Animals use several biological processes that lead to sexual reproduction. Humans and most other land animals use an internal fertilisation reproductive strategy in which the male deposits sperm directly inside the female. In external fertilisation, on the other hand, the male deposits sperm into a body of water. The sperm travel until they reach an egg deposited into the water by a female.
Number of Gametes
Species that use internal reproduction produce relatively few gametes. Because the male directly deposits sperm into the female's body, fewer gametes are needed. External fertilisation requires the male and female animals to produce larger numbers of gametes. Males must produce a large number of sperm to ensure that they travel throughout the body of water to meet an egg. Females also must deposit dozens or hundreds of eggs to ensure reproductive success. Producing a large quantity of gametes requires extra energy, which can be disadvantageous to an animal. However, the large number of gametes may result in a large generation of offspring, improving an organism's chance of passing on its genes.
External fertilisation strategies require a body of water to be successful. Sperm have small tails that propel them through water; they would die on land. While depositing gametes in water does not pose a problem for fish, aquatic invertebrates and other animals that live in the water, it can be a disadvantage for other species. Amphibians and creatures that dwell on land must return to water to deposit their gametes.
Success of Fertilization
The close proximity of egg and sperm in internal fertilisation increases the likelihood of a successful reproductive cycle. In external reproduction, animals disperse their gametes through a body of water. This dispersal reduces the likelihood that a sperm will find an egg. Many sperm and eggs die before achieving fertilisation. The low success rate of external fertilisation puts animals at a reproductive disadvantage compared to internal fertilisation.
External fertilisation strategies are behaviorally simpler than internal fertilisation. A male and female may deposit their gametes at a slightly different time or place without jeopardising their reproductive success. In contrast, animals using an internal fertilisation strategy rely on hormones, mating rituals and behavioural factors to ensure that the male and female have sexual intercourse. The external fertilisation strategy does not require these adaptations, making it a simpler reproductive strategy.