Defining characteristicsThe 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. A female individual cannot reproduce sexually without access to the gametes of a male (an exception is parthenogenesis). Some organisms can reproduce both sexually and asexually.
There is no single genetic mechanism behind sex differences in different species and the existence of two sexes seems to have evolved multiple times independently in different evolutionary lineages. The repeated pattern is sexual reproduction in isogamous species with two or more mating types with gametes of identical form and behavior (but different at the molecular level) to anisogamous species with gametes of male and female types to oogamous species in which the female gamete is very much larger than the male and has no ability to move. 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.
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.
Etymology and usageThe word female comes from the Latin femella, the diminutive form of femina, meaning "woman," which is not actually related to the word "male." In the late 14th century, the English spelling was altered so that the word paralleled the spelling of "male."
Mammalian femaleThe distinguishing characteristic of the class Mammalia is the presence of mammary glands. The mammary glands are modified sweat glands that produce milk, which is used to feed the young during the period of time shortly after birth. Only mammals have the capacity to produce milk. The presence of mammary glands is most obvious on humans, due to the tendency of the female human body to store large amounts of fatty tissue near the nipples, resulting in prominent breasts, although today some human females also surgically augment their breast size. However, mammary glands are present in all mammals, although they are vestigial in the male of the species.
The mammalian female is characterized by having two copies of the X chromosome as opposed to the male which carries only one X and one smaller Y chromosome. To compensate for the difference in size, one of the female's X chromosomes is randomly inactivated in each cell. In birds, by contrast, it is the female who is heterozygous and carries a Z and a W chromosome whilst the male carries two Z chromosomes.
Mammalian females are characterized in that they all bear live young (with the rare exception of monotremes, which lay eggs). This is not totally unique, as some animals, such as guppies have analogous reproductive structures. In addition, some other non-mammalian animals, such as sharks, whose eggs hatch inside their bodies also have the appearance of bearing live young.