Final answer:
Animal reproduction can involve either internal or external fertilization. External fertilization is common in aquatic environments and involves the release of gametes into the water, while internal fertilization occurs primarily in land animals and offers protection for developing embryos.
Step-by-step explanation:
During sexual reproduction in animals, the process of fertilization can occur internally or externally. External fertilization typically happens in aquatic environments where eggs and sperm are released into the water and fertilization occurs by chance. Many aquatic animals, such as fish, amphibians, and aquatic invertebrates engage in this process, often through spawning triggered by environmental cues like water temperature.
Internal fertilization, on the other hand, most commonly occurs in land-based animals, though some aquatic animals exhibit this reproductive strategy as well. The main advantages include protection of the fertilized eggs from dehydration, limited predation on embryos, and enhanced fertilization by a specific male. Internal fertilization leads to relatively fewer offspring, but with higher survival rates. Examples of animals that utilize internal fertilization include most birds, reptiles, and mammals, with some laying eggs (oviparity) and others giving birth to live young (viviparity).