The Development and Modification of Object Preferences in Domestic White Leghorn Chicks ALBERT A. EINSIEDEL, JR. Psychology Department indiana University-Purdue University Indianapolis, Indiana
This study examined in domestic chicks the time-course of adaptation to a new object after having established strong preferences for a previous stimulus (during the first 3 days of fife). By interchanging the stimuli to which the animal was exposed, and by confining the animal exclusively with the new object for some period of time, the new preference was expected t o develop as a function of the duration of exposure. The results confirm that expectation with a monotonic increase in preference as a function of hours of exposure. The establishment of a preference for the new object was rapid, as indicated by a significant shift in preference after 5 hr.
One criterion for imprinting formulated by Lorenz (1937) appears to have become the subject of much more controversy than the others, namely, the claim that imprinting is irreversible. Reversibility of social attachments implies that the originally established social bonds are modifiable, but does not specify what changes might occur. Traditionally, “irreversibility” implies that the 1st imprinting experience has priority over the subsequent one; its effects, therefore, are resistant to change (Hess, 1959). The data concerning the reversibility assumption have not always been supportive of this view (e.g., Hinde, 1970). Salzen (1966) argued that the available evidence on the irreversibility of imprinted preferences in the social and sexual behavior of birds suffers from methodological problems, thus weakening the support it purports to give this widely held assumption. Salzen offered his own interpretation, the neuronal model, emphasizing imprinting to be an experiential, rather than a maturational, phenomenon; i.e., an imprinted preference should be capable of being reversed if the imprinted bird is confined with a new stimulus in the absence of the original object. (Salzen and Meyer [1968] subsequently demonstrated the reversal of the social preferences of domestic chicks in the 1st week after hatching by substituting a new object for the original one.) Whether the reversibility assumption is only partially or wholly wrong is still unclear. Reports by Bateson (1974), Connolly (1968), Salzen (1962), Sluckin (1962), Sluckin and Salzen (1961), Zajonc, Reimer, and Hausser (1973), and Zajonc, Marcus, and Wilson (1974) lend support to the idea that the longer the bird has been exposed to the object, the better able it is to respond positively to that object. However, some questions remain Received for publication 17 October 1974 Revised for publication 31 January 1975 Developmental PsychobioEogy, 8(6): 533-540 (1975) 0 1 9 7 5 by John Wiley & Sons, Inc.
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to be addressed more fully: How much exposure to the new object is required to significantly alter existing preferences? How rapidly will these changes take place? Will the new experience require “reversal,” in the classic sense, of the old preferences or will the chick learn to accept new objects without necessarily rejecting the old one? Thus, strong preferences for certain stimuli must be established and attempts then made to modify these preferences by changing the stiinulus to which the bird is forcibly exposed. By testing the effects of this new experience at various times after the interchanges, the development of the new preference can be plotted as well as changes in the old. The expectation was simple and straightforward: the longer the new experience, the stronger the new preference.
Method
Subjects The eggs, purchased from the Indianapolis Farm Bureau Co-op, were brought from storage and incubated in our laboratory using a Humidair #50 incubator. They were hatched in the dark in a Humidair #SO hatcher. The temperature for both units ranged from 37.5” t o 38.3”C. The humidity level, measured only in the case of the incubator, was set at 85.5%. Approximately 10-12 hr after hatching, the chicks (Callus gullus) were transferred to the isolation-rearing or “imprinting” cages. No attempt was made to physically isolate individual eggs or newly hatched chicks during the hatching period or to identify subjects by sex.
Rearing and Maintenance Each isolation-rearing cage, 35 x 45 x 40 cm, was constructed of sheet metal, equipped with a wire mesh floor, and provided with individual water and food dispensers. Continuous illumination in each cage was provided by a 25-W white incandescent bulb mounted on the rear wall approximately 35 cm above the wire mesh floor. In addition to the heat generated by this light source, heat was provided by a black 25-W incandescent bulb. The ambient temperature in all the units was regulated thermostatically between 33.3” and 35.0”C. Once placed in their isolation-rearing units, the chicks were visible to the experimenter only through a “fish-eye” viewer mounted 8 cm from the top and in the center of the front wall of each cage.
