Organisms that metamorphose undergo radical changes over the course of their life cycle. A frog egg hatches a tadpole that metamorphoses into an adult frog within a few days or weeks. A fruit fly egg hatches a larva that feeds for a few hours or days and then enters the pupal stage during which it develops a protective covering. The changes that occur during the metamorphosis of a single species may be so great that the species occupies two separate and very different niches or places in an environment at different times. In fact, the larvae of two species may be more similar to each other than to the corresponding adult forms of their own species. Organisms that utilize different resources at different stages of their life cycle face an unusual evolutionary problem, exploiting different niches may be difficult with a single body plan. The solution is a juvenile (immature) form specialized for one niche, followed by metamorphosis to an entirely new body plan, adapted to a different niche in the adult. Clearly, species that metamorphose must undertake complex genetic and physiological processes in the transformation. These changes require complex regulatory mechanisms that involve turning on and off many genes at appropriate times. In addition, the reorganization of the body plan in a metamorphic species entails considerable energy costs. What sorts of ecological advantages could outweigh these complications? One prevailing hypothesis is that metamorphic species specialize so as to exploit habitats with high but transient (short term) productivity – and hence high potential for growth. Part of this strategy is that specializations for feeding, dispersal, and reproduction are separated across stages. A frog tadpole occupies an aquatic environment (such as a pond) with extremely high potential for growth. The existence of the pond or its high production may be transient, however. Whereas an aquatic larva is not capable of dispersal to new ponds if its habitat becomes unsuitable, the adult frog is. In this case rapid growth in the larva is separated from dispersal and reproduction in the adult. Although the adult feeds, its growth rate is far less than that of the tadpole. The energy adults obtain from feeding is dedicated to dispersal and reproduction. Many insects benefit from the same strategy. Although a butterfly larva feeds voraciously, often on a very specific set of host plant species, the adult does not grow. If it feeds, it does so only to maintain energy reserves required for dispersal and reproduction. The monarch butterfly exemplifies this strategy. Its larvae feed specifically on milkweeds. Monarch pupae also develop on this host plant. The emerging adults migrate long distances – from all over eastern North America to nine small sites in the Sierra Madre mountains of Mexico. There, females become sexually mature and migrate north, mating along the way and feeding only to maintain energy reserves. In this example, the feeding specialist stage is again separated from the dispersal and reproduction stages. In the previous examples, the reproductive function is delegated to the adult. Under certain ecological conditions, however, it is apparently advantageous for reproduction to occur in the larval stage. Thus, even the reproductive function typically fulfilled by the adult can apparently be modified under certain circumstances. Species that show this modification of a metamorphic life cycle are said to demonstrate neoteny, a life cycle in which the larvae of some populations or races become sexually mature and no longer metamorphose into adult. Some populations of the salamander Ambystoma maculatum show this trait. In fact, the larvae of this species were originally classified as a separate species before it was recognized that they are neotenic forms. The selective factors leading to neoteny are not well understood. We know, however, that neotenicforms are more frequently found in extreme environments, often high altitudes or latitudes. High-altitude populations of certain salamanders have higher frequencies of neoteny than do low-elevation population of these species. If the larval environment is rich compared to the harsh adult environment, selection may favor neoteny. One research study, has ruled out simple food effects, supplemental food did not increase the frequency with which organisms reached the adult stage. This suggests that neoteny may be a genetically determined feature of some amphibian life histories.