1. Chapter 46
Animal Reproduction

2. Overview: Pairing Up for Sexual Reproduction
Each earthworm produces sperm and eggs; in a few weeks, new worms will hatch from fertilized eggs
Animal reproduction takes many forms
Aspects of animal form and function can be viewed broadly as adaptations contributing to reproductive success

3. Fig. 46-1
Figure 46.1 How can each of these earthworms be both male and female?

4. Concept 46.1: Both asexual and sexual reproduction occur in the animal kingdom
Sexual reproduction is the creation of an offspring by fusion of a male gamete (sperm) and female gamete (egg) to form a zygote
Asexual reproduction is creation of offspring without the fusion of egg and sperm

5. Mechanisms of Asexual Reproduction
Many invertebrates reproduce asexually by fission, separation of a parent into two or more individuals of about the same size
Video: Hydra Budding

6. Fig. 46-2
Figure 46.2 Asexual reproduction of a sea anemone (Anthopleura elegantissima)

7. In budding, new individuals arise from outgrowths of existing ones
Fragmentation is 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 is the development of a new individual from an unfertilized egg

8. Sexual Reproduction: An Evolutionary Enigma
Sexual females have half as many daughters as asexual females; this is the “twofold cost” of sexual reproduction
Despite this, almost all eukaryotic species reproduce sexually

9. Asexual reproduction Sexual reproduction Female Female Male
Fig. 46-3
Asexual reproduction
Sexual reproduction
Female
Generation 1
Female
Generation 2
Male
Generation 3
Figure 46.3 The “reproductive handicap” of sex
Generation 4

10. Sexual reproduction results in genetic recombination, which provides potential advantages:
An increase in variation in offspring, providing an increase in the reproductive success of parents in changing environments
An increase in the rate of adaptation
A shuffling of genes and the elimination of harmful genes from a population

11. Reproductive Cycles and Patterns
Ovulation is the release of mature eggs at the midpoint of a female cycle
Most animals exhibit reproductive cycles related to changing seasons
Reproductive cycles are controlled by hormones and environmental cues
Animals may reproduce asexually or sexually, or they may alternate these methods

12. Several genera of fishes, amphibians, and lizards reproduce only by a complex form of parthenogenesis that involves the doubling of chromosomes after meiosis
Asexual whiptail lizards are descended from a sexual species, and females still exhibit mating behaviors

13. Fig. 46-4
Ovary size
Ovulation
Ovulation
Estradiol
Figure 46.4 Sexual behavior in parthenogenetic lizards
Progesterone
Hormone level
Time
Behavior
Female
Male- like
Female
Male- like

14. Fig. 46-4a
Figure 46.4 Sexual behavior in parthenogenetic lizards

15. Ovary size Ovulation Ovulation Estradiol Progesterone Hormone level
Fig. 46-4b
Ovary size
Ovulation
Ovulation
Estradiol
Progesterone
Hormone level
Time
Figure 46.4 Sexual behavior in parthenogenetic lizards
Behavior
Female
Male- like
Female
Male- like

16. Sexual reproduction is a special problem for organisms that seldom encounter a mate
One solution is hermaphroditism, in which each individual has male and female reproductive systems
Some hermaphrodites can self-fertilize

17. Individuals of some species undergo sex reversals
Some species exhibit male to female reversal (for example, certain oysters), while others exhibit female to male reversal (for example, a coral reef fish)

18. Video: Hydra Releasing Sperm
Concept 46.2: Fertilization depends on mechanisms that bring together sperm and eggs of the same species
The mechanisms of fertilization, the union of egg and sperm, play an important part in sexual reproduction
In external fertilization, eggs shed by the female are fertilized by sperm in the external environment
Video: Hydra Releasing Sperm

19. Fig. 46-5
Figure 46.5 External fertilization
Eggs

20. In internal fertilization, sperm are deposited in or near the female reproductive tract, and fertilization occurs within the tract
Internal fertilization requires behavioral interactions and compatible copulatory organs
All fertilization requires critical timing, often mediated by environmental cues, pheromones, and/or courtship behavior

21. Ensuring the Survival of Offspring
All species produce more offspring than the environment can handle, and the proportion that survives is quite small
Species with external fertilization produce more gametes than species with internal fertilization

