Mammalian reproduction

Most mammals are viviparous, giving birth to live young.^[1] However, the five species of monotreme, the platypuses and the echidnas, lay eggs. The monotremes have a sex determination system different from that of most other mammals.^[2] In particular, the sex chromosomes of a platypus are more like those of a chicken than those of a therian mammal.^[3]

Goat kids will stay with their mother until they are weaned.

The mammary glands of mammals are specialized to produce milk, a liquid used by newborns as their primary source of nutrition. The monotremes branched early from other mammals and do not have the teats seen in most mammals, but they do have mammary glands. The young lick the milk from a mammary patch on the mother's belly.

Viviparous mammals are in the subclass Theria; those living today are in the Marsupialia and Placentalia infraclasses. 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. Some placentals, e.g. guinea pig, give birth to fully developed (precocial) young, usually after long gestation periods, while some others, e.g. mouse, give birth to underdeveloped young.

Maturity and reproductive age

Sexual maturity and thus the earliest age at which mammals can reproduce varies dramatically across species. Members of the rodent family Cricetidae can reach sexual maturity in 1–2 months, e.g. the Norway lemming (Lemmus lemmus) in 39 days. Many dogs (family Canidae) and bovids (Bovidae) take about a year to reach maturity while primates (including humans) and dolphins (Delphinidae) require more than 10 years. Some whales take even longer, with the longest duration being recorded for the bowhead whale (Balaena mysticetus), which reaches maturity at an age of only about 23 years.^[4]

Reproductive system

Further information: Reproductive system § Other mammals, Sex organ § Mammals, Rut (mammalian reproduction), Carnivora § Reproductive system, and Human reproductive system

Placental mammals

Male placentals

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.^[5] In humans, both of these organs are outside the abdominal cavity, but they can be primarily housed within the abdomen in other animals. For instance, a dog's penis is covered by a penile sheath except when mating. Having the testicles outside the abdomen best facilitates temperature regulation of the sperm, which require specific temperatures to survive. 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. The produced sperm are stored in the epididymides until ejaculation through the vasa deferentia. The sperm cells are motile and they swim using tail-like flagella to propel themselves towards the ovum. The sperm follows temperature gradients (thermotaxis)^[7] and chemical gradients (chemotaxis) to locate the ovum.

Female placentals

The mammalian female reproductive system contains three main divisions: the vagina and uterus, which act as the receptacle for the sperm, the ovaries, which produce the female's ova, and the vulva, which consists of the labia and clitoris. The vagina, uterus and ovaries are always internal while the vulva is external. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the oviducts. At certain intervals, the ovaries release an ovum, which passes through the oviduct into the uterus.

If, in this transit, it meets with sperm, the egg selects sperm with which to merge; this is termed fertilization. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then implants itself in the wall of the uterus, where it begins the processes of embryogenesis and morphogenesis. 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. They are mostly fixed in location within the ovary until their transit to the uterus, and contain nutrients for the later zygote and embryo. Over a regular interval, in response to hormonal signals, a process of oogenesis matures one ovum which is released and sent down the oviduct. 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

The initial stages of human embryogenesis.

Main articles: Gestation and Pregnancy (mammals)

Gestation, called pregnancy in humans, is the period of time during which the fetus develops, dividing via mitosis inside the female. During this time, the fetus receives all of its nutrition and oxygenated blood from the female, filtered through the placenta, which is attached to the fetus' abdomen via an umbilical cord. This drain of nutrients can be quite taxing on the female, who is required to ingest slightly higher levels of calories. In addition, certain vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. The length of gestation, called the gestation period, varies greatly from species to species; it is 40 weeks in humans, 56–60 in giraffes and 16 days in hamsters.

Birth

Main article: Birth

Once the fetus has sufficiently developed, chemical signals start the process of birth. This begins with contractions of the uterus and dilation of the cervix. The fetus then descends to the cervix, where it is pushed out into the vagina, and eventually out of the female. The newborn, which is called an infant in humans, should typically begin respiration on its own shortly after birth. Not long after, the placenta is passed as well.

Human births

Human babies are unique in the animal kingdom due to their large head size relative to their bodies. This has an effect on the birthing process for humans as the bipedal gait of a human causes the birthing canal to be relatively narrow and twisted in the middle. As a result, the vast majority of human babies must rotate inside the birth canal in order to squeeze through the birthing canal and fit through the pelvic planes. This process is known as a rotational birth, and while it is not a process unique to humans, humans are unique in that nearly all human babies undergo this process out of necessity. 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.^[8]

Monotremes

Further information: Monotreme § Reproductive system

Monotremes, only five species of which exist, all from Australia and New Guinea, are mammals that lay eggs. They have one opening for excretion and reproduction called the cloaca. They hold the eggs internally for several weeks, providing nutrients, and then lay them and cover them like birds. Like marsupial "joeys", monotreme "puggles" are larval and fetus-like,^[9] as like them they cannot expand their torso due to the presence of epipubic bones, forcing them to produce undeveloped young.

