Sexual Reproduction in Sexual Reproduction in AnimalsAnimals

The animal kingdom includes a wide variety The animal kingdom includes a wide variety of organisms with different body forms and of organisms with different body forms and

ways of livingways of living

Same, DifferenceSame, Difference

Despite the difference among animals, Despite the difference among animals, the fundamental sequence that allows the fundamental sequence that allows them to reproduce sexually is the same.them to reproduce sexually is the same.

1.1. Meiosis produces gametes.Meiosis produces gametes.2.2. A male gamete (sperm) combines with a A male gamete (sperm) combines with a

female gamete (egg).female gamete (egg).3.3. A zygote is produced and develops into an A zygote is produced and develops into an

embryoembryo..4.4. The embryo develops through mitosis and The embryo develops through mitosis and

cell division into a mature offspring.cell division into a mature offspring.

Reproductive cycleReproductive cycle

The CycleThe Cycle

The first event in the cycle depends on The first event in the cycle depends on meiosis taking place in the gonads of the meiosis taking place in the gonads of the parents.parents.

The final event in the cycle depends on The final event in the cycle depends on repeated cell division, which enables the repeated cell division, which enables the zygote’s single cell to divide by mitosis into zygote’s single cell to divide by mitosis into two cells, then four, then eight, then so on.two cells, then four, then eight, then so on.

Successful reproductionSuccessful reproduction

For sexual reproduction to be successful the For sexual reproduction to be successful the following requirements must be met:following requirements must be met:

1.1. Both male and female gametes must arrive in the Both male and female gametes must arrive in the same place at the same time for fertilization.same place at the same time for fertilization.

2.2. The zygote must receive adequate food, moisture, The zygote must receive adequate food, moisture, warmth, and protection to developwarmth, and protection to develop

Different species meet these requirements Different species meet these requirements through a wide variety of reproductive patterns.through a wide variety of reproductive patterns.

Mating PatternsMating Patterns

Mating—the process by which two Mating—the process by which two members of a population come together to members of a population come together to combine their gametes for fertilization.combine their gametes for fertilization.

There is only one mating season each There is only one mating season each year—for some animals.year—for some animals.

Mating cont…Mating cont…

Times so that the offspring will hatch or be born Times so that the offspring will hatch or be born at the best time of the year.at the best time of the year.

Mating once a year is not the only pattern.Mating once a year is not the only pattern. Grunions (a fish) mate when the tides are highest, Grunions (a fish) mate when the tides are highest,

around the time of the full moon or new moon.around the time of the full moon or new moon.

Honeybees mate only once in a lifetime. But boy do Honeybees mate only once in a lifetime. But boy do they mate!!!they mate!!!

Fertilization patternsFertilization patterns

Fertilization occurs only if a sperm meets an egg Fertilization occurs only if a sperm meets an egg from the same species.from the same species.

Both sperm and egg cells are very delicate and Both sperm and egg cells are very delicate and will die if they dry out, so a moist environment is will die if they dry out, so a moist environment is an important requirement.an important requirement.

Moisture also keeps the egg’s cell membrane Moisture also keeps the egg’s cell membrane supple, so that a sperm can pierce it.supple, so that a sperm can pierce it.

Sperm can only swim in a wet environment.Sperm can only swim in a wet environment.

Fertilization cont…Fertilization cont… There are two main patterns of fertilization: external There are two main patterns of fertilization: external

and internal.and internal.

1.1. External—the sperm and egg meet outside the bodies of both External—the sperm and egg meet outside the bodies of both parents. This pattern is common in water-dwelling animals parents. This pattern is common in water-dwelling animals such as fish.such as fish.

2.2. Internal—for most land animals, the sperm travels from the Internal—for most land animals, the sperm travels from the male’s body into the female’s body to meet the eggmale’s body into the female’s body to meet the egg

Fertilization is only the beginning of animal Fertilization is only the beginning of animal reproduction. The resulting zygote must develop into reproduction. The resulting zygote must develop into an independent individual.an independent individual.

External fertilization External fertilization Most water-dwelling animals reproduce through external Most water-dwelling animals reproduce through external

fertilization, perhaps the simplest method of EF occurs in fertilization, perhaps the simplest method of EF occurs in animals such as the sea anemone.animals such as the sea anemone.

Adult anemones cannot move around to mate with each Adult anemones cannot move around to mate with each otherother

They can reproduce sexually by releasing their eggs and They can reproduce sexually by releasing their eggs and sperm directly into the water.sperm directly into the water.

This method relies on water currents to bring gametes This method relies on water currents to bring gametes together.together.

Fertilization of the sea anemoneFertilization of the sea anemone

EF cont…EF cont…

The resulting zygotes develop into free The resulting zygotes develop into free swimming, self-feeding larvaeswimming, self-feeding larvae

They may travel some distance before settling They may travel some distance before settling down and developing into non-swimming adults.down and developing into non-swimming adults.

Many free-swimming animals reproduce by a Many free-swimming animals reproduce by a less random form of EF.less random form of EF.

Ex—a female fish lays a cluster of eggs, Ex—a female fish lays a cluster of eggs, the the male then releases sperm directly onto the male then releases sperm directly onto the

egg cluster. This is called spawning.egg cluster. This is called spawning.

