Sexual reproduction

(biology) Reproduction involving the paired union of special cells (gametes) from two individuals.

Concept

Sexual reproduction is one of the two major ways, along with asexual reproduction, that plants and animals create offspring and thus propagate the species. Critical to sexual reproduction is the process of fertilization, whereby the male and female sex cells fuse, or bond. Fertilization may be of two types, either internal or external, and though humans normally fertilize by the first of those means, they may use the second, in the form of in vitro fertilization. Humans, of course, have by far the most complicated reproductive process, inasmuch as it is surrounded by a vast societal, interpersonal, and moral framework that is not a factor in animal reproduction. Part of that framework is the activity that precedes sexual reproduction: attraction, courtship, and so forth. Although no other animal's courtship rituals rival those of humans for sophistication, some of them are quite impressive in their complexity.

How It Works

The Reproductive System

The contrast between sexual and asexual reproduction is examined in Reproduction, an essay that also provides examples of plant reproduction through pollination. The present essay is concerned primarily with human sexual reproduction and secondarily with animal sexual reproduction. Some technical aspects of reproduction at the cellular level require consultation of processes explained in Genetics; here we confine our technical discussion to reproduction at the level of organs, fluids, and other bodily components. Reproduction is facilitated by the reproductive system, a group of organized structures that can be subdivided into male and female reproductive systems. During puberty, which typically occurs between the ages of 10 and 14 years, the reproductive systems of both sexes mature. This phase is marked in part by the release of eggs (female sex cells) in the female ovary and the formation of sperm (male sex cells) in the male testes. Reproduction can take place only when a sperm unites with an egg, a process called fertilization.

The Male Reproductive System

The testes are the pair of male reproductive glands located in the scrotum, a skin-covered sac that hangs from the groin. Each testis produces sperm cells, while the testes as a whole secrete testosterone. Testosterone is a hormone—a type of molecule that sends signals to spots remote from its point of origin to induce specific effects on the activities of other cells. Testosterone is associated with masculinity, though females secrete it in much smaller quantities as well. In males, testosterone secretion is critical to the development of secondary sexual characteristics—those unique traits that mark a person as a male or female, though they do not occur in the sexual organs themselves. A deepened voice is an example of a male secondary sex characteristic evident at puberty.

Sperm cells produced in the testes move to the epididymis, a coiled tube at the base of the penis where they are stored and matured. During ejaculation, or the ejection of sperm from the penis during orgasm, sperm travel from the epididymis through a long tube called the vas deferens to the urethra. This single tube, which extends from the bladder to the tip of the penis, is also the means by which urine passes out of the body. Liquid secretions from various glands combine with sperm (itself a gooey substance that is barely liquid) to form the semen, or seminal fluid. Ejaculated semen may contain as many as 400 million sperm.

The Female Reproductive System

The female system is much more complicated than the male version and has a role in all stages of reproduction. Whereas the male system primarily delivers semen to the vagina, the female system plays a critical part from fertilization until long after the birth of offspring. It produces ova, or eggs, receives sperm from the penis, houses and provides nutrients to the developing zygote (fertilized egg) and later the embryo and fetus, gives birth to offspring, and feeds those offspring after birth.

The visible part of the female reproductive system, which, of course, is not even half of the entire picture, includes the opening of the vagina and the external genital organs, or vulva. The vagina, a muscular tube extending from the uterus to the outside of the body, is the receptacle for sperm ejaculated during sexual intercourse and also forms part of the birth canal that will be used later, when the offspring comes to term. The external genital organs, known collectively as the vulva, include the labia, folds of skin on both sides of the openings to the vagina and urethra; the clitoris, a small, sensitive organ that is comparable to the male penis inasmuch as it swells when stimulated; and the mons pubis, a mound of fatty tissue above the clitoris.

The Ovaries and Menstruation

Eggs are produced in the ovaries, oval-shaped organs in the groin that also generate sex hormones. At birth, a female's ovaries contain hundreds of thousands of undeveloped eggs, each surrounded by a group of cells to form a follicle, or sac; however, only about 360-480 follicles reach full maturity. During puberty the action of hormones causes several follicles to develop each month. Normally, just one follicle fully matures, rupturing and releasing an ovum through the ovary wall in a process called ovulation. The mature egg enters one of the paired fallopian tubes, where it may be fertilized by a sperm and move on to the uterus to develop into a fetus. The lining of the uterus, called the endometrium, prepares for pregnancy each month by thickening, but if fertilization does not take place, the endometrium is shed during menstruation.

