The term “imprinting” refers to the rapid acquisition by young animals of the primary social bond to their parents during a limited period very early in life. This imprinting phenomenon can be most clearly seen in precocial bird species, whose young are hatched at a relatively advanced stage of development and are able to move about independently rather soon after hatching. Such species include ducks and other waterfowl, as well as chickens and turkeys. Imprinting also appears to exist in some precocial mammal species, such as the guinea pig (Hess 1959a; Shipley 1963). In all of these cases the attachment of the young to the mother is evident when he follows her about. As Morgan (1896) remarked, there is evidently an innate tendency to follow but there is no requirement that the object to be followed be the biological mother. After the infantile following behavior has been outgrown, the attachment continues to exist and forms the basis of later social preferences (Lorenz 1935).
It also appears that the primary socialization processes in other social animals contain features highly similar to those of imprinting. It is now well known that early life experiences play a decisive role in the formation of an animal’s or a person’s affectional system (e.g., Bowlby 1951; Harlow 1958; Harlow & Harlow 1962). Under normal conditions the first social experiences are with the parents and, in many species, often also with siblings. It is through these early social experiences that individuals become attached to members of their own species. On the other hand, under unusual circumstances, produced either experimentally in a laboratory or accidentally, social attachments to members of an alien species or even to inanimate objects can be formed during the earliest life period.
Primary socialization is of extreme importance not only in determining the cohesiveness of animal groups, important for immediate survival, but also in the continuation of the species, for it often influences the nature of sexual behavior, in particular with respect to the object chosen or accepted (Lorenz 1935). Precocial species provide the clearest instances of imprinting. In the case of species that are not precocial, primary socialization may have different grades of similarity with the classical imprinting phenomenon. One of the most important differences is that the time period involved is not so dramatically short as it is in imprinting and may extend throughout the period of association between young and parents (e.g., Kling-hammer 1962; Klinghammer & Hess 1964). But in all cases the primary socialization occurs during the first life period.
History. The importance of a relatively limited period in early life for the formation of social bonds has been well noted. Pliny the Elder, for example, told the tale of a goose that followed Lacydes faithfully (Naturalis historia, x). Both D. A. Spalding (1873) and C. L. Morgan (1896) reported that when they hand-reared chicks away from the mother, the chicks completely refused to have anything to do with her when they finally met. C. O. Whitman (Craig 1908) turned this phenomenon to a practical use with nonprecocial, or altricial, species: whenever he wanted to cross two species of pigeons, he would rear the young of one species with foster parents of the other species. When fully grown, pigeons reared with foster parents of a different species preferred to mate with members of that species rather than of their own. Later, Heinroth and Heinroth (1924-1928) hand-reared the young of almost every species of European bird, and noted that many of the social responses of these birds were transferred to their human caretaker. Indeed, interspecies sexual fixation has been observed in other birds, some fishes, and two mammals, the alpaca and vicuñna (Goodwin 1948; Baerends & Baerends-van Roon 1950; Hodge 1946).
It was Konrad Lorenz, the eminent European zoologist, who first looked at the phenomenon of imprinting scientifically and postulated some laws governing its occurrence. In a classic paper in 1935, Lorenz described it and gave it its name (in German, Pragung). Lorenz pointed out for the first time that if imprinting is to occur, the young animal must be exposed to its object during a critical period early in its life. He postulated that the first object to elicit a social response on the part of a young animal later released not only that response but also related ones, such as sexual behavior.
The first systematic investigations on imprinting were published in 1951. The independent. work of Ramsay (1951) in the United States and of Fabri-cius (1951a; 1951b) in Europe gave the first indications of some of the important factors in imprinting. Most of Ramsay’s experiments dealt with exchange of parents and young, although he also imprinted some waterfowl with such objects as a football or a green box. He worked with several species of ducks and a variety of chicken breeds. He noted the importance of auditory stimulation in imprinting and the effect of changes in coloring on recognition of the young by the parents, as well as of the parents by the young. His findings indicated that color is an essential element in recognition, while size or form seemed to be of less importance. Fabricius studied several species of ducks, including tufted ducks, eider ducks, and shovellers. He observed that newly hatched ducklings have a very strong tendency to follow the first moving objects with which they come in contact, for when they were exposed to older ducklings of a different species, they followed them persistently even though they were rewarded only with harassment and violent nipping. Fabricius found that there was a sensitive period during which imprinting occurred most easily: it had its peak at the age of 12 hours and then decreased until, after the age of 24 hours, imprinting was more or less impossible, for the animals became increasingly fearful of new objects. Movement was discovered to be a great influence in eliciting imprinting, as also were rhythmic calls.
