For many creatures, the transformation from infant to adult is a gradual process. In many mammals and birds, for example, development unfolds over the course of years. This is not to say that the transformation is constant. The color of a young bird's plumage often changes when the bird molts, and human children periodically experience growth spurts. However, none of the changes that occur in these animals is as dramatic as the metamorphosis that takes place between the larval stage and adulthood in amphibians and most insects.
What's so remarkable about the process of metamorphosis is that the same animal at each stage looks and behaves nothing alike. Bullfrog larvae, better known as tadpoles, are strict herbivores, while adult bullfrogs are carnivorous. Tadpoles, at least before they grow legs, look more like fish than frogs. In fact, they even breathe with gills. Adult frogs, by contrast, have powerful back legs and webbed feet and breathe with lungs.
Like bullfrog tadpoles, dragonfly larvae, called nymphs, are fully aquatic. The nymphs lack the long slender wings of their adult counterparts and, like tadpoles, acquire oxygen from the water through gills. Similarly, monarch butterflies undergo dramatic changes in their transition from larva to adult. Both phases are highly specialized and very different from each other. While the larva is specialized for eating and growing, gaining as much as 2,700 times its original weight, the adult only sips nectar as it searches for mates and good locations to lay eggs.
The level of specialization of the larval and adult phases of these animals suggests a reason for metamorphosis. Many scientists hypothesize that specialization minimizes competition between larvae and adults and thus allows these species to exploit their environment more effectively than other animals.