Design Each bird had 2 periods which we shall hereafter refer to as Phase I (Primacy) and Phase 11 (Recency) exposures. In all cases, Phase I began 10-12 hr after hatching and lasted for 72 hr, after which each bird was given a choice test. Phase I1 began immediately following that test. The 140 birds were equally and randomly assigned to 1 of 5 Phase I1 conditions: 5 min, 5 hr, 10 hr, 25 hr, or 72 hr. Each bird was tested again at the end of Phase 11. Within each exposure condition, the 28 subjects were further divided into 2
OBJECT PREFERENCES IN CHICKS
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subgroups, the Interchange and the Noninterchange groups. Interchange subjects were those who, after the 1st test, were given new training objects for Phase TI, whereas the Noninterchange subjects retained the same object which they had in Phase I. For the Interchange subjects, the 1st object was called the Primacy Object and the 2nd one, the Recency Object. For the Noninterchange subjects, the 2 test objects involved were referred to as Novel or Familiar. Of the 14 subjects in each cell in the 2 x 5 design, half were exposed initially to a blue object and the remaining half to the green. Thus, 70 chicks were exposed t o blue and 70 to green during Phase I. Among the Interchange subjects, therefore, those that had blue in Phase I were given green in Phase 11, and those given green in Phase I were given blue in Phase 11. For Noninterchange subjects, half were completely blue-reared and the other half, green-reared.
Training Stimuli The object to which the animal was exposed in its home cage was either a blue or a green pillow-like cloth disc whch was approximately 5 cm in diameter. The disc hung freely from the center of the cage ceiling by a string such that the bottom of the disc was approximately 3.8 cm from the wire mesh floor. The choice of blue and green as the colors to be used in this study was based upon previously reported results of experiments in which these colors were found to be more or less equally effective in eliciting approach responses (Kovach, 1971, Schaefer & Hess, 1959).
Home Observation Pretest home observations (5-min) were conducted to measure the time spent by the subject near the training stimulus while in a familiar home environment. The observation period was started by a quick removal and replacement of the training object.
Testing The test apparatus consisted of an enclosed chamber, 76 x 71 x 40 cm, constructed out of sheet metal, and lined on the outside with acoustical board to minimize vibration and transmission of extraneous noise. The objects used for testing were a green and a blue disc identical to the training stimuli. They were suspended from an arm at the top of the test box and hung freely, approximately 40 cm apart and 3.8 cm from the wire mesh floor. The left-right position of these discs was changed by simply rotating the arm to which the discs were attached. The position of the objects remained stationary during each trial, although the objects moved upon being hit by the subject. The temperature in the test box ranged from 29.4" to 35.0"C. A 25-W black bulb remained illuminated throughout the test to supplement the heat from a white 25-W bulb that was controlled from the outside. In a typical testing situation, the chick was placed manually through a small door onto a starting position equidistant to, and facing, both the test objects. The location and behavior of the chick were recorded manually at 5-sec intervals. Testing consisted of 2 consecutive 5-min trials with a 5-sec intertrial pause, during which time the chick was removed from the box and the position of the test objects reversed. Observations of the
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chicks could be made only through a “fish-eye” viewer mounted at the center of the front wall. Of major importance in this study was the amount of time the chick spent with the test object. The chick was said to be with an object if any part of its body was judged to be within a circle 12.7 cm in diameter drawn on the paper beneath each of the 2, test objects, and remaining in that circle at least 3 sec during each 5-sec interval. The combined floor area of the 2 circles made up less than 3% of the total floor area (5396 sq cm) .