Ramsay and Hess (1954) reported a method of studying imprinting in the laboratory, using a runway and a model fitted with a loudspeaker. Later the apparatus was modified as seen in Figure 1. Then Hinde (1955), using moorhens and coots, confirmed the importance of motion in eliciting imprinting responses. He also reported that following
was most easily elicited during the first day after hatching; older birds usually fled from the model he presented to them, thus showing the role of fear in ending the sensitive period for imprinting. Furthermore, he noted that if birds were exposed to different models, persistent following took place only with a familiar model.
Since then there have been an increasing number of experimenters who have attempted to assay the imprinting phenomenon, and their work has brought about an increasing understanding of the imprinting process and of the socialization process in general.
The critical period. The most salient feature of the imprinting phenomenon is that it occurs so early in life. Very soon after hatching, chicks and ducklings can follow mother objects and, in fact, show a strong desire to do so. At the very outset, Lorenz (1935) postulated that imprinting could “only take place within a brief critical period in the life of an individual,” which was “a very specific physiological state in the young animal’s development.” He noted that Greylag geese were imprinted during the first few hours after hatching and suggested that imprinting could occur as early as the first few minutes after hatching. He also recorded his observation that some partridges he happened to come across in a field when they were only a few hours old had obviously already been imprinted to their parents and could not be induced to approach him. Other researchers have noted that the readiness to follow an object wanes with increasing age (Alley & Boyd 1950; Hinde 1955; Hinde et al. 1956; Fabricius 1951b; Fabricius & Boyd 1954).
However, it was Ramsay and Hess (1954) who determined in the laboratory that there is indeed a rather limited age period during which mallard ducklings can be well imprinted, and that maximum imprinting occurs consistently only in ducklings imprinted at the age of 13 to 16 hours. The existence of an optimum developmental stage for imprinting has also been confirmed by Gottlieb (1961a), who used special procedures in order to pinpoint the age of his ducklings in days and hours from the very beginning of their embryonic development. It was determined that the age of 27 to 27½ days from the beginning of incubation was indisputably the most sensitive time for imprinting, and that by the age of 28½ days very little imprinting could take place.
Fear has been suggested by many writers as the factor that ends the period of imprintability, and Fabricius (1951a) has suggested that the initial inability of birds to locomote could account for the first rise in imprintability. Hess (1959b) actually plotted locomotor ability and fearfulness of chicks as a function of age and found that the emergence of fearfulness coincided exactly with the limits of the critical period for imprinting. However, initial imprintability is higher than increasing locomotor skill would suggest.
Interspecies and intraspecies differences. Species and breed differences are emerging as an increasingly important factor in assessing the various parameters of the imprinting phenomenon. According to Hess (1959a), some species and breeds, such as wild mallard ducks, show an extremely high degree of imprintability and usually respond quite vigorously to the first imprinting object they meet. They also retain the effects of the experience quite firmly. Leghorn chicks, in comparison with Vantress broiler chicks, may not show as much responsiveness in a laboratory imprinting situation, and even when they have had relatively extensive exposure to an imprinting object, this experience may not be as effective. In spite of the fact that chicks and ducklings have the same critical-period ages for imprinting, the peak of the strength-of-imprinting curve is much lower for chicks than it is for ducklings, as is the general curve itself. There are probably also differences among different species in the type of object that is adequate to arouse imprinting. Other differences between species and between breeds will be brought up as the topic is further pursued. It is, therefore, well to remember that while the socialization processes in different animals may resemble imprinting, generalization between, and even within, species must be cautious and limited.
Prior social and sensory experiences. Many researchers have found that housing chicks communally prior to the imprinting experience decreases imprintability or responsiveness (Guiton 1958; 1959; Sluckin & Salzen 1961; Hess 1962; 1964). It appears that under these circumstances they become imprinted to each other, and that this prior imprinting hinders the formation of imprinting toward a new object. The inhibition of imprinting to a new object when imprinting has already occurred appears to be a basic characteristic of the imprinting phenomenon and has been confirmed by Hess (1959a), who found that ducklings imprinted more strongly to models to which they had been first exposed than to a model they met immediately afterward.