Ef cont…Ef cont… Frogs have a similar form of EF, however, during Frogs have a similar form of EF, however, during

mating the male frog embraces the female. As mating the male frog embraces the female. As the female releases her eggs, the male releases the female releases her eggs, the male releases sperm over them.sperm over them.

The young that hatch from anemones, fish, or The young that hatch from anemones, fish, or frogs have little resemblance to their parents.frogs have little resemblance to their parents.

All must develop through several stages before All must develop through several stages before they become adult individuals capable of they become adult individuals capable of reproduction. reproduction.

Adult anemones and fish normally spend their Adult anemones and fish normally spend their entire lives in the water, but most frogs spend at entire lives in the water, but most frogs spend at least some time on land.least some time on land.

Life cycle of a frogLife cycle of a frog

Internal fertilizationInternal fertilization Most land animals reproduce through internal Most land animals reproduce through internal

fertilization.fertilization. To accomplish this, a specialized structure is To accomplish this, a specialized structure is

often used by the male to transfer sperm directly often used by the male to transfer sperm directly into the female.into the female.

Snakes and turtles reproduce through IF.Snakes and turtles reproduce through IF. Males transfer sperm into the female’s Males transfer sperm into the female’s cloacacloaca, a , a

chamber where the reproductive, urinary, and chamber where the reproductive, urinary, and digestive ducts leave the body.digestive ducts leave the body.

The sperm travel up the reproductive duct to The sperm travel up the reproductive duct to meet the egg from the female’s ovary.meet the egg from the female’s ovary.

IF cont…IF cont… Most reptiles lay eggs with tough, leathery Most reptiles lay eggs with tough, leathery

shells.shells.

Inside each shell, a fluid-filled sac surrounds and Inside each shell, a fluid-filled sac surrounds and protects the zygote as it becomes an embryo.protects the zygote as it becomes an embryo.

The egg also contains a large supply of food for The egg also contains a large supply of food for the developing embryo.the developing embryo.

When development is complete, the young When development is complete, the young reptile makes its way out of the shell.reptile makes its way out of the shell.

If cont…If cont…

Like reptiles, birds reproduce by IF, however, Like reptiles, birds reproduce by IF, however, few bird species have a specialized structure for few bird species have a specialized structure for transferring sperm.transferring sperm.

Both males and females have a cloaca and IF is Both males and females have a cloaca and IF is accomplished through close body contact. accomplished through close body contact.

The sperm swim from the male’s cloaca into the The sperm swim from the male’s cloaca into the female’s cloaca to fertilize her gametes.female’s cloaca to fertilize her gametes.

Birds also lay eggs, but the shells are hard Birds also lay eggs, but the shells are hard rather than leathery.rather than leathery.

If cont…If cont… Unlike most fish, amphibians, and reptiles; birds care Unlike most fish, amphibians, and reptiles; birds care

for their young. They sit on the eggs to keep them for their young. They sit on the eggs to keep them warm and guard them from predators.warm and guard them from predators.

Both parents invest a great deal of energy into feeding Both parents invest a great deal of energy into feeding the young.the young.

Killdeers can run however they cannot feed Killdeers can run however they cannot feed themselves or fly, although they are miniature themselves or fly, although they are miniature versions of their parents; whereas the American Robin versions of their parents; whereas the American Robin is blind, featherless and needs to be completely cared is blind, featherless and needs to be completely cared for before they can leave the nest.for before they can leave the nest.

Incomplete metamorphosisIncomplete metamorphosis

Insects such as Insects such as grasshoppers and grasshoppers and crickets pass through crickets pass through different stages of different stages of development; this development; this process is called…process is called…

incomplete incomplete metamorphosismetamorphosis..

Complete metamorphosisComplete metamorphosis

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

Mammals and marsupialsMammals and marsupials We are mammals and we fertilize our eggs internally.We are mammals and we fertilize our eggs internally.

Most female mammals do not lay eggs, rather they retain Most female mammals do not lay eggs, rather they retain and nourish the embryo(es) within their bodies.and nourish the embryo(es) within their bodies.

This ensures additional protection and development of This ensures additional protection and development of the young, before birth.the young, before birth.

Also, mammals produce milk for nourishment for their Also, mammals produce milk for nourishment for their young.young.

Marsupials, such as the kangaroo and the Marsupials, such as the kangaroo and the opossum, give birth to offspring in a very opossum, give birth to offspring in a very early stage of development.early stage of development.

The young animal uses a hook-like claw to The young animal uses a hook-like claw to crawl to its mother’s pouch (ouch!) (the crawl to its mother’s pouch (ouch!) (the marsupium). There it latches onto a nipple marsupium). There it latches onto a nipple to obtain milk for completion of its to obtain milk for completion of its development.development.

hermaphroditeshermaphrodites Animals that have both female and male Animals that have both female and male

reproductive organs in each individual are reproductive organs in each individual are hermaphrodites.hermaphrodites.

During mating each planarian injects sperm into During mating each planarian injects sperm into a reproductive pore on the other flatworm’s a reproductive pore on the other flatworm’s body. Each planarian then lays fertlized eggs.body. Each planarian then lays fertlized eggs.

The common earthworm is a hermaphrodite. IF The common earthworm is a hermaphrodite. IF ensures that sperm have a moist environment in ensures that sperm have a moist environment in which to travel and increases the chances that which to travel and increases the chances that all the eggs will be fertilized.all the eggs will be fertilized.