Female

The female is one of the two possible sexes in nature, the other being the male. Characteristically, mature females produce oocytes. In species with intracorporal fertilization, the most predominant feature of a female is the limited number of gametes that can be produced. For example in mammals, primordial germ cells divide by mitosis in females until a certain number (200,000) of oocytes is reached. After birth, these mitotic divisions come to a standstill, and meiosis and further maturation of oocytes occurs after puberty at ovulation. Ovulation is not a continuous production of eggs, but involves a selection process in which certain ones are able to become dominant and cause atresia in others. These events typically occur in cycles, with a duration of 21 to 28 days. However, spontaneous or inducable ovulations can also take place, as in the cat or rabbit. Although the number of oocytes produced seems to be more than sufficient, it is apparent, for example, in humans, that the fertile period in the life span of a female is, unlike in males, restricted to a certain age and, within this age, to certain days periodically. This fact is probably the most typical feature of females with regard to reproduction.

 Considering that each individual strives to transfer its genes to the next generation, the limited number of gametes with a chance to be fertilized causes females to take greater care of each individual gamete than males do. In females, natural selection therefore proceeded in a different way than in the male. Reproductive success in females is optimized when the offspring are actually raised and attain the ability to reproduce in the next generation. As a consequence, females not only use substantial resources in producing a single gamete, by adding nutrition (yolk) (1) and all necessary substances for initial cell divisions and growth of the cells, but, especially in higher chordates, they also provide basic nutrition for the offspring born (2). This is especially the case in mammals, where nutrition and protection is provided for a prolonged time after birth during childhood by the mammary glands, which are a typical female feature. Also, as can be seen in primates, raising of the offspring is done mostly by the female, although there are exceptions (3). Natural selection does not occur in a competition in which genes must be spread, but the most successful female is the one that is best able to protect and raise the offspring. In this respect, it is evident why females in the animal kingdom are better adapted to the environment with regard to hair or feathers, because this serves as a protection against predators. The appearance of this kind of sexual dimorphism depends on the degree of specialization among the two sexes.

 In species with extracorporal fertilization, such as frogs and fishes, differences between the two sexes are smaller than in mammals. In these species, both the male and female produce a vast number of gametes to form numerous new individuals. In these species, parental care for the offspring is not the rule. As a consequence, the phenotypically visible differences between the sexes vanish. Nevertheless, for the sake of reproductive success, females invest more than males, because the cost of producing eggs is greater than that of producing sperm.

The common biological necessity in females of a greater investment in reproduction has evolved common characteristic features among females of different species. For example, the genitalia of the female consist of germ-cell conducting parts (tubes) as well as of germ-cell preserving parts ) uterus). Although both male and female genital cells have the same origin in the primordial urogenital tract and in the primordial germ cells, the lack of male hormones due to the absence of the testis-determining factor allows the development of mullerian ducts. The fate of these organs is to develop finally to an uterus and a vagina during embryogenesis. Also, the hormonal situation is different in females and males. After puberty, cyclic hormonal actions take place in the female. The expression of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are directed by FSH-releasing hormone and LH-releasing hormone, respectively. FSH and LH induce ovulation and the expression of estrogen and progesterone, which occurs in the follicle.

 In the cascade of sex determination and in adults, hormones are also responsible for the expression of the female phenotype. This includes psychological behavior. Hormones in the female direct fertilization events like ovulation and behavior, eg to indicate the mating time for males.

 In higher organisms, the female is the homogametic sex XX, the male the heterogametic XY. Sex determination occurs by the dependence of the presence or absence of the testis-determining factor ( SRY), a gene located on the Y-Chromosome in mammals. In these species, therefore, sex determination is dependent on the sperm that successfully fertilizes the oocyte. In birds, some amphibians, and insects, the situation is reversed: The homogametic sex is male, the heterogametic sex is the female. Instead of two X-chromosomes, the chromosomes in the female are ZW, and sex determination occurs during ovulation.

 Parthenogenesis is possible only with eggs and occurs in some genera of lizards. In some species of fish, spermatozoa are necessary only for the induction of the egg, not for a genetic contribution.

References

1. T. D. Williams (1994) Biol. Rev. Camb. Phil. Soc. 69, 35–59. 

2. G. N. Wade and J. E. Schneider (1992) Neurosci. Biobehav. Rev. 16, 235–272.

3. J. W. Makin and R. H. Porter (1984) Behav. Neural. Biol. 41, 135–151.