Under natural conditions it does appear that imprinting to siblings can occur either simultaneously with or immediately after imprinting to the mother; this might account for “flocking” behavior. The nature of the effect of exposure to siblings before the laboratory imprinting experience has been found to be related both to species membership and to age at exposure to the imprinting experience (Hess 1964). In chicks and ducklings the effect of prior socialization was found to lower imprintability to the model. However, in the case of chicks the amount of following of the model was increased as a result of the socialization experience, especially when these chicks were well past the critical-period age, 36 hours. With ducklings, however, socialization decreased the amount of following as well as of imprintability.
Moltz and Stettner (1961) varied the visual stimulation of some ducklings with hoods that permitted them only diffuse light stimulation. When subjects were tested for imprintability, those that had experienced diffuse light were found to show greater imprintability when first exposed to the imprinting model at the ages of 12, 24, and 48 hours than did those that had lived in normal light conditions. The animals raised in diffuse light were most responsive at the age of 24 hours, while the normal animals were most responsive at the age of 12 hours and responded very little at the age of 48 hours, when the diffuse-light subjects still showed strong responsiveness. Hess (1964), however, has reported somewhat different results with chickens. Dark-reared chicks (like dark-reared ducklings) have maximum responsiveness at the age of 13 to 16 hours after hatching. However, socially isolated chicks exposed for two hours to patterned light stimulation prior to the imprinting experience behaved no differently from completely dark-reared animals if they were placed in the imprinting situation at the age of 16 hours; but if they were exposed to imprinting at the age of 48 hours, they followed somewhat better than the dark-reared ones. This difference, nevertheless, is much less than that resulting from prior social experience.
Experimental variables. Hess (1957) has postulated a law of effort in imprinting, which states that the more effort a young chick or duckling expends while following or attempting to be with the imprinting model, the more strongly it will be imprinted, as shown by later preference behavior when both the imprinting model and an unfamiliar one are offered. Imprinting-strength scores were obtained for 12- to 17-hour-old ducklings, who were made to follow an imprinting model for different distances in a ten-minute period. The greater the distance the ducklings had to follow, the better they were imprinted. This held true up to 50 feet, after which further distances did not materially increase imprinting strength. Moreover, when ducklings were allowed different amounts of time to follow the model for the same distance, the imprinting-strength scores for the same following distances were essentially identical. Evidently time in itself has no effect on the strength of imprinting.
Furthermore, muscle relaxation prevents imprinting, as shown by the failure of chicks and ducks that had been imprinted while under the influence of either meprobamate or carisoprodol to show any effect of the experience when tested later. If animals are imprinted normally but tested under drug conditions, there is no effect of the drug (Hess 1957; Hess et al. 1959). This, together with the fact that socialized chicks follow well but do not imprint well when an attempt is made to imprint them after the critical period, demonstrates that the law of effort applies only to socially naive normal animals exposed to the imprinting situation at the critical-age period.
It also appears, on the basis of several experiments (Hess 1957; 1959a; 1959c), that some of the research that has attempted to assess the law of effort by preventing animals from following the imprinting model have failed to take into account the effort expended by subjects struggling to escape restraint or approach the model. The effectiveness of struggle when animals cannot move is shown by the fact that chicks and ducklings have a greater imprintability in the first few hours after hatching than their locomotor skill would seem to indicate. Furthermore, Gottlieb (1961b) has shown that equal imprinting strength in two different duck varieties does not necessarily reflect the same amount of following; thus, if the amount of following is to be taken as a valid indicator of probable able imprinting strength, any comparisons must be between animals of the same variety.
The effect of painful stimulation during the imprinting experience is also an important factor. Fabricius (1951a) early reported the fact that his ducklings persisted in following older ducklings who maltreated them vigorously. Hess (1959c) observed that when ducklings were being imprinted to a human being who carelessly stepped on their toes, they did not run away in fear but stayed even closer. Kovach and Hess (1963) have demonstrated experimentally that during the critical-age period the administration of painful electric shocks enhances the amount of following during the imprinting experience, while after the critical age electric shocks depress following. These results emphasize the importance of the critical-age period; during it imprinting is enhanced by punishment, while after it imprinting is hindered, suggesting that the processes occurring after the critical period conform far more closely to the laws of conventional association learning than do the processes occurring during the critical period.
Other variables. While a perusal of the research literature shows that young birds can respond to an amazingly wide variety of different objects in the same way in which they would respond to their own mother, there are also indications that some characteristics of a potential imprinting object are more effective in eliciting imprinting than are others. Ramsay (1951), for example, indicated early that color was important, while size and shape appeared to be less so. Others (Lorenz 1935; Fabricius 1951a) have shown that rhythmic sounds are more effective than other types of sounds, that low-pitched ones arouse imprinting more than high-pitched ones (Collias & Collias 1956), and so forth. Schaefer and Hess (1959) have demonstrated that certain colors are more capable of eliciting imprinting, and that certain shapes are more potent than others in terms of both following behavior during imprinting and test scores when the chicks were confronted at a later age with the imprinting model and unfamiliar ones.