Marsupials

Further information: Marsupial reproductive system

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.^[12] During embryonic development, a choriovitelline placenta forms in all marsupials. In bandicoots, an additional chorioallantoic placenta forms, although it lacks the chorionic villi found in eutherian placentas.

• 
  Bull
• 
  Stallion
• 
  Male domestic cat
• 
  Female domestic cat
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  Female raccoon
• 
  Male raccoon
• 
  Male and female spotted hyena
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  Female marsupials
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  Male rabbit
• 
  Female rabbit

Gametogenesis

Animals, including mammals, produce gametes (sperm and egg) through meiosis in gonads (testicles in males and ovaries in females). Sperm are produced by the process of spermatogenesis and eggs are produced by oogenesis. These processes are outlined in the article gametogenesis. During gametogenesis in mammals many genes encoding proteins that take part in DNA repair mechanisms show enhanced or specialized expression ^[13] These mechanisms include meiotic homologous recombinational repair and mismatch repair.

Copulation

This section is an excerpt from Copulation (zoology) § In mammals.[edit]

Mating postures of mammals

Sexual behavior can be classified into behavioral states associated with reward motivation ("wanting"), reward consummation also known as pleasure ("liking"), and satiety ("inhibition");^[14] these behavioral states are regulated in mammals by reward-based sexual learning, fluctuations in various neurochemicals (i.e., dopamine − sexual desire also known as "wanting"; norepinephrine − sexual arousal; oxytocin and melanocortins − sexual attraction), and gonadal hormone cycles and further influenced by sex pheromones and motor reflexes (i.e., lordosis behaviour) in some mammals.^[14][15]

These behavioral states correlate with the phases of the human sexual response cycle: motivation − excitement; consummation − plateau and orgasm; satiety − refraction.^[14] Sexual learning (a form of associative learning) occurs when an animal starts to associate bodily features, personality, contextual cues, and other stimuli with genitally-induced sexual pleasure.^[14][15] Once formed, these associations in turn impinge upon both sexual wanting and sexual liking.

In most female mammals, the act of copulation is controlled by several innate neurobiological processes, including the motor sexual reflex of lordosis.^[16] In males, the act of copulation is more complex, because some learning is necessary, but the innate processes (retrocontrol of penis intromission in the vagina, rhythmic movement of the pelvis, detection of female pheromones) are specific to copulation. These innate processes direct heterosexual copulation.^[17] Female lordosis behaviour became secondary in Hominidae and is non-functional in humans.^[18] Mammals usually copulate in a dorso-ventral posture, although some primate species copulate in a ventro-vental posture.^[19][20]

Most mammals possess a vomeronasal organ that is involved in pheromone detection, including sex pheromones.^[21] Despite the fact that humans do not possess this organ, adult humans appear to be sensitive to certain mammalian pheromones that putative pheromone receptor proteins in the olfactory epithelium are capable of detecting.^{[note 1][21]} While sex pheromones clearly play a role in modifying sexual behavior in some mammals, the capacity for general pheromone detection and the involvement of pheromones in human sexual behavior has not yet been determined.^[14]

The duration of copulation varies significantly between mammal species,^[25] and may be correlated with body mass, lasting longer in large mammals than in small mammals.^[26] The duration of copulation may also be correlated with the length of the baculum in mammals.^[27]

Male mammals ejaculate semen through the penis into the female reproductive tract during copulation.^[28][29] Ejaculation usually occurs after only one intromission in humans, canids, and ungulates, but occurs after multiple intromissions in most mammal species.^[30][31]

Copulation can induce ovulation in mammal species that do not ovulate spontaneously.^[32]

• Copulating brown bears
• Copulating lions
• Copulating grey kangaroos
• Copulating Aldabra giant tortoises
• Copulating Magellanic penguins

See also

• Mammal reproductive system
• Evolution of descended testes in mammals
• Sexual reproduction § Mammals
• Sex organ § Mammals
• Human reproduction
• Animal sexual behavior § Mammals
• Pregnancy (mammals)
• Equine reproductive system
• Carnivora § Reproductive system
• Canine reproduction
• Dolphin § Reproduction and sexuality
• Llama § Reproduction
• Domestic sheep reproduction

Notes

1. In humans and other animals, trace amine-associated receptors (TAARs) that are expressed in the olfactory epithelium function as olfactory receptors that detect volatile amine odorants, including certain pheromones;^[22][23] these TAARs putatively function as a class of pheromone receptors involved in the olfactive detection of social cues.^[22][23]
   
   A review of studies involving non-human animals indicated that TAARs in the olfactory epithelium can mediate attractive or aversive behavioral responses to an agonist.^[22] This review also noted that the behavioral response evoked by a TAAR can vary across species.^[22] For example, TAAR5 mediates attraction to trimethylamine in mice and aversion to trimethylamine in rats.^[22] In humans, hTAAR5 presumably mediates aversion to trimethylamine, which is known to act as an hTAAR5 agonist and to possess a foul, fishy odor that is aversive to humans;^[22][24] however, hTAAR5 is not the only olfactory receptor that is responsible for trimethylamine olfaction in humans.^[22][24] As of December 2015,^[update] hTAAR5-mediated trimethylamine aversion has not been examined in published research.^[24]