To survive, eggs and sperm require a wet environment. In aquatic organisms like fish and amphibians, eggs and sperm can be released directly into the water by external fertilization. External fertilization, as you might imagine, is a chance affair: Somehow, sperm must meet egg in a large environment. Animals that release gametes externally therefore have developed strategies to increase the probability of fertilization. Some species release large numbers of gametes. Many female fish, including salmon and perch, lay eggs on or near a substrate, such as a rock, and males release their sperm onto the eggs to increase the probability of fertilization. Externally fertilizing animals may also come close together physically to improve chances that the sperm will fertilize the eggs. Male frogs grasp females, even though fertilization occurs externally, releasing sperm as the female lays her eggs (Fig. 42.7).

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A challenge for land-dwelling organisms is keeping gametes from drying out. Internal fertilization, in which fertilization takes place inside the body of the female, is an adaptation for living on land. Reptiles, birds, and mammals all use internal fertilization. However, this mode of fertilization is not exclusive to land dwellers. Marine mammals such as whales and dolphins use internal fertilization, as did the land-dwelling ancestors from which they evolved. Even some fish, such as guppies, reproduce by internal fertilization.