Fertilization

During sexual intercourse, a man releases approximately 300 million sperm into a woman's vagina, but only one of the sperm can fertilize the ovum. The successful sperm cell must enter the uterus, swim up the fallopian tube (a trumpet-shaped passageway between the ovary and the uterus) to meet the ovum, and then pass through a thick coating, known as the zona pellucida, that surrounds the egg. The head of the sperm cell contains enzymes (a type of protein that speeds up chemical reactions—see Enzymes) that break through the zona pellucida and allow the sperm to penetrate the egg. Once the head of the sperm is inside the egg, the tail falls off, and the outside of the egg thickens to prevent another sperm from entering. Many variables affect whether fertilization occurs after intercourse among humans. One factor is a woman's ovulatory, or menstrual, cycle. Human eggs can be fertilized for only a few days after ovulation, which typically occurs only once every 28 days. (To learn about what happens after fertilization, see Pregnancy and Birth.)

Real-Life Applications

External Fertilization

Most land animals use some form of internal fertilization similar to that which we have described for humans. External fertilization, on the other hand, is more common among aquatic animals, who simply dump their sperm and eggs into the water and let currents mix the two male and female cells together. The sea urchin is a typical example: a male sea urchin releases several billion sperm into the water, and these sperm then swim toward eggs released in the same area. Fertilization occurs within seconds when sperm come into contact and fuse with eggs. As noted in Reproduction, external fertilization is essentially sexual reproduction without sexual intercourse. For humans the process of reproduction by external means may lack the intimacy of internal reproduction, but since 1978 a form of external fertilization has offered the opportunity of conceiving children to couples who otherwise might have remained childless.

In Vitro Fertilization

This form of external fertilization is known as in vitro, or "in glass"—that is, in a glass test tube or petri dish. The term is contrasted with in vivo, meaning "in a living organism," or in utero, "in the uterus." During in vitro fertilization, eggs are removed surgically from a female's reproductive tract. They then can be fertilized in a test tube or petri dish by sperm taken from the woman's partner or another male. After the fertilized eggs have divided twice, indicating that the operation has "taken" and an embryo is starting to form, they are reintroduced to the female's body. If all goes well, the embryo and fetus eventually result in a normal birth.

In vitro fertilization has been performed successfully on a variety of domestic animals since the 1950s but on humans only since the late 1970s. Two English physicians, Patrick Steptoe (1913-1988) and Robert G. Edwards (1925-), developed a method for stimulating ovulation with hormone treatment and then retrieving the nearly mature eggs and placing them in a petri dish to mature. In their breakthrough 1978 operation, they added male sperm to the egg in the petri dish, where fertilization took place, and then implanted the eight-celled embryo in the mother's body. The result of this extraordinary operation was a healthy baby named Louise Brown.

A World Without Sex?

The birth of Louise, whose twentieth birthday was celebrated in Britain with great fanfare, excited fears and controversy similar to those surrounding cloning (see Genetic Engineering). As with cloning, the fear was that the technology of in vitro fertilization would lead to the depersonalized manufacturing of human beings associated with some nightmarish future society, but this has not come to pass for several reasons. One is that in vitro fertilization is successful only about 15% of the time. Another, much more significant reason is that there are few women capable of producing a child through internal fertilization who would want to conceive without the intimacy of sexual intercourse.

There are exceptions, of course, in real life (two women in a same-sex relationship who want a child, for example) as well as in fiction. In the latter category, the character of Jenny Fields in the American novelist John Irving's World According to Garp—published, ironically, in the same year as the birth of Louise Brown—would undoubtedly have conceived a child by in vitro means if those means had been available to her. As it was, Jenny, who conceived the book's title character during World War II, manages to do so by having intercourse purely for the purpose of producing offspring. She does this by choosing a wounded, brain-damaged airman who will never remember having had sex with her and who dies shortly thereafter.