Results In the 1st home observation, the overall average time spent with the object was 15.27 intervals out of a maximum of 60 5-sec intervals. The Interchange and Noninterchange subjects were no different from each other based on this observation. Test I data give an indication of the strength of the preference for the familiar and the novel objects. Regardless of the color in Phase I, all subjects spent a mean of 41.53 5-sec intervals near the familiar object, and 1.1 intervals near the unfamiliar object ( t = 17.89, df = 139, p < .001; 2-tailed test). On the average, the chcks spent 34.6% of the total test time with the familiar object and virtually the remainder of the time in the “field,” but not near the novel object. Blue-reared subjects spent an average of 40.79 intervals with the blue object, whereas the green-reared subjects spent 42.27 intervals with the green stimulus. This difference was not significant, indicating no “color bias”, an important factor to consider when modifying color preference by substitution (e.g., Taylor, Sluckin, & Hewitt, 1969). The 2nd home observation showed no effects attributable to the length of exposure to the training object in Phase I1 nor to whether the subjects were in the Interchange or Noninterchange conditions. On the average, the subjects in all conditions spent 19.42 intervals with the object (see Fig. 1). With the exception of the 5-Min groups, the tendency to remain beside the training object was fairly stable for different age animals. Merely a few hours, perhaps as few as 5, were required for the chicks in the Interchange condition to adapt to a strange object in their cage (Fig. 1). After the initial “acquaintance period”, which took anywhere from a few minutes to a few hours, the subjects appeared to have returned to their “normal” pattern of responding, including the normal level of interaction with the object, as evidenced by the absence of a difference between the Interchange and Noninterchange groups. An attempt was made to match the Interchange and Noninterchange subjects within each Phase I1 exposure condition on the basis of Test I scores. However, due t o scheduling constraints and mortality, systematic assignments were not always possible. Nevertheless, a post hoe comparison of the scores on the familiar object during Test I1 by the Interchange and Noninterchange subjects revealed that the 2 groups were comparable in terms of their responsiveness toward the familiar test object. For the blue-reared subjects, Interchange subjects spent an average of 41.17 intervals with the blue object whereas the Noninterchange subjects spent an average of 40.40 intervals. For green-reared chicks, the Interchange subjects had a mean score on the green object of 41 2 3 whereas the Noninterchange had 43.31 (differences were not significant). In the case of the Interchange subjects, an increased preference for the Recency
_
OBJECT PREFERENCES IN CHICKS
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LENGTH OF PHASE I1 EXPOSURE (2ND O B s ~ A T I c N l
Fig. 1. Time spent by Interchange and Noninterchange subjects with the training object during the 5-min home-cage observation preceding Test 11.
Object and a concurrent trend toward a diminished preference for the Primary Object apparently exists (see Fig. 2). A significant Exposure Condition x Object Condition interaction is clearly seen by the crossover of the 2 Interchange curves in Figure 2. Whether the object is the Primacy or the Recency stimulus affected the level of responsiveness of the subject toward it, as shown by the significant within-subjects Object factor ( F = 15.58, df = 4/135, p < .Ol). The longer the exposure to the Recency Object, the greater the preference that was developed for it; alternately the greater the exposure to the Recency Object and, thus, the longer the absence of the Primacy Object, the lesser the preference exhibited toward the latter. After 72 hr, the Recency Object was preferred over the Primacy Object, the difference between the 2 means being highly significant ( t = 3.07, df= 13, p < .Ol). The test data of the Noninterchange groups (see Fig. 2) show unequivocal support for the view that exposure leads to the development of strong preference for the familiar object. Noninterchange subjects spent more time with the Familiar Object than with the Novel Object (F = 350.19, df = 1/135, p < .001). In fact, the Noninterchange groups remained unresponsive to the object never encountered outside the testing situation, i.e., the Novel stimulus. This low level of responsiveness was consistent over all groups (Fig. 2). On the other hand, the Interchange subjects’ scores on the Recency Object show a trend toward preference as a function of exposure length. Comparing the Novel Object scores of the Interchange groups, no reliable difference was found between the groups given
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INTERCHANGE GROUPS' RECENCY OBJECT SCORES INTERCHANGE GROUPS' PRIMACY OBJECT SCORES NON-INTERCHANGE GROUPS' FAMILIAR OBJECT SCORES NON-INTERCHANGE GROUPS' NOVEL OBJECT SCORES
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Fig. 2. Time spent by Interchange and Noninterchange subjects with the Familiar/Primacy and Novel/Recency objects.
only 5 min of Phase I1 experience (t = .04, df= 26, p > .05).After 5 hr, however, the difference between these scores was marginally significant (t = 1.86, df = 26, p < -10) and the direction of the difference was as predicted. The difference between these scores continued to increase with longer exposure to the Recency stimulus, reaching its biggest difference after 72 hr ( t = 9.74, d f = 26, p < .OOl>. The Interchange animals' hghest level of responsiveness toward the new object (at 72 hr) was lower than the level of responsiveness of the Noninterchange controls a t the same age (t = 6.13, df = 26, p < .OOl) to the Familiar stimulus.