Food imprinting. Hess (1962; 1964) has suggested that imprinting, conceived of as the rapid acquisition of an object toward which an innate response could be directed, is not limited to the formation of social bonds but also applies to the learning of food objects by chicks at the third day of age. He carried out experiments in which chicks were given food reward for pecking at specific stimuli and were not rewarded for pecking at others. In the face of withdrawal of food, the effects of this experience were strongest when the rewarding had occurred during the third day of age; the greater the time interval between that age and the time of rewarding, the less was the effect of the experience. When the rewarding had occurred at the age of one day or at more than five days, there was no effect on the pecking behavior; but when the chicks had been rewarded at the age of three days, they continued to peck at the specific stimuli for at least ten days without any signs of decreasing intensity, even though there was no longer any food reward.
Furthermore, Hess (1962; 1964) found that if the food-reward experience took place while the three-day-old chicks were under the influence of either meprobamate or carisoprodol, when food reward was removed the chicks behaved essentially as if they had never had the reward experience. Hess therefore concluded that this phenomenon showed much more similarity to imprinting than to more common association-learning processes and must represent true imprinting of food objects. He did not, of course, claim food imprinting and social imprinting to be alike in all respects, for they are related to two very different vital functions—social cohesiveness, necessary for the survival of a social species, and ingestion of nutritious material, necessary for the survival of the individual.
Imprinting and learning. While in both imprinting and learning, a relationship or “connection” is established between an object and a response, there is a basic distinction between the two processes. In imprinting there is a critical period, developmentally timed, during which certain wide classes of stimuli act as releasers or unconditioned stimuli for certain types of innate responses; whereas, in ordinary association learning, the object in question does not act as an unconditional stimulus for the response but is initially neutral in its effect on the animal’s behavior. There is no critical period for association learning, although cases have been found where learning ability increases with age, in contrast to the decreasing of imprintability as the animal grows older.
Association learning is more effective when practice trials are spaced than when they are massed, whereas in imprinting massed effort has been found to facilitate the formation of imprinted social bonds.
Furthermore, Hess (1962; 1964) has reported that meprobamate and carisoprodol do not at all hinder either the learning or the retention of ordinary visual-discrimination learning with food as a reward, whereas it impedes both social and food imprinting.
Finally, it is well established that in association learning it is what has been most recently learned that is remembered the best. However, in imprinting, what has been learned first is the strongest, as shown by experiments in which ducklings were exposed to two different imprinting models during the critical-age period (Hess 1959a; 1959b; 1959c). The studies of the effects of socialization with siblings on imprintability to a parent model also demonstrate the importance of primacy in imprinting (Hess 1964).
It is of extreme importance to be aware of these differences between imprinting and association learning, because once the appropriate critical-age period has passed without imprinting’s having occurred, for lack of exposure to a suitable object, it is possible to use any stimulus to which the animal could have responded earlier as a potential conditioned stimulus, to which it may be trained, through conventional means, to make conditioned responses. Here the animal can readily generalize to other objects, thus increasing the range to which it can make conditioned responses, in contrast to the fact that when an animal has actually been imprinted to an object during the critical period, only this particular object or ones very much like it will, from then on, act as unconditional stimuli, though the range to which the animal was first capable of responding was very broad. What is more, even if imprinting has already occurred during the critical period, the animal can still be trained, through association learning, to make conditioned responses to objects to which it has not been imprinted. In such a case the response to the imprinted object may seem, superficially, to be just like the conditioned responses that the animal has been trained to make to a conditioned stimulus. But these two categories of responses are completely different in terms of the conditions of their origins and also in terms of their long-range effects on the character of the animal’s behavior.
Both social imprinting and food imprinting have counterparts in association learning. Taming is the association-learning counterpart of social imprinting; humans can tame an animal that has already been imprinted to its own species. But the two social bonds are not alike, for a tamed animal will court and attempt to mate with members of the opposite sex of its own species, not with human beings. Skinnerian experiments, in which birds are trained to peck at colored lights in order to obtain food or water, reflect the association-learning counterpart of food imprinting. But upon complete withdrawal of food reward, the bird’s pecking response to a colored light will soon disappear, while chicks imprinted to certain food objects when three days old do not lose their acquired pecking habits, even after long experience with no food reward.
