Unit 4: Reproduction                                                                1   2   3 

2.2. Sexual reproduction in animals

Most part of animals reproduce sexually. However, some groups, such as

some cnidarians have alternation of generations.

They have a complex life cycle which includes a sexual stage

formed by individuals that reproduce sexually (medusa) and an asexual stage formed by individuals that reproduce asexually (polyps).

Sexual reproduction in animals usually requires two members

of the opposite sex, a male and a female. Each sex has different reproductive organs (gonads) which produce gametes.

Depending on the type of gonads present, animals can be:

- Unisexual.

Each individual has only one type of gonad.

Most animals are unisexual and individuals are male or female.

Sexes show usually differences in their appearance. 

This is called sexual dimorphism.

The differences in size or colour or the presence of some structures,

such as horns, allow animals to recognise potential partners.

- Hermaphrodites.

         Each individual has both types of gonads. So that it is male and female

         at the same time.

a) Fertilisation

Fertilisation is the union of an ovum and a spermatozoon. When they fused, they form a zygote or fertilised egg cell.

In many animals fertilisation is preceded by courtship. It is special behavior, a sequence of actions that allows the male and the female come together before mating.

There are two types of fertilisation:

- External fertilisation.

    It occurs when the gametes join outside the female’s body.

    It is typical of aquatic animals and some terrestrial ones such as amphibians

    and some insects which expends a lot of time in water.

    First, the female releases the unfertilised eggs (ova) into water

    and then the male releases sperm (full of spermatozoa) over them.

- Internal fertilisation

    It occurs when the gametes join inside the female’s body.

    It involves copulation, which is the transmission of sperm by copulative

    organs into the female reproductive system.

b) Embryonic development

Embryonic development includes the processes from the formation

of the zygote to the birth of the new individual.

After fertilization, the zygote undergoes through many cellular changes

and becomes a developing organism, called an embryo.

Depending on where the embryonic development takes place,

animals can be classified as  viviparous, oviparous and ovoviviparous.

- Viviparous

     The embryo develops inside the female’s body where it receives

     nourishment and protection.

     In this case, the embryo grows inside an organ in the abdomen

     called the uterus (womb).

There, a special organ called the placenta feeds the embryo,

which is connected to it through the umbilical cord.

Birth occurs when the new being leaves the mother’s body during labour.

      Mammals, except Monotremes, are viviparous.

- Oviparous

         The embryo develops inside an egg outside of the female’s body.

         Most oviparous species lay eggs in nests and incubate them,

         in order to keep them warm and protected from predators.

         Egg contains all the nutritive substances embryo needs to finish

         its development.

         In this case, birth is called hatching and occurs when the egg hatches,

         that is when the new being breaks it and emerges.

         There are two types of eggs:

            - Eggs without a hard shell, such as those of Amphibians and Fish,

              which must be laid in water or in wet environment,

              so that they do not dry up.

            - Eggs with a hard shell and impermeable membranes (Amniotic egg),

              such as those of Birds and Reptiles, can be laid on land

              as they do not dry up.

- Ovoviviparous

The embryo develops inside an egg that remains inside of the female’s

body until it hatches.

This is characteristic of Reptiles such as some snakes, Amphibians

such as salamanders and Fish such as some sharks.

c) Post-embryonic development

    Post-embryonic development begins when the individual is born

    and it ends when it becomes adult and is able to reproduce.

    In animals it can be:

    - Direct development.

       The new-born individual is similar to the adult, but smaller.

       This post-embryonic development consists on the growth of the body

       and maturity of the sexual organs.

       It is typical of viviparous (Mammals) and oviparous animals

       with a large amount of nutritive substances in the egg (Birds, Reptiles

       and some Insects) that are born very developed.

    - Indirect development

       The new-born individual (larva) is different to the adult.

       The transformation of the larva into an adult takes place

       through a series of transformations called metamorphosis.

       It is typical of Amphibians and most part of Invertebrates.

Animation: Sexual reproduction in Animals (Edumedia)

Animation: Relaciones alimentarias (Edu.ar)

2.3. Explain why:

a. Why is the fertilisation, in terrestrial animals, internal?

b. Why is necessary sexual dimorphism in unisexual species?

c. Why some animals (e.g. amphibians) only can lay eggs in aquatic

    environments?

d. Why some species (e.g. frogs) undergo metamorphosis?

2.4. The following pictures represent the ladybug life cycle. Order them

        and relate each one with the descriptions below.

1. The larvae form a cocoon where it completes its development.

2. After fertilisation, ladybug lays a large quantity of eggs.

3. Once the transformation is completed, an adult individual emerges.

4. Mating and fertilisation.

5. The larvae that are born are quite different to the adults.

6. When the embryo has completed its development, hatching takes place.

7. The new individual is a mature adult able to reproduce.