Conclusions The results of this study support the expectation that the longer the animal i s exposed to a novel object, the stronger its preference for that object. This outcome is consistent with the previous findings by Salzen and Meyer (1968), Sluckin and Salzen (1961), Zajonc et al. (1973, 19741, and within our own laboratory. The new information presented here shows that new preferences develop rapidly in the course of an animal's exposure to a new object. Approach and subsequent interactive responses (e.g., pecking) become organized with respect to the new object withm 5 hr after the initial presentation of that object. Although this may not be ilfimediately obvious in the 2-object contrast test situation, it is evident in the home-cage behavior of the animal. This evidence does
OBJECT PREFERENCES IN CHICKS
539
not support the assumption of “irreversibility” in the sense that new preferences cannot be established, and casts doubt upon the idea of a critical period as being an exclusive phase for the development of differential approach in the case of chicks. Although the present data appear to show evidence for “reversibility” as advanced by Salzen and Meyer (1968), i.e., an increase in the preference for the new object and a concomitant decrease in the preference for the old one, whether such would be true for another testing procedure that measures “independent” preferences of the animal is not certain. In a contrast test involving the simultaneous presentation of the test objects such as the one used in this study, the animal may approach and spend some time with one or the other object, with both, or with neither one. If the animal spends time with both objects, it may do so by distributing the time equally between them or, as is more often the case, spend more time with one than with the other. Because of the nature of the contrast test procedure, we do not know just what became of the preference for the original object. The present data merely indicate an apparent loss of preference. Future research might find the use of a noncontrast test appropriate or, alternatively, a 2-object test for the Primacy or Recency preference by using a novel color as the 2nd stimulus. However, complete “reversibility” may nevertheless occur after considerable periods of enforced confmement with a new object, and such a phenomenon may be demonstrated with an appropriate test. The overall conclusion drawn from this research is that the young animal is continuously adapting its responses to the events that are present in its environment. The effects of earlier experience are important but so, too, are the effects of recent ones in the establishment of object preference in domestic chicks.
Notes Request reprints from: Dr. Albert A. Einsiedel, Jr., Psychology Department, Mather Memorial Building, Case Western Reserve University, Cleveland, Ohio 44106, U.S.A.
References Bateson, P. P. G. (1974). Length of training, opportunity for comparison, and imprinting in chicks. J. Comp. Physiol Psychol., 86: 586-589. Connolly, K. J. (1968). Imprinting and the following response as a function of training in domestic chicks. Br. J. Psychol, 5: 453-460. Hess, E. H. (1959). Two conditions limiting critical age for imprinting. J. Comp. Physiol. PsychoL, 52: 515-5 18. Hinde, R. A. (1970). Animal Behaviour. New York: McGraw-Hill. Kovach, J. K. (1971). Effectiveness of different colors in the elicitation and development of approach behavior in chicks. Behaviour, 38: 154-168. Lorenz, K. (1937). The companion in the bird’s world.Auk, 54: 245-273. Salzen, E. A. (1962). Imprinting and fear. Symp. Zool. SOC.L o n d , 8: 199-218. Salzen, E. A. (1966). The interaction of experience, stimulus characteristics and exogenous androgen in the behavior of domestic chicks. Behaviour, 26: 286-322. Salzen, E. A., and Meyer, C. C. (1968). Reversibility of imprinting. J. Comp. Physiol. PsychoZ., 66: 269-275. Schaefer, H. H., and Hess, E. H. (1959). Color preferences in imprinting objects. 2. Tierpsychol, 16: 161-172.