Eckhard H. Hess
Alley, R.; and Boyd, H. 1950 Parent-Young Recognition in the Coot. Ibis 92:46-51.
Baerends, G. P.; and Baerends-van Roon, J. M. 1950 An Introduction to the Study of the Ethology of Cichlid Fishes. Behaviour Supplement no. 1. Leiden (Netherlands): Brill.
Bowlby, john 1951 Maternal Care and Mental Health. Geneva: World Health Organisation.
Collias, Nicholas E.; and Collias, Elsie C. 1956 Some Mechanisms of Family Integration in Ducks. Auk 73: 378-400.
Craig, Wallace 1908 The Voices of Pigeons Regarded as a Means of Social Control. American Journal of Sociology 14:86-100.
Fabricius, Eric 1951a Zur Ethologie junger Anatiden. Acta zoologica fennica 68:1-178.
Fabricius, Eric 1951b Some Experiments on Imprinting Phenomena in Ducks. Pages 375-379 in International Ornithological Congress, Tenth, Uppsala, 1950, Proceedings. Uppsala (Sweden): Almqvist & Wiksell.
Fabricius, Eric; and Boyd, H. 1954 Experiments on the Following Reactions of Ducklings. Wildfowl Trust, Annual Report 6:84-89.
Goodwin, D. 1948 Some Abnormal Sexual Fixations in Birds. Ibis 90:45-48.
Gottlieb, Gilbert 1961a Developmental Age as a Baseline for Determination of the Critical Period in Imprinting. Journal of Comparative and Physiological Psychology 54:422-427.
Gottlieb, Gilbert 1961b The Following-response and Imprinting in Wild and Domestic Ducklings of the Same Species. Behaviour 18:205-228.
Guiton, Philip 1958 The Effect of Isolation on the Following Response of Brown Leghorn Chicks. Royal Physics Society of Edinburgh, Proceedings 27:9-14.
Guiton, Philip 1959 Socialization and Imprinting in Brown Leghorn Chicks. Animal Behavior 7:26-34.
Harlow, Harry F. 1958 The Nature of Love. American Psychologist 13:673-685.
Harlow, Harry F.; and Harlow, Margaret K. 1962 Social Deprivation in Monkeys. Scientific American 207, Nov.: 136-146.
Heinroth, Oskar; and Heinroth, Magdalena 1924-1928 Die Vaögel Mitteleuropas. 3 vols. Berlin: Bermühler.
Hess, Eckhard H. 1957 Effects of Meprobamate on Imprinting in Waterfowl. New York Academy of Sciences, Annals 67:724-732.
Hess, Eckhard H. 1959a Imprinting. Science 130:133-141.
Hess, Eckhard H. 1959b The Relationship Between Imprinting and Motivation. Volume 7, pages 44-77 in Nebraska Symposium on Motivation. Edited by Marshall R. Jones. Lincoln: Univ. of Nebraska Press.
Hess, Eckhard H. 1959c Two Conditions Limiting Critical Age for Imprinting. Journal of Comparative and Physiological Psychology 52:515—518.
Hess, Eckhard H. 1962 Imprinting and the “Critical Period” Concept. Pages 254-263 in Eugene L. Bliss (editor), Roots of Behavior. New York: Harper.
Hess, Eckhard H. 1964 Imprinting in Birds. Science 146:1128-1139.
Hess, Eckhard H.; Polt, James M.; and Goodwin, Elizabeth 1959 Effects of Carisoprodol on Early Experience and Learning. Pages 51-64 in James G. Miller (editor), The Pharmacology and Clinical Usefulness of Carisoprodol, Detroit, Mich.: Wayne State Univ. Press.
Hinde, R. A. 1955 The Following Response of Moorhens and Coots. British Journal of Animal Behaviour 3: 121-122.
Hinde, R. A.; Thorpe, W. H.; and Vince, M. A. 1956 The Following Response of Young Coots and Moorhens. Behaviour 9:214-242.
Hodge, W. H. 1946 Camels of the Clouds. National Geographic Magazine 89:641-656.
Klinghammer, Erich 1962 Imprinting in Altricial Birds: The Ring Dove Streptopelia roseogrisia and the Mourning Dove Zenaidura macruora carolensis. Ph.D. dissertation, Univ. of Chicago.
Klinghammer, Erich; and Hess, Eckhard H. 1964 Imprinting in an Altricial Bird: The Blond Ring Dove Streptopelia risoria. Science 146:265-266.