Courtship

There is something far greater than mere biochemistry involved in sexual reproduction, which is why humans enjoy the intimacy of producing a baby. This is also why courtship and mating rituals are a significant part of human life, providing a means by which a male and female join as partners. Such is true even in modern America and the West in general, where the aftermath of the 1960s sexual revolution has left behind a world largely stripped of its former mystery. For better or worse, sex before marriage is a common part of life today in a way that it was not before the 1960s, with the advent of birth control and "free love," and today there is little stigma attached to the conception of a child outside wedlock. That much has changed—and changed dramatically—but humans still practice courtship.

Males still have to prove to females that they are suitable mates, usually by displaying their physical prowess or some other attribute associated with masculinity. Females still do most of the choosing, and despite all the changes in views toward male and female roles, the ideas of basic male and female differences seem to be hardwired into the minds of most people. This is particularly so of people who are heterosexual and especially those who either have children or intend to have them. Such attitudes are not surprising, since humans, while being something more than animals, are still animals as well.

Animal Courtship

Among animals, courtship is a complex set of behaviors that leads to mating. Courtship behavior communicates to each potential mate that the other is not a threat and serves to reveal to each that the species, gender, and physical condition of the other are suitable for mating. During courtship, animals use rituals, a series of behaviors for communication that is performed the same way by all the males or females in a species. They are governed by fixed-action patterns (FAPs), which are virtually identical with instinct (see Instinct and Learning). In courtship, some animals leap and dance, others sing, and still others ruffle their feathers or puff up pouches. The male peacock displays his glorious plumage to the female, and humpback whales advertise their presence under the sea by singing a song that can be heard hundreds of miles away. Courtship behavior enables an animal to find, identify, attract, and arouse a mate. Animals use signals, such as the release of pheromones, or scent signals, as well as visual displays to claim a particular mate or a territory.

Choosing a Mate

Usually, the females do the choosing. In some species of birds, males display themselves in a small communal area called a lek, where females select a mate from the parading males. Across the animal kingdom, males generally compete with one another for mates, either by fighting or by ritualized displays, and females pick the best quality of male available. Several basic factors influence afemale in her choice of mate. First, she wants amale who can provide for her offspring, which iswhy the female tern (a type of bird) selects a good fish catcher. As part of courtship amongcommon terns, the male birds display fish to thefemales and may even feed them to the females as a way of demonstrating their ability to feed young. Among the long-jawed long-horned beetles that live in the Arizona desert, males battle each other for saguaro cactus fruit, and the females mate in exchange for access to the fruit.

Genetic fitness, or the ability to survive—and to advance the survival of the species—is another important factor in mate selection. This is a large part of the reason why males may fight each other for a female—to show her that they are the most fit. Of course, the female animal does not know that she is choosing on this basis, but she is, and she is likely to select a partner with a striking appearance, capable of energetic displays—both of which are signs of good health.

Secondary Sex Characteristics, Evolution, and the Model of an Ideal Mate

Humans, too, have deeply ingrained ideas regarding what makes an attractive potential mate, though to some extent, those ideas are cultural. Most societies regard a muscular male as attractive, for obvious evolutionary reasons: a physically fit male can provide for his mate and offspring, both by acquiring food and other resources and by protecting the nest. On the other hand, the modern American ideal of feminine beauty is more removed from nature. For one thing, this image of an attractive female is a thin body but large breasts—two things that seldom go together in nature but which are possible in the modern world through breast implantation surgery. That is, the achievement of such an ideal is available to a woman with a naturally thin body who also possesses the financial resources (and, of course, the desire) to undergo such surgery.

A woman naturally gifted with large breasts is apt also to have large buttocks, hips, and thighs, and while some men may find such anatomical features (particularly the first) desirable, this more natural version of the female body is not the image usually promoted in advertising or other media. Therefore, if a woman with a full figure wishes to meet societal expectations regarding beauty, she must endure something much more strenuous than a mere operation—a strict low-fat diet and a great deal of exercise. Given these often unnatural expectations from society, it is no wonder that a great many women express frustration with their attempts to conform to them, nor is it any wonder that many women in the modern world simply stop trying to conform. In any case, the American ideal of a thin female is far from universal, in terms of either place or time. Many traditional cultures favor a more amply formed female body, as did Western civilization in the past—a fact exemplified by the paintings of seventeenth-century artists, such as Peter Paul Rubens, famous for his fleshy female subjects. This is an ideal much closer to nature, since our evolutionary lineage has tended to favor women with large hips who are capable of bearing many children.

