Animal Reproductive Cycle

4 Key excerpts on "Animal Reproductive Cycle"

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  eBook - ePub

  Reproductive Seasonality in Teleosts

  Environmental Influences

  • Angus D. Munro, Alexander P. Scott, T. J. Lam(Authors)
  • 2019(Publication Date)
  • CRC Press

    (Publisher)

  II. Reproductive Cycles

  In temperate and high latitudes, where most research is conducted, almost all animals and plants reproduce seasonally, and only a few species, many of which are domesticated or live in close association with man, breed throughout the year. In fact not only reproduction, but all other facets of life, including those linked to reproduction such as moulting, migration, food intake, growth, hibernation, and diapause, occur seasonally. The annual reproductive cycles of birds and mammals living in temperate and arctic climates are underpinned by pronounced changes in gonadotrophin secretion, leading to gonadal growth and greatly increased sex-steriod secretion. These changes can be very dramatic, especially in birds where, for example, the testes can increase in weight a hundredfold or more within a few weeks.1 By using appropriate experimental conditions, these cycles can be reproduced in the laboratory. Such studies have allowed researchers to unravel the physiological mechanisms underlying the cycles of certain species. These species, however, have been only those that are suitable for laboratory-based studies, and this has meant that only a few species have been intensively investigated, whereas nothing at all is known of the majority. Only about 20 species of birds, including sparrows, Japanese quail, starlings, and ducks and approximately the same number of species of mammals, including sheep, deer, mink, hamsters, and mice have been closely studied and even these to very different degrees. In both cases this represents less than 1% of the species within each class. Such a situation, which is even more extreme in the case of teleosts, which number over 20,000 species, means that it is difficult to make generalizations between species. Indeed, it can be difficult to generalize within a species. For example, northern populations of deer mice, Peromyscus maniculatus, have very discrete breeding seasons, whereas southern populations breed continuously2 (Figure 1 ). Not only is there considerable variation in the length of the breeding season in this species among populations living at different latitudes, but even at a single location there is also a great year-to-year variation in the length of the breeding season. The degree of variation both within and between locations is more pronounced in species of small animals than it is in larger ones, which is what is expected when energetics and life expectancies are considered.2

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  The Reproductive System

  • Britannica Educational Publishing, Kara Rogers(Authors)
  • 2010(Publication Date)
  • Britannica Educational Publishing

    (Publisher)

  CHAPTER 3 REPRODUCTION AND PREGNANCY

  T he process of reproduction begins with intercourse and concludes with the birth of live offspring at the end of pregnancy. However, there are many steps in between, including fertilization—the uniting of sperm and ovum (egg)—and implantation—the embedding of the fertilized ovum in the wall of the female uterus. Following implantation, a woman’s body undergoes a series of changes that are characteristic of pregnancy, including the development of the maternal placenta, the temporary tissue that nourishes the developing conceptus (the fertilized ovum that becomes the fetus). Prenatal care and testing is used to assess the health of the developing fetus during pregnancy, thereby helping to ensure successful reproduction.

  SEXUAL INTERCOURSE

  Sexual intercourse, also known as coitus (or copulation), is a reproductive act in which the male reproductive organ enters the female reproductive tract. If the reproductive act is complete, sperm cells are passed from the male body into the female, in the process fertilizing the female egg and forming a new organism. In some vertebrates, such as fish, eggs are laid outside of the body and fertilized externally.

  To accomplish internal copulation, certain body and organic adaptations are necessary. In the human male, the penis serves both excretory and reproductive functions. During intercourse, the blood flow is temporarily increased and trapped in the penis so that it becomes enlarged and elevated, a condition known as erection. Erection changes the normally soft and flaccid organ to one of greater size and rigidity to permit easier penetration into the reproductive tract of the female. Sexual intercourse both culminates and terminates in orgasm, a process in which the male expels semen—containing sperm cells, which may unite with and fertilize the female egg, and a seminal plasma that contains cell nutrients, water, salts, and metabolites—into the female’s vaginal canal.

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  Biology

  A Self-Teaching Guide

  • Steven D. Garber(Author)
  • 2020(Publication Date)
  • Jossey-Bass

    (Publisher)

  If a sperm cell reaches the egg and penetrates the egg's membrane, resulting in fertilization, the fertilized egg, or zygote, passes down the oviduct to the uterus, where development continues until the baby is born. Most other mammals – in fact, most other vertebrates – have sex during specific times. Unlike humans, some animals have mating seasons that occur just once a year. It is during these times that the animals become receptive or stimulated, and the appropriate nuptial behavioral patterns begin. Sexual displays of many types, those specific to each species, are elicited when males and females encounter one another. In contrast, instead of having sex only during specific times in a cycle, humans are receptive all year long. Menstrual Cycle Puberty is the point at which an individual is first capable of reproduction. Girls usually reach this stage when about 12 years old, and boys reach puberty about two years later. When a girl reaches puberty, she has her first menstrual period, and her periods recur every 28 days or so during the rest of her child-bearing years. It is the female monthly cycle that is known as the menstrual cycle. Blood and cells that had lined the uterus are expelled in the menstrual flow, which is a result of the breakdown of the uterine lining. The outer cells slough off in a readying process that allows the uterus to regenerate a new lining receptive to implantation of a fertilized egg, creating a proper environment where an embryo can develop to full term. During estrus, when the cells are sloughed off, old cells and accompanying blood drain from the uterus through the vagina, out of which the discharge flows. This bleeding is controlled and normal, and is referred to as a woman's period. In humans, the entire menstrual cycle of uterine buildup and breakdown, involving the release of one egg, takes 28 days. The term “menstrual cycle” originated from the root menses, which means months (mensis means month)

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  Principles Of Comparative Psychology

  • Nicky Hayes(Author)
  • 2014(Publication Date)
  • Psychology Press

    (Publisher)

  last chapter , we saw how genes become mixed up into different combinations through sexual reproduction: the larger the gene pool, the more likely it is that “aberrant” genes, which carry physical disorders, will be paired with “healthy” ones. In such cases, it is the “healthy” genes that are expressed in the phenotype, and the disorder doesn't develop. But if the species has only a limited range available in the gene pool, then the chances of two aberrant genes pairing together are much higher, so that, over generations, the individuals of the species are likely to become much less healthy as these disorders accumulate. This makes it important that, whatever reproductive strategy the species uses, it is open to genetic contributions from individuals who are not part of the immediate family group.

  These two principles—that enough of the new generation must survive to reproduce in their turn, and that the gene pool should be as wide as possible—have produced a huge diversity in reproductive strategies. In this chapter, we will be looking at some of the different aspects of reproductive behaviour that have been studied by comparative psychologists; but we have to remember that we are only touching on the very tip of the iceberg—that there are far more reproductive strategies taking place in the animal world than early comparative psychologists ever suspected.

  Courtship and mating

  Animal species take a number of different approaches to the question of mating. In some species, two animals will pair together for life—a process known as pair-bonding

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