Asexual reproduction is relatively easy to achieve because it involves only a single animal. Sexual reproduction is much more complex because the partners often have to find each other and precisely coordinate their reproductive behavior. In most cases, each partner is either male or female, but in some animals—such as earthworms, slugs, and snails–each one is a hermaphrodite, an animal that has both male and female organs. Hermaphrodites usually fertilize each other, with both partners producing young (see Hermaphroditism).
During the spring, western grebes perform spectacular courtship dances. In the “rushing” display, the mating pair swim side-by-side with their wings held back, their long necks arched, and their yellow beaks angled upward. They swim so quickly that their bodies are pushed up out of the water and they appear to run across the surface. After courtship the male and female build a floating nest out of plant material.
Most aquatic animals shed their eggs and sperm into the water, where external fertilization takes place. In corals and many other sessile species, the moment of spawning is often triggered by the tides, maximizing the chances that the egg and sperm will meet. In a minority of marine animals, fertilization is internal, meaning that the male mates with the female, inserting his sperm into her body. For this to work, the male needs special adaptations to make the transfer. Male sharks and rays use special claspers that are attached to their pelvic fins, while barnacles, which are often hermaphrodites, use a thread-like penis that can be almost as long as their bodies.
Terrestrial vertebrates clasp each other tightly during copulation, the act by which the male deposits his sperm into the female’s reproductive tract. In the giant Galápagos tortoises pictured here, mating may take hours.
On land, external fertilization is rare because egg and sperm cells cannot survive for long in the open. As a result, almost all land animals must mate to trigger internal fertilization in order to reproduce. Different groups of animals have evolved a wide variety of mechanisms to make sure that males and females manage to locate suitable partners. Some female insects emit chemicals called pheromones, which guide males towards them, while others use sound signals or biochemically produced light (see Bioluminescence). In birds, elaborate plumage and courtship displays help to attract females towards the males (see Animal Courtship and Mating).
The queen honey bee may lay 1500 eggs in a single day. Worker bees feed the wormlike larva constantly—as many as 1300 times a day—after it hatches, sealing the cell when the grub has grown to fill it. The larva pupates in about 12 days, and the adult bee chews through the wax cap of its cell approximately three weeks after the eggs were first laid. Newly emerged adults perform various maintenance tasks until they are ready to begin foraging outside the hive.
The males of many insects and virtually all mammals use a penis to transfer sperm to the female, who harbors the eggs, in a process known as copulation. The penis ensures that sperm is transferred successfully without being carried away by wind, water, or other environmental elements. Most birds and reptiles mate using a cloaca, a single opening located on the lower abdomen. During mating, these animals align their cloacas for transfer of sperm. Some birds, such as bald eagles, can perform this feat in mid-air.
Once a female has mated, egg development can proceed in two different ways. In oviparous species, which include the majority of vertebrates except mammals, and also most insects, the fertilized eggs are laid and develop outside the mother's body. In viviparous animals, which include nearly all mammals together with some reptiles and sharks, the young develop inside the mother and are born live.
Most animals that are born live look similar to their parents, although they are not fully developed. By contrast, many egg-laying invertebrates look completely different from their parents when they hatch and often live in a completely different way. Known as larvae, these young change shapes as they grow up, during a process called metamorphosis. Larvae are also found in some fish and most amphibians.