Evolution and the Sensitive Man

As with much else about sexual reproduction and the courtship rituals associated with it, the idea of beauty—itself an expression of secondary sex characteristics—is much simpler for men. As we noted earlier, physical fitness in men almost always is regarded as attractive, but women often value men for other traits, most notably intelligence and sense of humor. Whereas many men place an emphasis on physical appearance in choosing a mate, women's expectations are much more complex and they are also likely to be much more forgiving toward members of the male population who do not look like Tom Cruise or some other Hollywood image of attractiveness.

In defense of modern men, many have complained that modern women do not always want what they say they want. For example, from the early 1970s onward, it often was said in public discussions of sexuality on TV or in magazines that women wanted men to be more sensitive. That is, they wanted men who were more verbal and given to talking about their feelings and who were more aware of the woman and her needs. It is not surprising that men were failing to meet these expectations, since the idea of the "strong, silent type" is a masculine ideal in many cultures. Furthermore, not only civilization but also evolution has tended to favor men who are given more to actions than to words—men who can make war, either on the battlefield or in the business world.

Such aggressive characteristics are all well and good among animals or in a human society just struggling to survive. But in the modern West, where most material needs are easily met, women have sought and expressed a desire for something more from men. Such was the situation that emerged in the wake of the sexual revolution, when people became more open not only about sex itself but about their feelings as well. Although the sexual revolution yielded a number of negative effects, including the loss of mystery associated with sex and an increase in out-of-wedlock pregnancies and cases of sexually transmitted disease, it also made it possible for people to be frank in a way that they had never been before. One outgrowth of this was the revelation, in many women's magazines, that women were no longer satisfied or impressed with strong, silent men.

During the 1970s the model of the sensitive man emerged, and for the first time it became possible for men to talk about their feelings. And they did, in books, in movies such as those of Woody Allen, and in therapist-led encounter groups or men's groups. Gradually, however, a surprising fact emerged: women did not want men to be too sensitive and certainly not too nonmasculine. They wanted a man who would talk about his feelings but not a man who wore his heart on his sleeve. They wanted to be treated as equals in the workplace, but if there was a strange noise in the house at night, they wanted the man to go see what it was. They wanted a man who would respect them—but not a man who was so polite and predictable that he possessed no air of mystery.

All of this reveals a great deal about humans and sexual reproduction. First of all, mating, or the interaction between males and females, is about far more than reproduction or even sex. It is an interaction that involves the whole person. Second, human sexuality is surrounded by webs of psychological, social, and spiritual complexity that make it a phenomenon quite different from animal sexuality, which tends to be about little more than procreation. And, third, for all the progress humans have made, and for all the levels of civilization that separate us from our evolutionary roots, there is still something in human sexuality that responds to very basic ideas about sex, the sexes, and the need to propagate the species through mating and reproduction.

Where to Learn More

Avraham, Regina. The Reproductive System. Philadelphia: Chelsea House Publishers, 2001.

Cool Nurse (Web site). <http://www.coolnurse.com/>.

Francoeur, Robert T. Taking Sides: Clashing Views on Controversial Issues in Human Sexuality. Guilford, CT: Dushkin Publishing Group/Brown and Benchmark, 1996.

"How Human Reproduction Works." How Stuff Works (Web site). <http://www.howstuffworks.com/human-reproduction.htm>.

Kimball, Jim. "Sexual Reproduction in Humans." Kim ball's Biology Pages (Web site). <http://www.ultranet.com/~jkimball/BiologyPages/S/Sexual_Reproduction.html>.

Parker, Steve. The Reproductive System. Austin, TX: Raintree Steck-Vaughn, 1997.

"Puberty Guide in Adolescents." Keep Kids Healthy (Website). <http://www.keepkidshealthy.com/adolescent/puberty.html>.

