Internal & External Fertilisation D. Crowley, 2007

Internal & External Fertilisation Sunday, April 02, 2017 Internal & External Fertilisation To know the difference between internal and external fertilisation

Identical? Look at the following - it shows two sequences how identical and non-identical twins are formed Can you tell which is which? Can you explain how you know?

Identical? Identical twins can form because once a sperm has fertilised the egg, the ball of cells (embryo) can split into two. If this happens you get two identical twins Non-identical twins can form, because sometimes a female can ovulate and produce two eggs in a month. These are then fertilised by a different sperm each, so you produce non-identical twins Unique individuals are due to individual sperm and eggs fertilising one another

Fertilisation Different organisms fertilise in different ways Can you think of any organisms which fertilise differently to humans (e.g. think about frogs and fish) For fertilisation to occur, the male sperm cells must get to the female egg cells. There are two main ways for this to occur - internal and external fertilisation

External Fertilisation External fertilisation occurs outside the female - the male releases sperm cells onto the egg cells as the female lays them External fertilisation occurs in fish and most water dwelling animals (e.g. amphibians). These organisms usually produce a large number of sex cells which are released into the water. There are usually courtship rituals which make sure the male and female sex cells are released near each other. However, the chances of fertilisation are very low, so huge numbers of eggs and sperm are needed to ensure enough offspring are produced

Internal Fertilisation Internal fertilisation occurs inside the female Internal fertilisation occurs in most land dwelling organisms (such as mammals) - this is necessary as sperm need fluid in order to be able to swim to the egg (the fluid comes from semen, ejaculated along with the sperm from the male). These organisms produce far fewer sex cells, as the chances of successful fertilisation are much higher (the sex cells are much closer together when they are released)

How Many? Why is it that organisms which produce by external fertilisation produce so many more sex cells? The chances of fertilisation are much lower in externally fertilised organisms, as the sex cells are further apart from each other: - This means the sex cells are more likely to be predated They can also be separated from one another (due to currents) In external fertilisation there is also less parental care, so the offspring are more likely to be killed (hence producing lots of them)

How Many? Why then are there so few sex cells produced in internal fertilisation? As the likelihood of successful fertilisation is higher in internally fertilised organisms, because the sex cells are so much closer together, producing a great many sex cells would cause problems because: - There is only a limited amount of space within the female’s uterus to accommodate a growing foetus In internal fertilisation there is a greater deal of parental care, so the offspring are more likely to survive (hence there is no need to produce lots of them)

Understanding Task - using exploring science book, read pages 18 and 19 and answer (in full sentences) questions 4, 5, 6, 7 and 8 Questions: - In external fertilisation not all the egg cells are fertilised, and some are subjected to predation and being separated from one another. Offspring which develop inside the mother have the advantages of protection, warmth and by getting everything they need from the mother. A baby giraffe is extremely vulnerable as soon as it is born (from predators), so being able to get up and run quickly after it is born will reduce its risk of being predated upon.

Understanding A bullfrog produces 20’000 eggs - because it reproduces externally, so needs many eggs as a great deal will be predated / may become separated The Lake Malawi mouthbrooder probably got its name because it cares for its young (after they have been fertilised) within its mouth, where they grow. Brood can mean to be anxious about something, as well as meaning the young that are all born and reared at a similar time. So the mouthbrooder is anxious for her young which have all been born at a similar time, and looks after them in her mouth! Animal Number of egg cells Fertilisation type Bat 1 Internal Bullfrog 20’000 External Cat 4 Cod 5’000’000 Tiger 2

Internal & External Internal Fertilisation External Fertilisation etc… Look at the worksheet, and cut out all the cards Organise the cards into two groups - internal fertilisation and external fertilisation Stick these into your book Internal Fertilisation External Fertilisation etc… etc…

Internal & External Internal Fertilisation Eggs produced in small numbers Method usually by land animals (water is not available for the sperm to swim) Method has a better chance of the offspring surviving due to improved protection Human, butterfly, horse External Fertilisation Eggs produced in very large numbers Used by fish because water is available for sperm to swim in Sperm can get eaten by aquatic predators Fish, frog