Sexual Reproduction in Animals

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Sexual Reproduction in Animals
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The animal kingdom includes a wide variety of
organisms with different body forms and ways of
living
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Same yet Different

• 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.

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Reproductive cycle
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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 zygotes single
  cell to divide by mitosis into two cells then
  four then eight then so on.

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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.

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Mating Patterns

• Matingthe process by which two members of a
  population come together to combine their gametes
  for fertilization.
• Populationa group of individuals of the same
  species that live and reproduce in the same area.
• There is only one mating season each yearfor
  some animals.

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Mating cont

• It is timed so that the offspring will hatch or
  be born when environmental conditions are
  favourable for their growth and development.
• 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.
• Whereas the honeybee mates only once in a
  lifetime. The queen mates with several males
  (drones) on her single mating flight. The sperm
  she receives during this flight will fertilize
  all the eggs she produces for the rest of her
  life.

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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 eggs cell membrane
  supple so that a sperm can pierce it.
• Sperm can only swim in a wet environment.

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Fertilization cont

• There are two main patterns of fertilization
  external and internal.
• Externalthe sperm and egg meet outside the
  bodies of both parents. This pattern is common in
  water-dwelling animals such as fish.
• Internalfor most land animals the sperm travels
  from the males body into the females body to
  meet the egg
• Fertilization is only the beginning of animal
  reproduction. The resulting zygote must develop
  into an independent individual.

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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 however they can reproduce sexually
  by releasing their eggs and sperm directly into
  the water.
• This method relies on water currents to bring
  gametes together.

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Fertilization of the sea anemone
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EF cont

• The resulting zygotes develop into free swimming
  self-feeding larvae (an immature form of the
  organism).
• 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.
• Exa female fish lays a cluster of eggs the male
  then releases sperm directly onto the egg
  cluster. This is called spawning.

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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.

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Life cycle of a frog
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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 females 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 females ovary.

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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.

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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 males cloaca into the
  females cloaca to fertilize her gametes.
• Birds also lay eggs but the shells are hard
  rather than leathery.

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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.

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Incomplete metamorphosis

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

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Complete metamorphosis

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

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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.

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• 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 mothers pouch (the marsupium). There it
  latches onto a nipple to obtain milk for
  completion of its development.

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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 flatworms 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.