"Puberty Information for Boys and Girls." American Academy of Pediatrics (Web site). <http://www.aap.org/family/puberty.htm>.

Whitfield, Philip. The Human Body Explained: A Guide to Understanding the Incredible Living Machine. New York: Henry Holt, 1995.

Winikoff, Beverly, and Suzanne Wymelenberg. The Whole Truth About Contraception: A Guide to Safe and Effective Choices. Washington, DC: Joseph Henry Press, 1997.

The production of a new living thing by two parent organisms, with each parent contributing half the material in the DNA of the offspring. The young, genetically different from either parent, can rapidly adapt to their environment by means of natural selection. (See chromosomes and meiosis.)

Sexual reproduction is characterized by processes that pass a combination of genetic material to offspring, resulting in diversity. The main two processes are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually cross over to achieve homologous recombination.

The evolution of sexual reproduction is a major puzzle. The first fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1 to 1.2 billion years ago.^[1] Sexual reproduction is the primary method of reproduction for the vast majority of macroscopic organisms, including almost all animals and plants. Bacterial conjugation, the transfer of DNA between two bacteria, is often mistakenly confused with sexual reproduction, because the mechanics are similar.

A major question is why sexual reproduction persists when parthenogenesis appears in some ways to be a superior form of reproduction. Contemporary evolutionary thought proposes some explanations. It may be due to selection pressure on the clade itself—the ability for a population to radiate more rapidly in response to a changing environment through sexual recombination than parthenogenesis allows. Alternatively, sexual reproduction may allow for the "ratcheting" of evolutionary speed as one clade competes with another for a limited resource.

In the first stage of sexual reproduction, "meiosis," the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During "fertilization," haploid gametes come together to form a diploid zygote and the original number of chromosomes (2n) is restored.

Contents 1 Plants 1.1 Flowering plants 1.2 Ferns 1.3 Bryophytes 2 Fungi 3 Insects 4 Mammals 4.1 Male 4.2 Female 4.3 Gestation 4.4 Birth 4.5 Monotremes 4.6 Marsupials 5 Fish 6 See also 7 Notes 8 References

Plants

Main article: Plant reproduction

Flowering plants

Flowers are the sexual organs of flowering plants.

Flowering plants are the dominant plant form on land and they reproduce by sexual and asexual means. Often their most distinguishing feature is their reproductive organs, commonly called flowers. The anther produces male gametophytes, the sperm is produced in pollen grains, which attach to the stigma on top of a carpel, in which the female gametophytes (inside ovules) are located. After the pollen tube grows through the carpel's style, the sex cell nuclei from the pollen grain migrate into the ovule to fertilize the egg cell and endosperm nuclei within the female gametophyte in a process termed double fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus two female cells) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The ovary, which produced the female gametophyte(s), then grows into a fruit, which surrounds the seed(s). Plants may either self-pollinate or cross-pollinate. Nonflowering plants like ferns, moss and liverworts use other means of sexual reproduction.

Ferns

Ferns typically produce large diploid sporophytes with rhizomes, roots and leaves; and on fertile leaves called sporangium, spores are produced. The spores are released and germinate to produce short, thin gametophytes that are typically heart shaped, small and green in color. The gametophytes or thallus, produce both motile sperm in the antheridia and egg cells in separate archegonia. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produce on the top side of the thallus, and swim in the film of water to the antheridia where they fertilize the egg. To promote out crossing or cross fertilization the sperm are released before the eggs are receptive of the sperm, making it more likely that the sperm will fertilize the eggs of different thallus. A zygote is formed after fertilization, which grows into a new sporophytic plant. The condition of having separate sporephyte and gametophyte plants is call alternation of generations. Other plants with similar reproductive means include the Psilotum, Lycopodium, Selaginella and Equisetum.