Kovach, Joseph; and Hess, Eckhard H. 1963 Imprinting: Effects of Painful Stimulation on the Following Response. Journal of Comparative and Physiological Psychology 56:461-464.
Lorenz, K. Z. 1935 Der Kumpan in der Umwelt des Vogels. Journal fiir Ornithologie 83:137-213, 289-413
Moltz, Howard 1960 Imprinting: Empirical Basis and Theoretical Significance. Psychological Bulletin 57: 291-314.
Moltz, Howard; and Stettner, L. Jay 1961 The Influence of Patterned-light Deprivation on the Critical Period for Imprinting. Journal of Comparative and Physiological Psychology 54:279-283.
Morgan, C. Lloyd 1896 Habit and Instinct. London and New York: Arnold.
Ramsay, A. O. 1951 Familial Recognition in Domestic Birds. Auk 68:1-16.
Ramsay, A. O.; and Hess, E. H. 1954 A Laboratory Approach to the Study of Imprinting. Wilson Bulletin 66:196-206.
Schaefer, Halmuth H.; and Hess, Eckhard H. 1959 Color Preferences in Imprinting Objects. Zeitschrift fur Tierpsychologie 16:161-172.
Shipley, William U. 1963 The Demonstration in the Domestic Guinea Pig of a Process Resembling Classical Imprinting. Animal Behaviour 11:470-474.
Sluckin, W. 1964 Imprinting and Early Learning. London: Methuen.
Sluckin, W.; and salzen, E. A. 1961 Imprinting and Perceptual Learning. Quarterly Journal of Experimental Psychology 13:65-77.
Spalding, Douglas A. (1873) 1954 Instinct, With Original Observations on Young Animals. British Journal of Animal Behaviour 2:2-11.
"Imprinting." International Encyclopedia of the Social Sciences. . Encyclopedia.com. (April 27, 2017). http://www.encyclopedia.com/social-sciences/applied-and-social-sciences-magazines/imprinting
"Imprinting." International Encyclopedia of the Social Sciences. . Retrieved April 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/social-sciences/applied-and-social-sciences-magazines/imprinting
Imprinting refers to the chemical modification of the DNA in some genes that affects how or whether those genes are expressed. One particular kind of DNA imprinting found in mammals is known as parental genomic imprinting, in which the sex of the parent from whom a gene is inherited determines how the gene is modified. While imprinting has been found in only about fifty human genes to date, some estimates suggest it may occur in several hundred more, in perhaps up to 1 percent of all genes. Imprinting defects are responsible for several human diseases, including some forms of cancer. Imprinting also occurs in other organisms, from yeast to plants to fruit flies.
Gene Expression in Imprinted and Nonimprinted Genes
Chromosomes, and the genes they contain, are inherited in pairs, with one copy of each supplied from each parent. For most genes, both members of the pair, called the maternal and paternal alleles, are used equally. Both are expressed (read by the transcription machinery to make protein) in roughly equal amounts.
In contrast, for most imprinted genes, only one allele is expressed, while the other copy is silenced by imprinting. For some genes it is the maternal copy, for others it is the paternal copy. This is an exception to the Mendelian assumption that the two parents contribute equally to the phenotype controlled by autosomal genes. For some genes, both alleles are expressed, but one copy is expressed much more than the other. For some genes, the silencing occurs in some tissues but not others.
Imprinted genes should not be confused with sex-linked genes, which are carried on the X or Y chromosome. Most imprinted alleles are located on autosomes , but are "stamped" with the sex of the parent that contributed them.
Imprinting should also not be confused with dominant and recessive alleles, in which one allele always controls the phenotype at the expense of the other, because of differences in the alleles themselves. The "dominance" seen in imprinting is determined by the sex of the parent contributing the allele, not any property of the allele itself. Thus, a particular allele will appear to be recessive when inherited from one parent, but dominant when inherited from the other. Such an effect, in which the expression difference is not due to the alleles but to forces acting on them from outside, is termed an "epigenetic effect."
Imprinting is thought to be responsible for many cases of incomplete penetrance, an inheritance pattern in which a dominant gene (as for a genetic disease) is not expressed in some individuals despite being present. Imprinting offers a mechanism by which a particular allele can be turned on or turned off as it is passed down through successive generations.
Timing and Mechanism of Imprinting
Although the details of imprinting are still unknown, it is clear that imprinting must occur either during the formation of the gametes or immediately after fertilization, while the two chromosome sets are still distinct. The imprint must be reliably passed on to each new daughter chromosome during DNA replication.
