How does internal fertilization differ from external fertilization?

The primary difference between internal and external fertilization lies in the location where the fusion of the sperm and egg occurs.

• Internal Fertilization: This process occurs inside the body of the female animal. It often involves specialized structures, such as intromittent (copulatory) organs, which facilitate the transfer of sperm into the female's reproductive tract. While many species use these organs, some animals achieve internal fertilization through other methods, such as the deposition of spermatophores that the female then collects. This method is universal among mammals and reptiles, and it is required for species that produce shelled eggs.12
• External Fertilization: In this process, the union of gametes takes place outside the female's body, typically in the surrounding environment, such as water. In many animals, this relies on random collisions between eggs and sperm, though gelatinous coats surrounding the eggs can help trap sperm to increase the chances of successful interaction. This method is common among many amphibians and various fish species.31