Bryophytes

The bryophytes, which include liverworts, hornworts and mosses, reproduce both sexually and vegetatively. They are small plants found growing in moist locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction. These plants start as a haploid spore that grows into the dominate form, which is a multicellular haploid body with leaf-like structures that photosynthesize. Haploid gametes are produced in antherida and archegonia by mitosis. The sperm released from the antherida respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells thus producing a zygote. The zygote divides by mitotic division and grows into a sporophyte that is diploid. The multicellular diploid sporophyte produces structures called spore capsules, which are connected by seta to the archegonia. The spore capsules produce spores by meiosis, when ripe the capsules burst open and the spores are released. Bryophytes show considerable variation in their breeding structures and the above is a basic outline. Also in some species each plant is one sex while other species produce both sexes on the same plant.^[2]

Fungi

Fungi are classified by the methods of sexual reproduction they employ. The outcome of sexual reproduction most often is the production of resting spores that are used to survive inclement times and to spread. There are typically three phases in the sexual reproduction of fungi: plasmogamy, karyogamy and meiosis.

Insects

Insects mating on a liatris flower head.

Insect species make-up more than two-thirds of all extant animal species, and most insect species use sex for reproduction, though some species are facultatively parthenogenetic. Many species have sexual dimorphism, while in others the sexes look nearly identical. Typically they have two sexes with males producing spermatozoa and females ovum, the ova develop into eggs that have a covering called the chorion, which forms before internal fertilization. Insects have very diverse mating and reproductive strategies most often resulting in the male depositing spermatophore within the female, which stores the sperm until she is ready for egg fertilization. After fertilization, and the formation of a zygote, and varying degrees of development; the eggs are deposited outside the female in many species, or in some, they develop further within the female and live born offspring are produced.

Mammals

There are three extant kinds of mammals: Monotremes, Placentals and Marsupials, all with internal fertilisation. In placental mammals, offspring are born as juveniles: complete animals with the sex organs present although not reproductively functional. After several months or years, the sex organs develop further to maturity and the animal becomes sexually mature. Most female mammals are only fertile during certain periods during their estrous cycle, at which point they are ready to mate. Individual male and female mammals meet and carry out copulation. For most mammals, males and females exchange sexual partners throughout their adult lives.

Male

For more details on this topic, see Male reproductive system (human).

The male reproductive system contains two main divisions: the penis, and the testicles, the latter of which is where sperm are produced. 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, in dogs, the penis is internal except when mating). Having the testicles outside the abdomen best facilitates temperature regulation of the sperm, which require specific temperatures to survive. 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. Prior to ejaculation the produced sperm are stored in the seminal vesicle, a small gland that is located just behind the bladder.
A sperm cell is motile and swims via chemotaxis, using its flagellum to propel itself towards the ovum.

Female

For more details on this topic, see Female reproductive system (human).

The female reproductive system likewise contains two main divisions: the vagina and uterus, which act as the receptacle for the sperm, and the ovaries, which produce the female's ova. All of these parts are always internal. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the Fallopian tubes. At certain intervals, the ovaries release an ovum, which passes through the fallopian tube into the uterus.

If, in this transit, it meets with sperm, the sperm penetrate and merge with the egg, fertilizing it. 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 with in the ovaries of the fetus before its birth. They are mostly fixed in location with in 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 Fallopian tube. If not fertilized, this egg is flushed out of the system through menstruation in humans and other great apes and reabsorbed in other mammals in the estrus cycle.

Gestation

Main articles: Pregnancy (mammals) and Pregnancy

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: Childbirth

Once the fetus is sufficiently developed, chemical signals start the process of birth, which begins with contractions of the uterus and the 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. Most mammals eat this, as it is a good source of protein and other vital nutrients needed for caring for the young. The end of the umbilical cord attached to the young’s abdomen eventually falls off on its own.

Monotremes

Monotremes, only five species of which exist, all from Australia and New Guinea, 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. After less than two weeks the young hatches and crawls into its mother’s pouch, much like marsupials, where it nurses for several weeks as it grows.

Marsupials

Marsupials reproductive systems differ markedly from those of placental mammals. Females have two vaginas, both of which open externally through one orifice but lead to different compartments within the uterus. Males generally have a two-pronged penis, which corresponds to the females' two vaginae^[3]. The penis is used only for discharging semen into females, and is separate from the urinary tract.^{[citation needed]} Both sexes possess a cloaca^[3], which is connected to a urogenital sac used to store waste before expulsion.