The exact molecular mechanism of imprinting is also unknown, but it is thought to involve the modification of a gene's promoter. The promoter is the upstream region to which RNA polymerase binds to begin transcription. Imprinting prevents or restricts binding of RNA polymerase, thus preventing gene transcription.
One method by which a gene becomes imprinted is believed to be by the addition of methyl groups (-CH3) to cytosine nucleotides in the promoter region. The evidence for methylation is strong. Methylation is a common mechanism for gene silencing, because these bulky side groups interfere with the efficient binding of the various transcription factors required to attract the polymerase enzyme. Methylation patterns are known to be altered during gamete formation, and are reliably passed on during replication. Further evidence comes from the observation that altered methylation patterns in some imprinted genes are associated with the aberrant expression of the normally silent allele.
Example of Imprinting: The IGF2 Gene
One of the best-studied imprinted genes is the one that encodes an insulin-like growth factor called growth factor 2 (IGF2). In this gene, the paternal copy is active, whereas the maternal copy is inactive. Imagine that two parents have produced a female child. During egg formation in the mother (or shortly after fertilization), the mother's copy of the IGF2 gene is methylated , rendering it transcriptionally silent. The child uses only the paternal allele to make the growth factor. However, when this child makes her own eggs, neither copy of the gene will remain active, because the alleles will have been "restamped" as coming from a female. The active allele she used throughout life is passed on in an inactive form to her children.
The protein encoded by the IGF2 gene is a growth factor, which stimulates the growth of target cells. Failure to properly imprint the maternal allele, or inheritance of two copies of the male allele, can have important consequences. For example, the expression of two copies of the IGF2 gene is associated with Beckwith-Wiedemann syndrome, a growth disorder, accompanied by an increase in a type of cancer called Wilms tumor . Other human cancers are also associated with improper imprinting (of other genes), causing either too much or too little gene expression.
Uniparental Disomy and Human Disease
Inheritance of two copies of one parent's chromosome (or part of it) is called uniparental disomy, a type of chromosome aberration. Detection of uni-parental disomy in individuals with genetic disorders was one of the first clues that imprinting had important developmental and medical consequences.
Prader-Willi syndrome and Angelman syndrome can both be caused by uniparental disomy of chromosome 15, which carries a maternally expressed, paternally imprinted gene. Two maternal copies of the gene causes Prader-Willi syndrome, which is marked by mild mental retardation, decreased growth of the gonads , and obesity. Two paternal copies of this same gene causes Angelman syndrome, marked by severe mental retardation, small head size, seizures, inappropriate laughter, and distinctive facial features. (The gene itself codes for a protein involved in degrading other proteins.) Imprinting defects can also cause these syndromes in the absence of uniparental disomy, since the result is the same: either zero or two copies of the gene are expressed.
The evolutionary reason for imprinting is not yet clear, although some scientists propose that, at least in mammals, it arose from an evolutionary tug of war between males and females. In this scheme, fathers (who contribute only sperm) benefit when the embryo grows as fast as possible. Thus, silencing genes that slow down embryonic growth is in their interest, even if it depletes resources from the mother. Mothers, on the other hand, need to conserve their resources. Silencing genes that promote rapid growth is therefore in their interest. Supporting this hypothesis is the fact that many of the known imprinted genes regulate growth. Paternally expressed (maternally imprinted) genes such as IGF2 tend to promote growth, whereas maternally expressed (paternally imprinted) genes tend to inhibit it.
see also Chromosomal Aberrations; Fertilization; Inheritance Patterns; Methylation; RNA Polymerases.
Everman, David B., and Suzanne B. Cassidy. "Genomic Imprinting: Breaking the Rules." Journal of the American Academy of Child and Adolescent Psychiatry 39, no. 3 (March 2000): 386-389.
Greally, John M., and Matthew W. State. "Genomic Imprinting: The Indelible Mark of the Gamete." Journal of the American Academy of Child and Adolescent Psychiatry 39, no. 4 (April 2000): 532-535.
Paulsen, Martina, and Anne C. Ferguson-Smith. "DNA Methylation in Genomic Imprinting, Development, and Disease." Journal of Pathology 195, no. 1 (2001): 97-110.
Yale University School of Medicine and Yale-New Haven Hospital. <http://info.med.yale.edu>.