22. Species with internal fertilization provide greater protection of the embryos and more parental care
The embryos of some terrestrial animals develop in amniote eggs with protective layers
Some other animals retain the embryo, which develops inside the female
In many animals, parental care helps ensure survival of offspring

23. Fig. 46-6
Figure 46.6 Parental care in an invertebrate

24. Gamete Production and Delivery
To reproduce sexually, animals must have systems that produce gametes
In most species individuals have gonads, organs that produce gametes
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

25. Most insects have separate sexes with complex reproductive systems
In many insects, the female has a spermatheca in which sperm is stored during copulation

26. (a) Male honeybee (drone) (b) Female honeybee (queen)
Fig. 46-7
Accessory gland
Ovary
Ejaculatory duct
Testis
Oviduct
Spermatheca
Penis
Vas deferens
Vagina
Seminal vesicle
Accessory gland
Figure 46.7 Insect reproductive anatomy
(a) Male honeybee (drone)
(b) Female honeybee (queen)

27. Even animals with simple body plans can have complex reproductive systems, for example parasitic flatworms

28. Sperm duct (vas deferens) 2 Oviduct
Fig. 46-8
Genital pore
(Digestive tract)
Male organs:
Female organs:
Uterus Yolk gland 4
Seminal vesicle 3
Yolk duct 3
Sperm duct (vas deferens) 2
Oviduct 1
Ovary
Figure 46.8 Reproductive anatomy of a hermaphrodite 2
Vas efferens
Seminal receptacle 1
Testis
(Excretory pore)

29. A cloaca is a common opening between the external environment and the digestive, excretory, and reproductive systems
A cloaca is common in nonmammalian vertebrates; mammals usually have a separate opening to the digestive tract

30. Monogamy is relatively rare among animals
Males and/or females of some species have evolved mechanisms to decrease the chance of their mate mating with another individual

31. RESULTS 30 20 Percentage of females lacking sperm in spermatheca 10
Fig. 46-9
RESULTS 30 20
Percentage of females lacking sperm in spermatheca 10
Figure 46.9 Why is sperm usage biased when female fruit flies mate twice?
Control; not remated
Remated to wild-type males
Remated to “no-sperm” males
Remated to “no-ejaculate” males

32. Concept 46.3: Reproductive organs produce and transport gametes
The following section focuses on the human reproductive system

33. Female Reproductive Anatomy
The female external reproductive structures include the clitoris and two sets of labia
The internal organs are a pair of gonads and a system of ducts and chambers that carry gametes and house the embryo and fetus
Animation: Female Reproductive Anatomy

34. Fig
Oviduct
Ovary
Uterus
(Urinary bladder)
(Rectum)
(Pubic bone)
Cervix
Urethra
Vagina
Shaft
Glans
Clitoris
Prepuce
Labia minora
Labia majora
Vaginal opening
Oviduct
Ovaries
Figure Reproductive anatomy of the human female
Follicles
Corpus luteum
Uterine wall
Uterus
Endometrium
Cervix
Vagina

35. Oviduct Ovary Uterus (Urinary bladder) (Rectum) (Pubic bone) Cervix
Fig a
Oviduct
Ovary
Uterus
(Urinary bladder)
(Rectum)
(Pubic bone)
Cervix
Urethra
Vagina
Figure Reproductive anatomy of the human female
Shaft
Glans
Clitoris
Prepuce
Labia minora
Labia majora
Vaginal opening

36. Oviduct Ovaries Follicles Corpus luteum Uterine wall Uterus
Fig b
Oviduct
Ovaries
Follicles
Corpus luteum
Uterine wall
Uterus
Endometrium
Cervix
Figure Reproductive anatomy of the human female
Vagina

37. Ovaries The female gonads, the ovaries, lie in the abdominal cavity
Each ovary contains many follicles, which consist of a partially developed egg, called an oocyte, surrounded by support cells
Once a month, an oocyte develops into an ovum (egg) by the process of oogenesis

38. Ovulation expels an egg cell from the follicle
The remaining follicular tissue grows within the ovary, forming a mass called the corpus luteum
The corpus luteum secretes hormones that help to maintain pregnancy
If the egg is not fertilized, the corpus luteum degenerates

