Based on Pough et. al, 1989. Vertebrate Zoology - 3rd Ed. MacMillan
Hickman et al., 1996. Integrated Principles of Zoology - 10th Ed. William C. Brown Publishers
There are two basic types of reproduction, asexual and sexual. Sexual reproduction is much more common among vertebrates; I don't actually know of any truly asexual reproduction in vertebrates (although a good biologist always leaves room for exceptions).
True asexual reproduction involves the direct reproduction of the organism without the use of specialized sexual organs. The offspring are exact copies of the original.
Sexual reproduction involves specialized reproductive cells or gametes. These cells are haploid, and are usually produced by meiosis. The male cell is called a sperm, it is smaller than the female cell (the egg) and it is motile (it can move). The egg is larger than the sperm, it is non-motile, and it carries nutrients to sustain the embryo until it can feed or absorb nutrition from the mother. The gametes are produced in the gonads or reproductive organs; the male organs are the testes and the female organs are called the ovaries. Either sex may have accessory sex organs such as the penis or the vagina; the accessory sex organs are usually involved in the mating process itself.
Sexual reproduction involves fertilization, the union of gametes to form a zygote. The zygote is a single cell, and it is genetically different from either of the parents due to recombination during meiosis, and due to the union of genetic material from two different organisms.
There are several patterns of sexual reproduction in vertebrates. Most familiar is biparental reproduction (sometimes called dioecious reproduction). Here there are two parents, one male and the other female. Dioecious reproduction leaves the greatest genetic variability among offspring (an advantage under changing conditions or in competition or when faced with disease), but the organisms must find mates.
A second pattern is parthenogenesis. Here a viable offspring is produced by the parent (a female) without mating or fertilization. Some people call this asexual reproduction, but it does not meet the strict definition used above since the sex organs are used to produce the egg. There are 2 forms of parthenogenesis:
A third pattern of sexual reproduction is hermaphroditism. This is the monecious pattern. The hermaphrodite posses both male and female organs, and, in some cases may fertilize itself. Other hermaphrodites mate. Some exhibit what is known as sequential hermaphroditism; here the organism starts out either male or female and later changes its sex.
Sexual reproduction has both advantages and disadvantages. Among the costs are: 1. the need to find a mate (hard to do for sessile organisms or organisms with low population densities; 2. the time and energy required to find a mate and to copulate (this is time that could be spent eating or acquiring resources); 3. the exposure to predation (sometimes from the mate!) that comes with mating; 4. exposure to disease from the mate; 5. when an animal mates, only 1/2 of its genes go into each of the offspring, so the genes are "diluted"; 6. because some proportion of the population is male, not all adult organisms are directly producing offspring, thus lowering the rate of population growth; and 7. adaptive combinations of genes are lost.
Benefits of sexual reproduction revolve around increased genetic diversity. This increased diversity is good when conditions change and a population needs new combinations of genes to do well under the new conditions; when predators or competitors are evolving rapidly and the new gene combinations may help the population "keep up", and when the population is threatened with disease. In the latter case, at least some members of a genetically diverse population will be able to resist the disease.
Overall, asexual reproduction seems to be favored when rapid population growth is needed, and sexual reproduction is favored under changing conditions. There are, in fact, populations of animals (mostly invertebrates) which modify their reproductive pattern is response to environmental conditions.
When sexual reproduction is favored, it begins with gametogenesis, the formation of gametes. Gametes are formed in the f gonads, which themselves arise from endoderm in the embryo. In mammals the gonads of the embryo are originally female, but in male embryos testosterone production causes masculinization and thus the conversion of ovaries into testes.
For most vertebrates, the next step in gametogenesis is meiosis. First, there is doubling of genetic material, then formation of bivalents as homologous chromosomes pair. This is the time when crossing-over occurs. Crossing-over puts genes from the father on the same chromosome with genes from the mother; these genes can be inherited together in the future. After crossing-over, the homologous chromosomes separate, and there are 2 subsequent cellular divisions. Each of these divisions produces 2 cells, so the end result is 4 cells. Thus, one cell with doubled genetic material gives rise to 4 cells, each with 1/2 of a normal body cell's complement of genetic material. Spermatogenesis is the formation of sperm; after meiosis the cells undergo the additional changes necessary to make the motile. Oogenesis is the formation of eggs; during mitosis in the ovaries the contents of the original cell are not divided equally. The divisions are unequal, with most of the protoplasm going into 1 cell, which will become the egg. The nuclear material is divided equally. At the end, there is 1 big cell and 3 small ones; the small ones are called polar bodies and are discarded.
In terms of actually producing the offspring, sexual reproduction has 3 main forms:
As you can see, "live birth" or viviparity, is not limited to mammals; it is found in reptiles, amphibians, and even sharks. There are species of sharks in which the young are nourished via a placenta. Note that all birds are oviparous (as are a few mammals). Among the vertebrates, reptiles show the greatest variability in producing young.
Many organisms show distinct breeding cycles. These are usually synchronized to particular seasons in an attempt to give the young the best chance at survival. Thus, young deer are born in spring after the cold of winter and at a time where there is abundant plant growth low to the ground. Breeding at a particular time of year implies mating at a particular time, and this helps to group parents only at mating time, and allows them to spread out at other times. Some organisms, such as box turtles, will mate whenever males and females encounter each other; but the females can store the male's sperm for several years. She will lay her eggs early in the summer regardless of when she mates. This system works well for box turtles, which are solitary and slow moving, an thus infrequently encounter potential mates.
In mammals, breeding is controlled by the estrous cycle. This cycle of hormones in the female's body means that there is only a limited period of female receptivity. Other mammals, however, are receptive to mating at other times. The menstrual cycle is limited to primates (the order of mammals to which humans belong). Among primates, there is a tendency to shed the uterine lining once each month or so. Some have suggested that this sloughing of the lining is an adaptation to reduce disease.
Read through all of Chapter 6. Most of the material is directly related to vertebrates. We will not go into great depth on the mammalian (and human) reproductive systems; instead we will take a more comparative approach as outlined here. You might want to take Human Physiology to learn more about reproduction in humans.
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