in the lower. Men, on average, have denser, stronger
boneBones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, 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,
tendonA tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fascia as they are all made of collagen except that ligaments join one bone to another bone, and fascia connect muscles to other...
s, and
ligamentIn anatomy, the term ligament is used to denote three different types of structures:# Fibrous tissue that connects bones to other bones. They are sometimes called "articular ligaments", "fibrous ligaments", or "true ligaments"....
s.
Men dissipate heat faster than women through their
sweat glandSweat glands are exocrine glands, found in the skin of all mammal species, that are used for body temperature regulation. In humans a system of apocrine - and merocrine sweat glands is the main method of cooling. Many other mammals rely on panting or other means as a primary source of cooling, but...
s. Women have a greater insulation and energy reserves stored in
subcutaneous fatSubcutaneous fat is found just beneath the skin as opposed to visceral fat which is found in the peritoneal cavity. Subcutaneous fat can be measured using body fat calipers giving a rough estimate of total body adiposity...
, absorbing endothermic heat less and retaining exothermic heat to a greater degree.
Men typically have larger
tracheaeThe trachea, or windpipe, is a tube that connects to the pharynx or larynx, allowing the passage of air to the lungs. It is lined with pseudostratified ciliated columnar epithelium cells with mucosal goblet cells which produce mucus...
and branching
bronchiA bronchus is a passage of airway in the respiratory tract that conducts air into the lungs.No gas exchange takes place in this part of the lungs.-Anatomy:...
, with about 30 percent greater
lung volumeLung volumes refer to physical differences in lung volume, while lung capacities represent different combinations of lung volumes, usually in relation to inhalation and exhalation....
per body mass. They have larger
heartThe heart is a muscular organ found in all vertebrates that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
s, 10 percent higher
red blood cellRed blood cells are the most common type of blood cell and the vertebrate body's principal means of delivering oxygen to the body tissues via the blood. They take up oxygen in the lungs or gills and release it while squeezing through the body's capillaries. The cells are filled with hemoglobin, a...
count, higher
hemoglobinHemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of vertebrates, and the tissues of some invertebrates....
, hence greater oxygen-carrying capacity. They also have higher circulating
clotting factorsCoagulation is a complex process by which blood forms clots. It is an important part of hemostasis , wherein a damaged blood vessel wall is covered by a platelet and fibrin-containing clot to stop bleeding and begin repair of the damaged vessel...
(
vitamin KVitamin K denotes a group of lipophilic, hydrophobic vitamins that are needed for the posttranslational modification of certain proteins, mostly required for blood coagulation...
, pro
thrombinThrombin also commonly called pro-thrombin is a coagulation protein in the blood stream that has many effects in the coagulation cascade...
and
plateletPlatelets, or thrombocytes , are small, irregularly-shaped anuclear cells , 2-3 µm in diameter, which are derived from fragmentation of precursor megakaryocytes. The average lifespan of a platelet is between 8 and 12 days...
s). These differences lead to faster healing of
woundIn medicine, a wound is a type of injury in which skin is torn, cut or punctured , or where blunt force trauma causes a contusion . In pathology, it specifically refers to a sharp injury which damages the dermis of the skin.-Types of wounds:-Open:Open wounds can be classified according to the...
s and higher peripheral pain tolerance.
Women typically have more
white blood cellWhite blood cells , or leukocytes , are cells of the immune system defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a...
s (stored and circulating), more
granulocyteGranulocytes are a category of white blood cells characterised by the presence of granules in their cytoplasm. They are also called polymorphonuclear leukocytes because of the varying shapes of the nucleus, which is usually lobed into three segments...
s and B and T
lymphocyteA lymphocyte is a type of white blood cell in the vertebrate immune system.By their appearance under the light microscope, there are two broad categories of lymphocytes, namely the large granular lymphocytes and the small lymphocytes. Functionally distinct subsets of lymphocytes correlate with...
s. Additionally, they produce more
antibodiesAntibodies are gamma globulin proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacteria and viruses...
at a faster rate than men. Hence they develop fewer
infectiousAn infection is the detrimental colonization of a host organism by a foreign species. In an infection, the infecting organism seeks to utilize the host's resources to multiply, usually at the expense of the host. The infecting organism, or pathogen, interferes with the normal functioning of the...
diseases and succumb for shorter periods.
EthologistsEthology is the scientific study of animal behavior, and a sub-topic of zoology....
argue that women, interacting with other women and multiple offspring in social groups, have experienced such traits as a
selectiveNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
advantage. Note that almost all examples of sexual dimorphism in humans are quantitative, and have some degree of overlap.
