External fertilization is common among aquatic animals, including fish, amphibians, and aquatic invertebrates. Following an elaborate ritual of mating behavior to synchronize the release of eggs and sperm, both males and females deposit their gametes in the water at approximately the same time in close proximity to each other. The water protects the sperm and eggs from drying out. Fertilization occurs when the sperm reach the eggs. Internal fertilization requires that sperm be deposited in or close to the female reproductive tract. It is most common among terrestrial animals that either lay a shelled egg, such as reptiles and birds, or when the embryo develops for a period of time within the female body. For example, certain sharks, skates, and rays have internal fertilization. The pelvic fins are specialized to pass sperm to the female. In most of these species, the embryos develop internally and are born alive.