An organism's sex is female (symbol: ♀) if it produces the ovum (egg cell), the type of gamete (sex cell) that fuses with the male gamete (sperm cell) during sexual reproduction.^[2][3][4]

A female has larger gametes than a male. Females and males are results of the anisogamous reproduction system, wherein gametes are of different sizes (unlike isogamy where they are the same size). The exact mechanism of female gamete evolution remains unknown.

In species that have males and females, sex-determination may be based on either sex chromosomes, or environmental conditions. Most female mammals, including female humans, have two X chromosomes. Female characteristics vary between different species, with some species having pronounced secondary female sex characteristics, such as the presence of pronounced mammary glands in mammals.

In humans, the word female can also be used to refer to gender in the social sense of gender role or gender identity.^[5][6]

Etymology and usage

The word female comes from the Latin femella, the diminutive form of femina, meaning "woman". It is not etymologically related to the word male, but in the late 14th century the English spelling was altered to parallel that of male.^[7][8] Female is also used as a noun meaning "a female organism", though describing women as females is often considered disparaging, as it makes no distinction between other animals and humans.^[9][10]

Biological sex is conceptually distinct from gender,^[11][12] although they are often used interchangeably.^[13][14] The adjective female can describe a person's sex or gender identity.^[15]

The word can also refer to the shape of connectors and fasteners, such as screws, electrical pins, and technical equipment. Under this convention, sockets and receptacles are called female, and the corresponding plugs male.^[16][17]

Defining characteristics

Females produce ova, the larger gametes in a heterogamous reproduction system, while the smaller and usually motile gametes, the spermatozoa, are produced by males.^[3][18] Generally, a female cannot reproduce sexually without access to the gametes of a male, and vice versa, but in some species females can reproduce by themselves asexually, for example via parthenogenesis.^[19]

Patterns of sexual reproduction include:

• Isogamous species with two or more mating types with gametes of identical form and behavior (but different at the molecular level),
• Anisogamous species with gametes of male and female types,
• Oogamous species, which include humans, in which the female gamete is much larger than the male and has no ability to move. Oogamy is a form of anisogamy.^[20] There is an argument that this pattern was driven by the physical constraints on the mechanisms by which two gametes get together as required for sexual reproduction.^[21]

Other than the defining difference in the type of gamete produced, differences between males and females in one lineage cannot always be predicted by differences in another. The concept is not limited to animals; egg cells are produced by chytrids, diatoms, water moulds and land plants, among others. In land plants, female and male designate not only the egg- and sperm-producing organisms and structures, but also the structures of the sporophytes that give rise to male and female plants.^{[citation needed]}

Females across species

Species that are divided into females and males are classified as gonochoric in animals, as dioecious in seed plants^[22] and as dioicous in cryptogams.^[23]: 82

In some species, female and hermaphrodite individuals may coexist, a sexual system termed gynodioecy.^[24] In a few species, female individuals coexist with males and simultaneous hermaphrodites; this sexual system is called trioecy.^[25] In Thor manningi (a species of shrimp), females coexist with males and protandrous hermaphrodites.^[26]

Mammalian female

A distinguishing characteristic of the class Mammalia is the presence of mammary glands. Mammary glands are modified sweat glands that produce milk, which is used to feed the young for some time after birth. Only mammals produce milk. Mammary glands are obvious in humans, because the female human body stores large amounts of fatty tissue near the nipples, resulting in prominent breasts. Mammary glands are present in all mammals, although they are normally redundant in males of the species.^[27]

Most mammalian females have two copies of the X chromosome, while males have only one X and one smaller Y chromosome; some mammals, such as the platypus, have different combinations.^[28][29] One of the female's X chromosomes is randomly inactivated in each cell of placental mammals while the paternally derived X is inactivated in marsupials. In birds and some reptiles, by contrast, it is the female which is heterozygous and carries a Z and a W chromosome while the male carries two Z chromosomes. In mammals, females can have XXX or X.^[30][31]

