Imprinting

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Imprinting describes a process in which newborn animals rapidly develop a strong attachment to a particular individual, often the mother. It is associated particularly with precocious bird species (species that mature early) such as chickens, ducks, and geese, in which the young hatch fairly well-developed.

Imprinting is advantageous because once offspring imprint on their mother, they will try to remain close to her and follow her around, behaviors that are beneficial in terms of the offspring's survival. The young also indicate distress when the mother is absent.

Imprinting was one of the first matters tackled by the field of ethology. Konrad Lorenz, one of the founders of ethology, studied imprinting to determine what controls and limits the behavior associated with imprinting. Lorenz showed that newly hatched birds imprint on practically any moving object to which they are close during their first day of life.

In natural conditions, of course, this object is almost certainly to be the mother. However, in a famous experiment, Lorenz was able to get birds to imprint on him. Interestingly, male birds that imprinted on Lorenz subsequently courted human beings when they tried to find mates, rather than courting members of their own species. This suggests that imprinting not only provides behavioral instructions to young birds soon after they hatch, but has important implications for future behavior as well.

Further work on imprinting in birds has revealed that species may respond preferentially to the appropriate stimulus. Although baby birds imprint on any moving object, they are also more likely to imprint on objects that have certain head and neck features corresponding to those itexpects to find in an adult of its own species. This makes it more likely that, in the wild, baby birds will imprint on the correct individual.

Two characteristics of imprinting are essential. First, imprinting describes an innate, preprogrammed response that is released by the appropriate stimuli. In the case of the baby birds, the presence of any mobile entity close to the chicks in the first hours or day of life is sufficient to release the response. In other species, different stimuli are required. Baby shrews also imprint on their mother, and will hold onto the fur of either the mother or another sibling when the mother wishes to move, so that the entire family is able to travel in caravan style. In shrews, the releasing stimulus for imprinting is suckling: Babies imprint on the odor of the female who suckles them.

A second feature of imprinting is that there is a very specific critical period when imprinting is possible. Goslings and other birds generally imprint in the first day of life and often within the first hours. For shrews, studies show that the critical period occurs between the fifth and fifteenth days of life. It is the female who nurses the babies during that time on whom they will imprint.

Imprinting is an example of a behavior that has both innate and learned components. Innate behaviors are preprogrammed, and appear fully developed in individuals. Innate behaviors tend to appear in situations in which the environment is fairly predictable. Learned behaviors are shaped by the environment. The advantage of learning is that it is flexible. Learned behaviors are suited to changing or uncertain environments.

Imprinting requires learning because young animals use cues from the environment in order to learn who is the parent. The behaviors that result, however, such as following behavior in precocious birds, is largely innate. The largely preprogrammed behavior that follows imprinting is believed tohave evolved because it is more efficient than learning, and because the flexibility that comes from learned behaviors is not advantageous in situations where imprinting occurs.

Some authors have extended the notion of imprinting to include other instances of preprogrammed behavior that require a releasing factor. Parental imprinting, for example, describes the imprinting of parents on their offspring. Parental imprinting is believed to be responsible for the success of brood parasites, bird species that lay their eggs in the nests of other species. The adoptive parents imprint on brood parasite young when they hatch, and then feed and raise them. Song imprinting has been studied in some bird species. In white-crowned sparrows, for example, young males imprint on the songs of adult conspecifics (members of the same species) that they hear sung around them, and sing similar songs when they mature and begin to look for mates.

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