5. neoteny
Jane Burton/Bruce Coleman Ltd.
Axolotls, a type of salamander, never fully metamorphosize. That is to say: they are essentially larvae for life, though they can reproduce. This phenomenon, known as neotenization, or the retention of juvenile features into adulthood, appears situationally in some other species of salamander. If fully maturing—and thus taking to the land—would put the salamanders at risk due to food shortages or high numbers of predators, they remain in a state of arrested development.
