IMPRINTING Simulation

The formation of social bonds in young precocial birds

A study of the formation of social or species bonds within the young of a species has occupied many ethnologists and psychologists. Accordingly, a wide variety of subjects have been observed, ranging along the phylogenetic scale from humans to insects. The most dramatic and clear-cut manifestations of affiliate tendencies, however, are seen in the young of certain ground-nesting fowl such as chickens, ducks, or quail. This early social behavior among precocial birds has been termed imprinting and has been defined as a process that causes the newly hatched to become rapidly and strongly attached to social objects such as parents or parental surrogates. That is to say, there is a marked tendency to approach, remain near, and follow such "targets". A complementary observation is that infant birds give evidence of distress when familiar targets are removed from them and contentment or pleasure when the target is returned.

In naturalistic settings the focus of investigation has been the formation and cohesion of family units. In the laboratory, work has centered on observations of the social responses of young birds to standardized or at least specifiable stimulus arrays. Students of the problem usually take the proximity of the young animal to the imprinting target as an index of the strength of the social bond. That is, the higher the proximity score during a test or observation, the greater the likelihood that the subject has imprinted.

The earliest and perhaps most straightforward theoretical analysis of imprinting was advanced by Konrad Lorenz in the 1930s. He had two major ideas about imprinting:

• Imprinting is the result of an instinct. That is, a genetically predetermined response that is "released" by a limited set of stimuli from the environment, the most effective stimuli probably being those that are represented in adults of a give species.

• Additionally, the "critical period" during which imprinting can occur was thought to be limited and severely restricted to the animals very early life.

Recently, however, researchers have wished to examine imprinting in the more rigorously controllable context of the laboratory. One purpose of the change in research modes has been to examine the role of experiential factors, such as special forms of learning, that may operate in addition to genetic influences. In general, animal psychologists have armed themselves with equipment that allows them to incubate, hatch and rear birds in the absence of any natural parent, and that also allows them to state with more or less specificity the total experience of subjects during investigations. Further, their selection of a variety of types of targets and treatments is thought to enable them to map the range of stimulus conditions that are sufficient to elicit the response.

Within exploratory work of this sort it is acceptable to examine variables that may or may not have face validity. Whatever simulation experiments are designed, though, it should be assumed that the rearing and testing situations for all subjects are precisely controlled and identical. Animals are to be hatched and reared in impoverished environments:

• the walls and floors of all apparatus are flat black

• no food or water will be available until the termination of an experiment

• handling will be kept to a minimum

• temperature held constant.

At the time of a scheduling of a test the subject will be removed from the rearing pen and placed immediately in a small open field situation. Suspended a few inches from the floor will be one or another of the imprinting targets, fastened to a rod. Through a mechanism attached to this rod the target can be moved about in the open field in standardized patterns. The floor of the field is marked off in a grid pattern (see Figure 1). A "time sampling" procedure will be employed to measure a given subject's tendency to remain near the target; every ten (10) seconds the proximity of the chick to the target will be noted. If the subject and target occupy the same grid, an arbitrarily large number (score) will be assigned for that observation. If the chick is in a adjacent grid, some smaller number will be assigned. If the chick is in an even more distant grid, a still smaller number will be assigned, and so on. All these observations will then be averaged to form a composite "imprinting" score for the entire test. Hence, fractional scores can be obtained.

The duration of a single test will be ten (10) minutes. Again, only animals that approach and stay near the target will be awarded imprinting scores. A high score indicates that the S has remained close to the target for relatively long periods and that imprinting has occurred. Low scores imply the opposite.

Grier, J., Counter, S. & Shearer, W. (1967). Prenatal auditory imprinting on chickens. Science, 155, 1962.

Guiton, P. (1959). Socialization and imprinting in brown leghorn chicks. Journal of Animal Behavior, 7, 26.

Jaynes, J. (1957). Imprinting: The interaction of learned and innate behavior. The critical periods. J. of Comparative and Physiological Psychology,50, 6.