39. Oviducts and Uterus
The egg cell travels from the ovary to the uterus via an oviduct, or fallopian tube
Cilia in the oviduct convey the egg to the uterus, also called the womb
The uterus lining, the endometrium, has many blood vessels
The uterus narrows at the cervix, then opens into the vagina

40. Vagina and Vulva
The vagina is a thin-walled chamber that is the repository for sperm during copulation and serves as the birth canal
The vagina opens to the outside at the vulva, which consists of the labia majora, labia minora, hymen, and clitoris

41. The clitoris has a head called a glans covered by the prepuce
The vagina, labia minora, and clitoris are rich with blood vessels; the clitoris also has many nerve endings

42. Mammary Glands
The mammary glands are not part of the reproductive system but are important to mammalian reproduction
Within the glands, small sacs of epithelial tissue secrete milk

43. Male Reproductive Anatomy
The male’s external reproductive organs are the scrotum and penis
Internal organs are the gonads, which produce sperm and hormones, and accessory glands
Animation: Male Reproductive Anatomy

44. Figure 46.11 Reproductive anatomy of the human male
Seminal vesicle (behind bladder)
(Urinary bladder)
Prostate gland
Bulbourethral gland
Urethra
Erectile tissue of penis
Scrotum
Vas deferens
Epididymis
Testis
(Urinary bladder)
(Urinary duct)
Figure Reproductive anatomy of the human male
Seminal vesicle
(Rectum)
(Pubic bone)
Vas deferens
Erectile tissue
Ejaculatory duct
Prostate gland
Urethra
Bulbourethral gland
Penis
Glans
Vas deferens
Epididymis Testis Scrotum
Prepuce

45. Seminal vesicle (behind bladder)
Fig a
Seminal vesicle (behind bladder)
(Urinary bladder)
Prostate gland
Bulbourethral gland
Urethra
Erectile tissue of penis
Figure Reproductive anatomy of the human male
Scrotum
Vas deferens
Epididymis
Testis

46. Epididymis Testis Scrotum
Fig b
(Urinary bladder)
(Urinary duct)
Seminal vesicle
(Rectum)
(Pubic bone)
Vas deferens
Erectile tissue
Ejaculatory duct
Prostate gland
Urethra
Penis
Bulbourethral gland
Glans
Vas deferens
Figure Reproductive anatomy of the human male
Epididymis Testis Scrotum
Prepuce

47. Testes
The male gonads, or testes, consist of highly coiled tubes surrounded by connective tissue
Sperm form in these seminiferous tubules
Leydig cells produce hormones and are scattered between the tubules
Production of normal sperm cannot occur at the body temperatures of most mammals

48. The testes of many mammals are held outside the abdominal cavity in the scrotum, where the temperature is lower than in the abdominal cavity

49. Ducts
From the seminiferous tubules of a testis, sperm pass into the coiled tubules of the epididymis
During ejaculation, sperm are propelled through the muscular vas deferens and the ejaculatory duct, and then exit the penis through the urethra

50. Accessory Glands
Semen is composed of sperm plus secretions from three sets of accessory glands
The two seminal vesicles contribute about 60% of the total volume of semen
The prostate gland secretes its products directly into the urethra through several small ducts
The bulbourethral glands secrete a clear mucus before ejaculation that neutralizes acidic urine remaining in the urethra

51. Penis
The human penis is composed of three cylinders of spongy erectile tissue
During sexual arousal, the erectile tissue fills with blood from the arteries, causing an erection
The head of the penis has a thinner skin covering than the shaft, and is more sensitive to stimulation

52. Human Sexual Response Two reactions predominate in both sexes:
Vasocongestion, the filling of tissue with blood
Myotonia, increased muscle tension
The sexual response cycle has four phases: excitement, plateau, orgasm, and resolution
Excitement prepares the penis and vagina for coitus (sexual intercourse)

53. Direct stimulation of genitalia maintains the plateau phase and prepares the vagina for receipt of sperm
Orgasm is characterized by rhythmic contractions of reproductive structures
In males, semen is first released into the urethra and then ejaculated from the urethra
In females, the uterus and outer vagina contract

