This chapter covers materials for the allied ﬁelds of ecology, evolution, and animal behavior. Ecology is ‘‘the study of the interrelationships between organisms and their natural environment, both living and nonliving’’ (Oxford Dictionary of Biology, 4th ed., 2000). Conservation biology and environmentalism are closely related, but not extensively covered in this chapter. Evolution is ‘‘the gradual process by which the present diversity of plant and animal life arose from the earliest and most primitive organisms.’’ See also Chapter 7, ‘‘Genetics, Biotechnology, and Developmental Biology,’’ for related materials. Animal behavior, ‘‘the activities that constitute an animal’s response to its external environment,’’ here encompasses all biological subdisciplines including ethology, sociobiology, and behavioral ecology. Human behavior and comparative psychology are largely excluded from consideration, and neurobiology is covered in Chapter 11, Anatomy and Physiology.