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Sluclan, W. (1962). Perceptual and association learning. Syrnp. Zool. SOC.Lond., 8: 193-198. Sluckin, W., and Salzen, E. A. (1961). Imprinting and perceptual learning. Q. J. Exp. PsychoZ., 13: 65-77. Taylor, A., Sluckin, W., and Hewitt, R. (1969). Changing colour preferences of chicks. Anim. Behav., 17: 3-8. Zajonc, R. B., Marcus, H., and Wilson, W. R. (1974). Object preference, and distress in the domestic chick. J. Comp. Physiol. Psychol., 86: 581-585. Zajonc, R. B., Reimer, D. J., and Hausser, D. (1973). Imprinting and the development of object preference in chicks by mere repeated exposure. J. Cornp. Physiol. PsychoZ., 83: 434440.
This study examined in domestic chicks the time-course of adaptation to a new object after having established strong preferences for a previous stimulus (during the first 3 days of fife). By interchanging the stimuli to which the animal was exposed, and by confining the animal exclusively with the new object for some period of time, the new preference was expected t o develop as a function of the duration of exposure. The results confirm that expectation with a monotonic increase in preference as a function of hours of exposure. The establishment of a preference for the new object was rapid, as indicated by a significant shift in preference after 5 hr.
One criterion for imprinting formulated by Lorenz (1937) appears to have become the subject of much more controversy than the others, namely, the claim that imprinting is irreversible. Reversibility of social attachments implies that the originally established social bonds are modifiable, but does not specify what changes might occur. Traditionally, “irreversibility” implies that the 1st imprinting experience has priority over the subsequent one; its effects, therefore, are resistant to change (Hess, 1959). The data concerning the reversibility assumption have not always been supportive of this view (e.g., Hinde, 1970). Salzen (1966) argued that the available evidence on the irreversibility of imprinted preferences in the social and sexual behavior of birds suffers from methodological problems, thus weakening the support it purports to give this widely held assumption. Salzen offered his own interpretation, the neuronal model, emphasizing imprinting to be an experiential, rather than a maturational, phenomenon; i.e., an imprinted preference should be capable of being reversed if the imprinted bird is confined with a new stimulus in the absence of the original object. (Salzen and Meyer [1968] subsequently demonstrated the reversal of the social preferences of domestic chicks in the 1st week after hatching by substituting a new object for the original one.) Whether the reversibility assumption is only partially or wholly wrong is still unclear. Reports by Bateson (1974), Connolly (1968), Salzen (1962), Sluckin (1962), Sluckin and Salzen (1961), Zajonc, Reimer, and Hausser (1973), and Zajonc, Marcus, and Wilson (1974) lend support to the idea that the longer the bird has been exposed to the object, the better able it is to respond positively to that object. However, some questions remain Received for publication 17 October 1974 Revised for publication 31 January 1975 Developmental PsychobioEogy, 8(6): 533-540 (1975) 0 1 9 7 5 by John Wiley & Sons, Inc.
5 33
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to be addressed more fully: How much exposure to the new object is required to significantly alter existing preferences? How rapidly will these changes take place? Will the new experience require “reversal,” in the classic sense, of the old preferences or will the chick learn to accept new objects without necessarily rejecting the old one? Thus, strong preferences for certain stimuli must be established and attempts then made to modify these preferences by changing the stiinulus to which the bird is forcibly exposed. By testing the effects of this new experience at various times after the interchanges, the development of the new preference can be plotted as well as changes in the old. The expectation was simple and straightforward: the longer the new experience, the stronger the new preference.
Method
Subjects The eggs, purchased from the Indianapolis Farm Bureau Co-op, were brought from storage and incubated in our laboratory using a Humidair #50 incubator. They were hatched in the dark in a Humidair #SO hatcher. The temperature for both units ranged from 37.5” t o 38.3”C. The humidity level, measured only in the case of the incubator, was set at 85.5%. Approximately 10-12 hr after hatching, the chicks (Callus gullus) were transferred to the isolation-rearing or “imprinting” cages. No attempt was made to physically isolate individual eggs or newly hatched chicks during the hatching period or to identify subjects by sex.
Rearing and Maintenance Each isolation-rearing cage, 35 x 45 x 40 cm, was constructed of sheet metal, equipped with a wire mesh floor, and provided with individual water and food dispensers. Continuous illumination in each cage was provided by a 25-W white incandescent bulb mounted on the rear wall approximately 35 cm above the wire mesh floor. In addition to the heat generated by this light source, heat was provided by a black 25-W incandescent bulb. The ambient temperature in all the units was regulated thermostatically between 33.3” and 35.0”C. Once placed in their isolation-rearing units, the chicks were visible to the experimenter only through a “fish-eye” viewer mounted 8 cm from the top and in the center of the front wall of each cage.