54. During the resolution phase, organs return to their normal state and muscles relax

55. Concept 46.4: The timing and pattern of meiosis in mammals differ for males and females
Gametogenesis, the production of gametes by meiosis, differs in females and males
Sperm are small and motile and are produced throughout the life of a sexually mature male
Spermatogenesis is production of mature sperm

56. Figure 46.12 Human gametogenesis
Fig a
Epididymis
Seminiferous tubule
Testis
Cross section of seminiferous tubule
Primordial germ cell in embryo
Mitotic divisions
Sertoli cell nucleus
Spermatogonial stem cell 2n
Mitotic divisions
Spermatogonium 2n
Mitotic divisions
Primary spermatocyte 2n
Meiosis I
Figure Human gametogenesis
For the Cell Biology Video Motion of Isolated Flagellum, go to Animation and Video Files.
For the Cell Biology Video Flagellum Movement in Swimming Sperm, go to Animation and Video Files.
Lumen of seminiferous tubule
Secondary spermatocyte n n
Meiosis II
Neck
Spermatids (at two stages of differentiation)
Early spermatid n n n n
Tail
Midpiece
Head
Plasma membrane
Differentiation (Sertoli cells provide nutrients)
Mitochondria
Sperm n n n n
Nucleus
Acrosome

57. Cross section of seminiferous tubule Secondary spermatocyte
Fig b
Epididymis
Seminiferous tubule
Sertoli cell nucleus
Spermatogonium
Testis
Primary spermatocyte
Cross section of seminiferous tubule
Secondary spermatocyte
Spermatids (two stages)
Figure Human gametogenesis
Lumen of seminiferous tubule
Sperm

58. Primordial germ cell in embryo
Fig c
Primordial germ cell in embryo
Mitotic divisions
Spermatogonial stem cell 2n
Mitotic divisions
Spermatogonium 2n
Mitotic divisions
Primary spermatocyte 2n
Meiosis I
Secondary spermatocyte n n
Meiosis II
Figure Human gametogenesis
Early spermatid n n n n
Differentiation (Sertoli cells provide nutrients)
Sperm n n n n

59. Figure 46.12 Human gametogenesis
Fig d
Neck
Tail
Midpiece
Head
Plasma membrane
Mitochondria
Nucleus
Acrosome
Figure Human gametogenesis

60. Eggs contain stored nutrients and are much larger
Oogenesis is development of mature oocytes (eggs) and can take many years

61. Figure 46.12 Human gametogenesis
Fig e
Ovary
Primary oocyte within follicle
In embryo
Growing follicle
Primordial germ cell
Mitotic divisions 2n
Oogonium
Mitotic divisions
Primary oocyte (present at birth), arrested in prophase of meiosis I 2n
Mature follicle
Ruptured follicle
Completion of meiosis I and onset of meiosis II
First polar body
Figure Human gametogenesis n n
Secondary oocyte, arrested at metaphase of meiosis II
Ovulated secondary oocyte
Ovulation, sperm entry
Completion of meiosis II
Second polar body
Corpus luteum n n
Fertilized egg
Degenerating corpus luteum

62. Primary oocyte within follicle
Fig f
Ovary
Ruptured follicle
Primary oocyte within follicle
Ovulated secondary oocyte
Growing follicle
Figure Human gametogenesis
Corpus luteum
Mature follicle
Degenerating corpus luteum

63. Figure 46.12 Human gametogenesis
Fig g
In embryo
Primordial germ cell
Mitotic divisions 2n
Oogonium
Mitotic divisions
Primary oocyte (present at birth), arrested in prophase of meiosis I 2n
Completion of meiosis I and onset of meiosis II
First polar body n n
Secondary oocyte, arrested at metaphase of meiosis II
Ovulation, sperm entry
Figure Human gametogenesis
Completion of meiosis II
Second polar body n
Fertilized egg n

64. Spermatogenesis differs from oogenesis:
In oogenesis, one egg forms from each cycle of meiosis; in spermatogenesis four sperm form from each cycle of meiosis
Oogenesis ceases later in life in females; spermatogenesis continues throughout the adult life of males
Oogenesis has long interruptions; spermatogenesis produces sperm from precursor cells in a continuous sequence