Some biologists theorise that a species' degree of sexual dimorphism is inversely related to the degree of paternal investment in
parentingParenting is the process of promoting and supporting the physical, emotional, social, and intellectual development of a child from infancy to adulthood...
. 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).
Although there are many biologically-determined, sexually-dimorphic behaviours in other species, these have few, if any, implications for human society. However, analysis of sexually dimorphic human behavior naturally provokes controversy. One less controversial, but still hypothetical, area with considerable discussion in academic literature concerns potential evolutionary advantages associated with sexual competition (both intrasexual and intersexual) and short- and long-term sexual strategies.
According to Daly and Wilson, "The sexes differ more in human beings than in monogamous mammals, but much less than in extremely polygamous mammals."
D.M. Buss stated that "Males should prefer attributes in potential mates associated with reproductive value
or fertility, depending on whether males in human evolutionary history have tended to seek long-term or short-term mating partners. Specifically, if males in our evolutionary past have tended to seek short-term mating partners, selection should have favoured male preferences for females in their early 20s who show cues positively correlated with fertility. If males in our evolutionary past tended to seek long-term mating partners, selection should have favoured preferences for females in their mid-teens who show cues indicative of reproductive value. Evolutionary theorists differ on which of these they judge to be most likely." However, there has not yet been a study to prove that men chose who they actually end up mating with or, rather that is is the women who chose the men.
Sexual dimorphism in birds
Sexual dimorphism varies in bird species from very obvious differences, such as the plumage of male Peacocks, the greatly larger form of female eagles, to very similar appearance (most
parrotParrots, also known as psittacines , are birds of the roughly 372 species in 86 genera that make up the order Psittaciformes, found in most warm and tropical regions. The order is subdivded in three families: the Psittacidae , the Cacatuidae and the Nestoridae...
s).
When the appearance of the male and female is very similar they are called 'Monomorphic birds'
http://birds.about.com/od/glossaryofavianterms/g/monomorphic.htm. The gender of Monomorphic birds is commonly revealed by endoscopic examination or surgical sexing
http://www.prettybird.com/researcharticles/genderarticle.htm
Male birds such as canaries or parrots are occasionally mentioned by fanciers as being better singers or mimics than females, perhaps due to stronger attention- and territory-seeking behaviors, and are commonly sought out more frequently in the pet trade as a result.
See also
- Sexual dimorphism in non-human primates
Sexual dimorphism in non-human primates has long been observed in the primate order, with numerous studies performed to document and explain the phenomenon. Recent studies have mainly used the technique of comparative analysis to examine both the variation in the expression of the dimorphism among...
- Bateman's principle
In biology, Bateman's principle is the theory that females almost always invest more energy into producing offspring than males, and therefore in most species females are a limiting resource over which the other sex will compete...
- Digit ratio
The digit ratio is the ratio of the lengths of different digits or fingers typically measured from the bottom crease where the finger joins the hand to the tip of the finger. It has been suggested by some scientists that the ratio of two digits in particular, the 2nd and 4th , is affected by...
- Operational sex ratio
In the evolutionary biology of sexual reproduction, the operational sex ratio is the ratio of sexually competing males to females that are ready to mate. It is different from the physical sex ratio in that physical sex ratio also takes into account sexually inactive individual organisms, and...
- Sexual selection
Sexual selection is the theory proposed by Charles Darwin that states that certain evolutionary traits can be explained by intraspecific competition. Darwin defined sexual selection as the effects of the "struggle between the individuals of one sex, generally the males, for the possession of the...
- Sexual differentiation
Sexual differentiation is the process of development of the differences between males and females from an undifferentiated zygote...
- Sexually dimorphic nucleus
Sexually dimorphic nucleus , is believed to be related to sexual behavior in animals. It is a cluster of cells located in the preoptic area of hypothalamus of the brain. The volume of SDN is significantly larger in males than in females, caused mainly by greater cell number and larger cell size,...
- Sexual dimorphism measures
Although the subject of sexual dimorphism is not in itself controversial, the measures by which it is assessed differ widely. Most of the measures are used on the assumption that a random variable is considered so that probability distributions should be taken into account...
- Sexual reproduction
Sexual reproduction is characterized by processes that pass a combination of genetic material to offspring, resulting in diversity. The main two processes are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration...
- Sex-limited genes
Sex-limited genes are genes which are present in both sexes of sexually reproducing species but turned on in only one sex. In other words, sex-limited genes cause the two sexes to show different traits or phenotypes. An example of sex-limited genes are genes which instructs male elephant seal to...
- Gender differences
A sex difference is a distinction of biological and/or physiological characteristics typically associated with either males or females of a species in general. This article focuses on quantitative differences which are based on a gradient and involve different averages...
- List of homologues of the human reproductive system