Mammalian females bear live young, with the exception of monotreme females, which lay eggs.^[32] Some non-mammalian species, such as guppies, have analogous reproductive structures; and some other non-mammals, such as some sharks, also bear live young.^[33]

In sex determination for mammals, female is the default sex, while in the poplar genus Populus the default is male.^[34]

Sex determination

The sex of a particular organism may be determined by genetic or environmental factors, or may naturally change during the course of an organism's life.^[24]

Genetic determination

The sex of most mammals, including humans, is genetically determined by the XY sex-determination system where females have XX (as opposed to XY in males) sex chromosomes. It is also possible in a variety of species, including humans, to have other karyotypes. During reproduction, the male contributes either an X sperm or a Y sperm, while the female always contributes an X egg. A Y sperm and an X egg produce a male, while an X sperm and an X egg produce a female. The ZW sex-determination system, where females have ZW (as opposed to ZZ in males) sex chromosomes, is found in birds, reptiles and some insects and other organisms.^[24]

Environmental determination

The young of some species develop into one sex or the other depending on local environmental conditions, e.g. the sex of crocodilians is influenced by the temperature of their eggs. Other species (such as the goby) can transform, as adults, from one sex to the other in response to local reproductive conditions (such as a brief shortage of males).^[35]

Evolution

The question of how females evolved is mainly a question of why males evolved. The first organisms reproduced asexually, usually via binary fission, wherein a cell splits itself in half. From a strict numbers perspective, a species that is half males/half females can produce half the offspring an asexual population can, because only the females are having offspring. Being male can also carry significant costs, such as in flashy sexual displays in animals (such as big antlers or colorful feathers), or needing to produce an outsized amount of pollen as a plant in order to get a chance to fertilize a female. Yet despites the costs of being male, there must be some advantage to the process.^[36]

The advantages are explained by the evolution of anisogamy, which led to the evolution of male and female function.^[37] Before the evolution of anisogamy, mating types in a species were isogamous: the same size and both could move, catalogued only as "+" or "-" types.^[38]: 216 In anisogamy, the mating cells are called gametes. The female gamete is larger than the male gamete, and usually immotile.^[39] Anisogamy remains poorly understood, as there is no fossil record of its emergence. Numerous theories exist as to why anisogamy emerged. Many share a common thread, in that larger female gametes are more likely to survive, and that smaller male gametes are more likely to find other gametes because they can travel faster. Current models often fail to account for why isogamy remains in a few species.^[36] Anisogamy appears to have evolved multiple times from isogamy; for example female Volvocales (a type of green algae) evolved from the plus mating type.^[38]: 222 Although sexual evolution emerged at least 1.2 billion years ago, the lack of anisogamous fossil records make it hard to pinpoint when females evolved.^[40]

Female sex organs (genitalia, in animals) have an extreme range of variation among species and even within species. The evolution of female genitalia remains poorly understood compared to male genitalia, reflecting a now-outdated belief that female genitalia are less varied than male genitalia, and thus less useful to study. The difficulty of reaching female genitalia has also complicated their study. New 3D technology has made female genital study simpler. Genitalia evolve very quickly. There are three main hypotheses as to what impacts female genital evolution: lock-and-key (genitals must fit together), cryptic female choice (females affect whether males can fertilize them), and sexual conflict (a sort of sexual arms race). There is also a hypothesis that female genital evolution is the result of pleiotropy, i.e. unrelated genes that are affected by environmental conditions like low food also affect genitals. This hypothesis is unlikely to apply to a significant number of species, but natural selection in general has some role in female genital evolution.^[41]

Symbol

The symbol ♀ (Unicode: U+2640 Alt codes: Alt+12), a circle with a small cross underneath, is commonly used to represent females. Joseph Justus Scaliger once speculated that the symbol was associated with Venus, goddess of beauty, because it resembles a bronze mirror with a handle,^[42] but modern scholars consider that fanciful, and the most established view is that the female and male symbols derive from contractions in Greek script of the Greek names of the planets Thouros (Mars) and Phosphoros (Venus).^[43][44]