Imprinting studies conducted with mammal and bird species are much easier because the researcher has the freedom to conduct controlled experiments that test many different variables, thereby identifying the factors that influence an individual animal's ability to imprint. For bird species, a famous experiment is the moving ball experiment. A newly hatched chick is isolated in a chamber within which a suspended ball revolves around the center of the chamber. The researcher can test not only movement as an imprinting trigger but also other variables, such as critical imprinting time after hatching, color as an imprinting factor, and variations in the shape of the ball as imprinting factors. Other experiments involve switching eggs between different species (for example, placing a duck egg among geese eggs).
For mammals, imprinting has been observed in many species, such as humans, chimpanzees, gorillas, dolphins, elephant seals, wolves, and cattle. In most of these species, the failure of a mother to come into contact with its newborn almost always results in rejection of the child. In species such as elephant seals, smell is the primary means by which a mother identifies its pups. Maternal imprinting is of critical importance in a mammalian child's subsequent social development. Replacement of a newborn monkey's natural mother with a "doll" substitute leads to irreparable damage; the infant is socially and sexually repressed in its later life encounters with other monkeys. These and other studies establish imprinting as a required learning behavior for the successful survival and socialization of all birds and nonhuman mammals.
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