Sexual Reproduction in Animals

1. Sexual Reproduction in Animals

2. The animal kingdom includes a wide variety of organisms with different body forms and ways of living

3. Same, Difference • Despite the difference among animals, the fundamental sequence that allows them to reproduce sexually is the same. • Meiosis produces gametes. • A male gamete (sperm) combines with a female gamete (egg). • A zygote is produced and develops into an embryo. • The embryo develops through mitosis and cell division into a mature offspring.

4. Reproductive cycle

5. The Cycle • The first event in the cycle depends on meiosis taking place in the gonads of the parents. • The final event in the cycle depends on repeated cell division, which enables the zygote’s single cell to divide by mitosis into two cells, then four, then eight, then so on.

6. Successful reproduction • For sexual reproduction to be successful the following requirements must be met: • Both male and female gametes must arrive in the same place at the same time for fertilization. • The zygote must receive adequate food, moisture, warmth, and protection to develop • Different species meet these requirements through a wide variety of reproductive patterns.

7. Mating Patterns • Mating—the process by which two members of a population come together to combine their gametes for fertilization. • There is only one mating season each year—for some animals.

8. Mating cont… • Times so that the offspring will hatch or be born at the best time of the year. • Mating once a year is not the only pattern. • Grunions (a fish) mate when the tides are highest, around the time of the full moon or new moon. • Honeybees mate only once in a lifetime. But boy do they mate!!!

9. Fertilization patterns • Fertilization occurs only if a sperm meets an egg from the same species. • Both sperm and egg cells are very delicate and will die if they dry out, so a moist environment is an important requirement. • Moisture also keeps the egg’s cell membrane supple, so that a sperm can pierce it. • Sperm can only swim in a wet environment.

10. Fertilization cont… • There are two main patterns of fertilization: external and internal. • External—the sperm and egg meet outside the bodies of both parents. This pattern is common in water-dwelling animals such as fish. • Internal—for most land animals, the sperm travels from the male’s body into the female’s body to meet the egg • Fertilization is only the beginning of animal reproduction. The resulting zygote must develop into an independent individual.

11. External fertilization • Most water-dwelling animals reproduce through external fertilization, perhaps the simplest method of EF occurs in animals such as the sea anemone. • Adult anemones cannot move around to mate with each other • They can reproduce sexually by releasing their eggs and sperm directly into the water. • This method relies on water currents to bring gametes together.

12. Fertilization of the sea anemone

13. EF cont… • The resulting zygotes develop into free swimming, self-feeding larvae • They may travel some distance before settling down and developing into non-swimming adults. • Many free-swimming animals reproduce by a less random form of EF. Ex—a female fish lays a cluster of eggs, the male then releases sperm directly onto the egg cluster. This is called spawning.

14. Ef cont… • Frogs have a similar form of EF, however, during mating the male frog embraces the female. As the female releases her eggs, the male releases sperm over them. • The young that hatch from anemones, fish, or frogs have little resemblance to their parents. • All must develop through several stages before they become adult individuals capable of reproduction. • Adult anemones and fish normally spend their entire lives in the water, but most frogs spend at least some time on land.

15. Life cycle of a frog

16. Internal fertilization • Most land animals reproduce through internal fertilization. • To accomplish this, a specialized structure is often used by the male to transfer sperm directly into the female. • Snakes and turtles reproduce through IF. • Males transfer sperm into the female’s cloaca, a chamber where the reproductive, urinary, and digestive ducts leave the body. • The sperm travel up the reproductive duct to meet the egg from the female’s ovary.

17. IF cont… • Most reptiles lay eggs with tough, leathery shells. • Inside each shell, a fluid-filled sac surrounds and protects the zygote as it becomes an embryo. • The egg also contains a large supply of food for the developing embryo. • When development is complete, the young reptile makes its way out of the shell.

18. If cont… • Like reptiles, birds reproduce by IF, however, few bird species have a specialized structure for transferring sperm. • Both males and females have a cloaca and IF is accomplished through close body contact. • The sperm swim from the male’s cloaca into the female’s cloaca to fertilize her gametes. • Birds also lay eggs, but the shells are hard rather than leathery.

19. If cont… • Unlike most fish, amphibians, and reptiles; birds care for their young. They sit on the eggs to keep them warm and guard them from predators. • Both parents invest a great deal of energy into feeding the young. • Killdeers can run however they cannot feed themselves or fly, although they are miniature versions of their parents; whereas the American Robin is blind, featherless and needs to be completely cared for before they can leave the nest.

20. Incomplete metamorphosis • Insects such as grasshoppers and crickets pass through different stages of development; this process is called… • incomplete metamorphosis.

21. Complete metamorphosis • Houseflies and butterflies go through complete metamorphosis in which the adult form has little resemblance to earlier stages.

22. Mammals and marsupials • We are mammals and we fertilize our eggs internally. • Most female mammals do not lay eggs, rather they retain and nourish the embryo(es) within their bodies. • This ensures additional protection and development of the young, before birth. • Also, mammals produce milk for nourishment for their young.

23. Marsupials, such as the kangaroo and the opossum, give birth to offspring in a very early stage of development. • The young animal uses a hook-like claw to crawl to its mother’s pouch (ouch!) (the marsupium). There it latches onto a nipple to obtain milk for completion of its development.

24. hermaphrodites • Animals that have both female and male reproductive organs in each individual are hermaphrodites. • During mating each planarian injects sperm into a reproductive pore on the other flatworm’s body. Each planarian then lays fertlized eggs. • The common earthworm is a hermaphrodite. IF ensures that sperm have a moist environment in which to travel and increases the chances that all the eggs will be fertilized.