The female develops a kind of yolk sack in her womb which delivers nutrients to the embryo. Embryos of bandicoots, koalas and wombats additionally form placenta-like organs that connect them to the uterine wall, although the placenta-like organs are smaller than in placental mammals and it is not certain that they transfer nutrients from the mother to the embryo.^[4]

Pregnancy is very short, typically 4 to 5 weeks. The embryo is born at a very young stage of development, and is usually less than 5 cm (2.0 in) long at birth. It has been suggested that the short pregnancy is necessary to reduce the risk that the mother's immune system will attack the embryo.

The newborn marsupial uses its forelimbs (with relatively strong hands) to climb to a nipple, which is usually in a pouch on the mother's belly. The mother feeds the baby by contracting muscles over her mammary glands, as the baby is too weak to suck. The newborn marsupial's need to use its forelimbs in climbing to the nipple has prevented the forelimbs from evolving into paddles or wings and has therefore prevented the appearance of aquatic or truly flying marsupials (although there are several marsupial gliders).

Fish

The vast majority of fish species lay eggs that are then fertilized by the male,^[5] some species lay their eggs on a substrate like a rock or on plants, while others scatter their eggs and the eggs are fertilized as they drift or sink in the water column. Some fish species use internal fertilization and then disperse the developing eggs or give birth to live offspring. Fish that have live-bearing offspring include the Guppy and Mollies or Poecilia. Fishes that give birth to live young can be ovoviviparous, where the eggs are fertilized within the female and the eggs simply hatch within the female body, or they can be viviparous, where the female supplies nourishment to the internally growing offspring. Some fish are hermaphrodites, where a single fish is both male and female and can produce eggs and sperm. In hermaphroditic fish, some are male and female at the same time while in other fish they are serially hermaphroditic; starting as one sex and changing to the other. In at least one hermaphroditic species, self-fertilization occurs when the eggs and sperm are released together. Internal self-fertilization may occur in some other species.^[6] One fish species does not reproduce by sexual reproduction but uses sex to produce offspring; Poecilia formosa is a unisex species that uses a form of parthenogenesis called gynogenesis, where unfertilized eggs develop into embryos that produce female offspring. Poecilia formosa mate with males of other fish species that use internal fertilization, the sperm does not fertilize the eggs but stimulates the growth of the eggs which develops into embryos.^[7]

See also

• Asexual reproduction
• Biological reproduction
• Mating in fungi
• Operational sex ratio
• Reptile
• Sex organ
• Sexual intercourse
• Anisogamy
• Isogamy
• Evolution of sexual reproduction

Notes

1. ^ Orgel, Leslie E. "The Origin of Life on Earth". Scientific American. http://www.eddieting.com/eng/originoflife/orgel.html. Retrieved 2009-09-25. 
2. ^ Lovett Doust, Jon, and Lesley Lovett Doust. 1988. Plant reproductive ecology: patterns and strategies. New York: Oxford University Press. P 290.
3. ^ ^{a b} Iowa State University Biology Dept. Discoveries about Marsupial Reproduction Anna King 2001. webpage (note shows code, html extension omitted)
4. ^ "Family Peramelidae (bandicoots and echymiperas)". http://animaldiversity.ummz.umich.edu/site/accounts/information/Peramelidae.html. 
5. ^ BONY FISHES - Reproduction
6. ^ Orlando EF, Katsu Y, Miyagawa S, Iguchi T. J Mol Endocrinol. 2006 Oct;37(2):353-65. Cloning and differential expression of estrogen receptor and aromatase genes in the self-fertilizing hermaphrodite and male mangrove rivulus, Kryptolebias marmoratus. Department of Biological Sciences, Florida Atlantic University, North, Ft. Pierce, Florida 34946, USA. eorlando@fau.edu
7. ^ I. Schlupp, J. Parzefall, J. T. Epplen, M. Schartl Limia vittata as host species for the Amazon molly: no evidence for sexual reproduction Journal of Fish Biology Volume 48 Issue 4 Page 792-795, April 1996 (1996) Journal of Fish Biology 48 (4), 792–795. doi:10.1111/j.1095-8649.1996.tb01472.x

References

1. Pang, K. "Certificate Biology: New Mastering Basic Concepts", Hong Kong, 2004
2. Journal of Biology of Reproduction, accessed in August 2005.

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)