REPRODUCTIVE STRATEGIES OF VERTEBRATES Successful reproductive strategies make sure that they produce many offspring ensuring the survival of the particular species

Why is the success of reproduction strategies important for each species? To produce the maximum number of surviving offspring, Ensuring that the species continuous in the future (no organism lives forever), While using the least amount of energy.

Different species living in different environments Reproductive strategies of different organisms are linked to the environment in which they live.

REPRODUCTIVE STRATEGIES TO BE STUDY External & Internal fertilisation Ovipary, ovovivipary and vivipary Precocial and Altricial development Amniotic egg Parental care

Internal and external fertilisation Over time the environment of the earth changed from an aquatic environment only to a terrestrial environment as well. Some animals with the desired traits evolved to be suited for the terrestrial environment. Their reproductive strategies also changed to suit the different environments. Two kinds of fertilisations developed, namely, External, and Internal fertilisation.

External feritisation External fertilisation is a strategy of  fertilisation in which a sperm cell fuses with an egg cell outside the body of organisms. Both gametes are released in water, because their is no danger of the gametes drying out. The sperm cell can swim through the water and fertilise the egg cell to form a diploid zygote. This is common in Aquatic animals

Frogs and fishes: Mating takes place, but gametes are released in water for external fertilisation

Disadvantages of external fertilisation Wasteful; huge loss of energy – many sperm cells and egg cells produced and released Fertilisation not certain – washed away by currents, eaten by predators. Harsh environmental conditions

How does external fertilisation maximise reproduction? Huge numbers of eggs increases probability of fertilisation. Courtship rituals ensure that males and females are closer to each other when releasing gametes. Spawning is timed to occur when ocean currents can disperse eggs e.g. sessile species. Young easily dispersed by sea currents; reduce competition. No complicated physical mating, using energy. Larval form gets food directly from environment; no energy input from parent. Female frogs sheds 2000 t0 3000 eggs in water at a time; males shed millions of sperm.

Internal fertilisation Fertilisation which takes place inside the female body is called internal fertilisation.   Internal fertilisation in animals is done through copulation, which involves the insertion of the male penis or other organ into the female vagina (in most mammals) or into the cloaca in most reptiles, birds, and some fish.

Internal fertilisation (Cont) Occurs in terrestrial vertebrates i.e. birds, reptiles and mammals. Marine mammals and some fish e.g. Sharks also have internal fertilisation. Mating and copulation occurs. Most animals have a cloaca – common opening for reproduction and egestion – during fertilisation cloacas are lined up. Male mammals have a penis – an organ to transfer sperm to body of female. Fluid inside female provide medium for sperm to swim towards egg cell(s) – internal fertilisation.

How does internal fertilisation maximise reproduction? Fertilisation is more certain – gametes placed closer together. Fewer egg cells needed. Energy saved in producing fewer egg cells / gametes - can be used for other purposes. Protect gametes inside body Fluid in female prevents drying out of gametes & provides medium through which sperm cells swim towards egg cell. Reproductive strategies of birds and reptiles – cloacas of male and female are lined up.

Energy saved by internal fertilisation can be used for ... Yolk to feed young. Shells that enclose eggs (oviparity); better protection increases survival rate. Some fertilised eggs very well-protected e.g. Sharks (ovoviviparity) and mammals (placenta). Less wastage of gametes – in humans one egg per month produced.

Disadvantages of internal fertilisation A cooperative partner is needed – female and male individuals needed Mates have to find each other Competition for mates occurs Reproductive behaviour has to be coordinated Producing offspring that will survive often requires parental care e.g. feeding and protection against predators

Examples of internal fertilisation – mating and copulation

Differences between internal & external fertilisation Internal fertilisation No copulation Copulation occurs (sexual intercourse) to transfer sperm cell Both gametes released outside body in environment Sperm cells released into the female body Less chance of fertilisation Greater chance of fertilisation Many egg cells released Few or single egg cell released Zygotes develop outside body of parents Zygote are protected inside the female body Common in aquatic animals Common in terrestrial animals No special organ for transferring sperm cell into water Special organ, penis, use to transfer sperm cells into vagina/cloaca In both cases sperm cells swim towards egg cells