A 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 for some time after birth. Only mammals produce milk. Mammary glands are most obvious in humans, as the female human body stores large amounts of fatty tissue near the nipples, resulting in prominent breasts. 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 has 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 and reptiles, 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 bear live young (with the rare exception of monotremes, which lay eggs). Some non-mammalian species, such as guppies, have analogous reproductive structures; and some other non-mammals, such as sharks, whose eggs hatch inside their bodies, also have the appearance of bearing live young.
Read more about this topic: Female
Famous quotes containing the words female and/or mammalian:
“I look on it as no trifling effort of female strength to withstand the artful and ardent solicitations of a man that is thoroughly master of our hearts. Should we in the conflict come off victorious, it hardly pays us for the pain we suffer from the experiment ... and I still persist in it that such a behaviour in any man I love would rob me of that most pleasing thought, namely, the obligation I have to him for not making such a trial.”
—Sarah Fielding (17101768)
“Specialization is a feature of every complex organization, be it social or natural, a school system, garden, book, or mammalian body.”
—Catharine R. Stimpson (b. 1936)