How does internal fertilization differ from external fertilization in animals?

In animals, the primary difference between internal and external fertilization lies in where the union of the sperm and egg occurs.1

• External Fertilization: In this process, the male releases sperm over eggs that have been shed by the female into the environment, typically in aquatic habitats. This method is common among many invertebrates as well as most fish and amphibians. Because the gametes are released into the water, successful fertilization often depends on random collisions, though the presence of gelatinous coats on eggs can help trap sperm and increase the chances of interaction.213

• Internal Fertilization: This occurs when sperm is introduced into the body of the female, where it unites with the egg. This strategy is standard for terrestrial breeders, as it protects the gametes from desiccation, though it also occurs in some aquatic species. Internal fertilization often involves specialized structures, such as intromittent (copulatory) organs, which facilitate the transfer of sperm, though some species achieve internal fertilization without them.214

Both methods serve the same fundamental purpose: the fusion of hereditary material to form a zygote.31