Design Each bird had 2 periods which we shall hereafter refer to as Phase I (Primacy) and Phase 11 (Recency) exposures. In all cases, Phase I began 10-12 hr after hatching and lasted for 72 hr, after which each bird was given a choice test. Phase I1 began immediately following that test. The 140 birds were equally and randomly assigned to 1 of 5 Phase I1 conditions: 5 min, 5 hr, 10 hr, 25 hr, or 72 hr. Each bird was tested again at the end of Phase 11. Within each exposure condition, the 28 subjects were further divided into 2
OBJECT PREFERENCES IN CHICKS
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subgroups, the Interchange and the Noninterchange groups. Interchange subjects were those who, after the 1st test, were given new training objects for Phase TI, whereas the Noninterchange subjects retained the same object which they had in Phase I. For the Interchange subjects, the 1st object was called the Primacy Object and the 2nd one, the Recency Object. For the Noninterchange subjects, the 2 test objects involved were referred to as Novel or Familiar. Of the 14 subjects in each cell in the 2 x 5 design, half were exposed initially to a blue object and the remaining half to the green. Thus, 70 chicks were exposed t o blue and 70 to green during Phase I. Among the Interchange subjects, therefore, those that had blue in Phase I were given green in Phase 11, and those given green in Phase I were given blue in Phase 11. For Noninterchange subjects, half were completely blue-reared and the other half, green-reared.
Training Stimuli The object to which the animal was exposed in its home cage was either a blue or a green pillow-like cloth disc whch was approximately 5 cm in diameter. The disc hung freely from the center of the cage ceiling by a string such that the bottom of the disc was approximately 3.8 cm from the wire mesh floor. The choice of blue and green as the colors to be used in this study was based upon previously reported results of experiments in which these colors were found to be more or less equally effective in eliciting approach responses (Kovach, 1971, Schaefer & Hess, 1959).
Home Observation Pretest home observations (5-min) were conducted to measure the time spent by the subject near the training stimulus while in a familiar home environment. The observation period was started by a quick removal and replacement of the training object.
Testing The test apparatus consisted of an enclosed chamber, 76 x 71 x 40 cm, constructed out of sheet metal, and lined on the outside with acoustical board to minimize vibration and transmission of extraneous noise. The objects used for testing were a green and a blue disc identical to the training stimuli. They were suspended from an arm at the top of the test box and hung freely, approximately 40 cm apart and 3.8 cm from the wire mesh floor. The left-right position of these discs was changed by simply rotating the arm to which the discs were attached. The position of the objects remained stationary during each trial, although the objects moved upon being hit by the subject. The temperature in the test box ranged from 29.4" to 35.0"C. A 25-W black bulb remained illuminated throughout the test to supplement the heat from a white 25-W bulb that was controlled from the outside. In a typical testing situation, the chick was placed manually through a small door onto a starting position equidistant to, and facing, both the test objects. The location and behavior of the chick were recorded manually at 5-sec intervals. Testing consisted of 2 consecutive 5-min trials with a 5-sec intertrial pause, during which time the chick was removed from the box and the position of the test objects reversed. Observations of the
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chicks could be made only through a “fish-eye” viewer mounted at the center of the front wall. Of major importance in this study was the amount of time the chick spent with the test object. The chick was said to be with an object if any part of its body was judged to be within a circle 12.7 cm in diameter drawn on the paper beneath each of the 2, test objects, and remaining in that circle at least 3 sec during each 5-sec interval. The combined floor area of the 2 circles made up less than 3% of the total floor area (5396 sq cm) .