"Imprinting." Genetics. . Encyclopedia.com. (April 27, 2017). http://www.encyclopedia.com/medicine/medical-magazines/imprinting
"Imprinting." Genetics. . Retrieved April 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/medical-magazines/imprinting
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 it expects 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 to have 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.
see also Behavior; Behavioral Ecology.
Alcock, John. Animal Behavior, 4th ed. Sunderland, MA: Sinauer Associates, 1989.
Curtis, Helena. Biology. New York: Worth Publishers, 1989.
Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W.W. Norton, 1996.
Halliday, Tim. Animal Behaviour. London: Blandford, 1994.
Krebs, John R., and Nicholas B. Davies. Behavioral Ecology: An Evolutionary Approach, 4th ed. Cambridge, MA: Blackwell Science, 1997.
"Imprinting." Animal Sciences. . Encyclopedia.com. (April 27, 2017). http://www.encyclopedia.com/science/news-wires-white-papers-and-books/imprinting
"Imprinting." Animal Sciences. . Retrieved April 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/imprinting
A type of learning characteristic of fowls that occurs only during a critical period of development soon after birth.
Imprinting is the process that prompts ducklings to form an attachment to their mothers—or whatever other moving object that appears—within the first two days of life. Ethologists, scientists who study the behavior of animals in their natural environment , noted the process of imprinting as they observed newly hatched ducklings. They discovered that if a duckling were introduced to another moving object, alive or not, during a critical period after birth , the duckling would follow that object as if it were the mother. Humans and even wooden decoys successfully served as maternal substitutes after as little as ten minutes of imprinting. It has been discovered that once the process takes place, the ducklings will follow the substitute, even through adverse circumstances, in preference to a live duck. Imprinting does not take place anytime after the first two days of life because by that time, it is believed, ducklings develop a fear of strange objects. There is little evidence that imprinting occurs in humans or most other animals. It has been noted to some
extent in dogs, sheep, and guinea pigs. The discovery and study of imprinting have prompted continued examination of the relative roles of instinct and acquired behavior in the process of learning.
Bower, Gordon H., and Ernest R. Hilgard. Theories of Learning. Englewood Cliffs, NJ: Prentice-Hall, 1981.
"Imprinting." Gale Encyclopedia of Psychology. . Encyclopedia.com. (April 27, 2017). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/imprinting
"Imprinting." Gale Encyclopedia of Psychology. . Retrieved April 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/imprinting
imprinting, acquisition of behavior in many animal species, in which, at a critical period early in life, the animals form strong and lasting attachments. Imprinting is important for normal social development. The term was first used by the zoologist Konrad Lorenz to describe the way in which the social characteristics of greylag geese and other fowl become instilled in their young offspring (see ethology). In natural circumstances imprinting, to the mother, food, or surroundings, occurs instinctively during a biologically fixed time span; it is very difficult to extinguish. Under experimental conditions chicks and ducklings readily become imprinted to an appropriate model such as a moving decoy or a human being. Subsequent learning may be tied to and reinforced by the imprinted object, and later social behaviors, such as the greeting ceremony and courtship, may be directed exclusively to the mother-substitute. In fowl, attachment increases with the amount of effort the offspring must exert to follow the imprinted object. The onset of fear in an organism is believed to end the period of imprintability. There is evidence that in fowl the imprinting period begins before hatching and is characterized by vocal communication between mother and unhatched ducklings.
"imprinting." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (April 27, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/imprinting
"imprinting." The Columbia Encyclopedia, 6th ed.. . Retrieved April 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/imprinting
"imprinting." A Dictionary of Zoology. . Encyclopedia.com. (April 27, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/imprinting-0
"imprinting." A Dictionary of Zoology. . Retrieved April 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/imprinting-0
"imprinting." A Dictionary of Ecology. . Encyclopedia.com. (April 27, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/imprinting
"imprinting." A Dictionary of Ecology. . Retrieved April 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/imprinting
1. (in behaviour) A specialized form of learning in which young animals, during a critical period in their early development, learn to recognize and approach some large moving object nearby. In nature this is usually the mother, though simple models or individuals of a different species (including humans) may suffice. Imprinting was first described by Konrad Lorenz, working with young ducks and geese. See learning .
2. (in genetics) See molecular imprinting.
"imprinting." A Dictionary of Biology. . Encyclopedia.com. (April 27, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/imprinting-1
"imprinting." A Dictionary of Biology. . Retrieved April 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/imprinting-1
"imprinting." World Encyclopedia. . Encyclopedia.com. (April 27, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/imprinting
"imprinting." World Encyclopedia. . Retrieved April 27, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/imprinting