Animal Reproduction

• Earthworm

  Produces sperm and eggs; new worms hatch from fertilized eggs
• Animal reproduction


  • Takes many forms
  • Aspects of animal form and function can be viewed as adaptations contributing to reproductive success
• Sexual reproduction


  Creation of an offspring by fusion of a male gamete (sperm) and female gamete (egg) to form a zygote

  
• Asexual reproduction


  Creation of offspring without the fusion of egg and sperm; one parent clones offspring
• Asexual reproduction in invertebrates

  • Fission = separation of a parent into two or more individuals
  • Budding = new individuals arise from outgrowths of existing ones
  • Fragmentation = breaking of the body into pieces, some or all of which develop into adults
• Fragmentation


  Must be accompanied by regeneration = regrowth of lost body parts
• Parthenogenesis


  Development of a new individual from an unfertilized egg
• Sexual reproduction


  • Females have half as many daughters as asexual females (twofold cost)
  • Despite this, almost all eukaryotic species reproduce sexually
• Advantages of sexual reproduction

  • Increase in variation in offspring
  • Increase in the rate of adaptation
  • Shuffling of genes and elimination of harmful genes from a population
• Ovulation


  Release of mature eggs at the midpoint of a female cycle
• Reproductive cycles


  • Related to changing seasons
  • Controlled by hormones and environmental cues
  • Animals may reproduce asexually or sexually, or alternate these methods
• Hermaphroditism


  Each individual has both male and female reproductive systems

  
• Sex reversal

  • Some species exhibit male to female reversal, while others exhibit female to male reversal
• External fertilization


  Eggs shed by the female are fertilized by sperm in the external environment
• Internal fertilization


  Sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract
• Fertilization


  • Requires critical timing, often mediated by environmental cues, pheromones, and/or courtship behavior
• Offspring survival

  • Species with external fertilization produce more gametes than species with internal fertilization
  • Species with internal fertilization provide greater protection of the embryos and more parental care
• Amniote eggs


  Embryos of some terrestrial animals develop in eggs with protective layers
• Gonads


  Sex organs that produce gametes
• Reproductive systems

  • Some simple systems do not have gonads, but gametes form from undifferentiated tissue
  • The most complex systems contain many sets of accessory tubes and glands that carry, nourish, and protect gametes and developing embryos

  
• Cloaca


  Common opening between the external environment and the digestive, excretory, and reproductive systems, found in nonmammalian vertebrates
• Female external reproductive structures

  Clitoris and two sets of labia
• Female internal reproductive organs

  Pair of gonads (ovaries) and a system of ducts and chambers that carry gametes and house the embryo and fetus
• Ovaries


  Female gonads that contain follicles, each with a partially developed egg (oocyte) surrounded by support cells
• Oogenesis


  Process by which an oocyte develops into an ovum (egg)
• Ovulation


  Expels an egg cell from the follicle
• Corpus luteum


  Remaining follicular tissue that grows within the ovary, secreting hormones to help maintain pregnancy
• Oviducts (fallopian tubes)


  Carry the egg cell from the ovary to the uterus
• Uterus


  Also called the womb, where the embryo and fetus develop
• Vagina

  Thin-walled chamber that is the repository for sperm during copulation and serves as the birth canal
• Vulva

  External female genitalia, including the labia majora, labia minora, hymen, and clitoris
• Mammary glands

  Secrete milk, important for mammalian reproduction but not part of the reproductive system
• Male external reproductive organs

  Scrotum and penis
• Male internal reproductive organs

  Gonads (testes) that produce sperm and hormones, and accessory glands
• Testes


  Male gonads containing seminiferous tubules where sperm form, and Leydig cells that produce hormones
• Production of normal sperm cannot occur at the body temperatures of most mammals, so the testes are held outside the abdominal cavity in the scrotum, where the temperature is lower
• Male reproductive organs

  • Seminal vesicle
  • Prostate gland
  • Bulbourethral gland
  • Erectile tissue of penis
  • Urethra
  • Scrotum
  • Vas deferens
  • Epididymis
  • Testis
• Testes

  • Highly coiled tubes surrounded by connective tissue
  • Sperm form in seminiferous tubules
  • Leydig cells produce hormones and are scattered between the tubules
  • Held outside the abdominal cavity in the scrotum where the temperature is lower
• Sperm production

  1. From seminiferous tubules to epididymis

  2. Propelled through vas deferens and ejaculatory duct

  3. Exit penis through urethra

  
• Accessory glands

  • Seminal vesicles
  • Prostate gland
  • Bulbourethral glands