Mammalian reproduction
[3] The mammary glands of mammals are specialized to produce milk, a liquid used by newborns as their primary source of nutrition.
A marsupial has a short gestation period, typically shorter than its estrous cycle, and gives birth to an underdeveloped (altricial) newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen.
The mammalian male reproductive system contains two main divisions, the penis and the testicles, the latter of which are where sperm are produced and usually held in a scrotum.
The external location may also cause a reduction in the heat-induced contribution to the spontaneous mutation rate in male germinal tissue.
[6] Sperm are the smaller of the two gametes and are generally very short-lived, requiring males to produce them continuously from the time of sexual maturity until death.
When developed enough to survive outside the womb, the cervix dilates and contractions of the uterus propel the fetus through the birth canal, which is the vagina.
The ova, which are the female sex cells, are much larger than the sperm and are normally formed within the ovaries of the fetus before its birth.
If not fertilized, this egg is released through menstruation in humans and other great apes, and reabsorbed in other mammals in the estrus cycle.
Gestation, called pregnancy in humans, is the period of time during which the fetus develops, dividing via mitosis inside the female.
In addition, certain vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits.
Human babies are unique in the animal kingdom due to their large head size relative to their bodies.
A primary hypothesis for why this process and others occur, causing human births to be drastically more difficult than other mammals is known as the obstetrical dilemma.
Marsupials' reproductive systems differ markedly from those of placentals,[10][11] though it is probably the plesiomorphic condition found in viviparous mammals, including non-placental eutherians.
Animals, including mammals, produce gametes (sperm and egg) through meiosis in gonads (testicles in males and ovaries in females).
[14][15] These behavioral states correlate with the phases of the human sexual response cycle: motivation − excitement; consummation − plateau and orgasm; satiety − refraction.
In most female mammals, the act of copulation is controlled by several innate neurobiological processes, including the motor sexual reflex of lordosis.