Results In the 1st home observation, the overall average time spent with the object was 15.27 intervals out of a maximum of 60 5-sec intervals. The Interchange and Noninterchange subjects were no different from each other based on this observation. Test I data give an indication of the strength of the preference for the familiar and the novel objects. Regardless of the color in Phase I, all subjects spent a mean of 41.53 5-sec intervals near the familiar object, and 1.1 intervals near the unfamiliar object ( t = 17.89, df = 139, p < .001; 2-tailed test). On the average, the chcks spent 34.6% of the total test time with the familiar object and virtually the remainder of the time in the “field,” but not near the novel object. Blue-reared subjects spent an average of 40.79 intervals with the blue object, whereas the green-reared subjects spent 42.27 intervals with the green stimulus. This difference was not significant, indicating no “color bias”, an important factor to consider when modifying color preference by substitution (e.g., Taylor, Sluckin, & Hewitt, 1969). The 2nd home observation showed no effects attributable to the length of exposure to the training object in Phase I1 nor to whether the subjects were in the Interchange or Noninterchange conditions. On the average, the subjects in all conditions spent 19.42 intervals with the object (see Fig. 1). With the exception of the 5-Min groups, the tendency to remain beside the training object was fairly stable for different age animals. Merely a few hours, perhaps as few as 5, were required for the chicks in the Interchange condition to adapt to a strange object in their cage (Fig. 1). After the initial “acquaintance period”, which took anywhere from a few minutes to a few hours, the subjects appeared to have returned to their “normal” pattern of responding, including the normal level of interaction with the object, as evidenced by the absence of a difference between the Interchange and Noninterchange groups. An attempt was made to match the Interchange and Noninterchange subjects within each Phase I1 exposure condition on the basis of Test I scores. However, due t o scheduling constraints and mortality, systematic assignments were not always possible. Nevertheless, a post hoe comparison of the scores on the familiar object during Test I1 by the Interchange and Noninterchange subjects revealed that the 2 groups were comparable in terms of their responsiveness toward the familiar test object. For the blue-reared subjects, Interchange subjects spent an average of 41.17 intervals with the blue object whereas the Noninterchange subjects spent an average of 40.40 intervals. For green-reared chicks, the Interchange subjects had a mean score on the green object of 41 2 3 whereas the Noninterchange had 43.31 (differences were not significant). In the case of the Interchange subjects, an increased preference for the Recency
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OBJECT PREFERENCES IN CHICKS
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LENGTH OF PHASE I1 EXPOSURE (2ND O B s ~ A T I c N l
Fig. 1. Time spent by Interchange and Noninterchange subjects with the training object during the 5-min home-cage observation preceding Test 11.
Object and a concurrent trend toward a diminished preference for the Primary Object apparently exists (see Fig. 2). A significant Exposure Condition x Object Condition interaction is clearly seen by the crossover of the 2 Interchange curves in Figure 2. Whether the object is the Primacy or the Recency stimulus affected the level of responsiveness of the subject toward it, as shown by the significant within-subjects Object factor ( F = 15.58, df = 4/135, p < .Ol). The longer the exposure to the Recency Object, the greater the preference that was developed for it; alternately the greater the exposure to the Recency Object and, thus, the longer the absence of the Primacy Object, the lesser the preference exhibited toward the latter. After 72 hr, the Recency Object was preferred over the Primacy Object, the difference between the 2 means being highly significant ( t = 3.07, df= 13, p < .Ol). The test data of the Noninterchange groups (see Fig. 2) show unequivocal support for the view that exposure leads to the development of strong preference for the familiar object. Noninterchange subjects spent more time with the Familiar Object than with the Novel Object (F = 350.19, df = 1/135, p < .001). In fact, the Noninterchange groups remained unresponsive to the object never encountered outside the testing situation, i.e., the Novel stimulus. This low level of responsiveness was consistent over all groups (Fig. 2). On the other hand, the Interchange subjects’ scores on the Recency Object show a trend toward preference as a function of exposure length. Comparing the Novel Object scores of the Interchange groups, no reliable difference was found between the groups given
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INTERCHANGE GROUPS' RECENCY OBJECT SCORES INTERCHANGE GROUPS' PRIMACY OBJECT SCORES NON-INTERCHANGE GROUPS' FAMILIAR OBJECT SCORES NON-INTERCHANGE GROUPS' NOVEL OBJECT SCORES
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LENGTI-' OF PHASE II EXPOSURE (TEST Ill
Fig. 2. Time spent by Interchange and Noninterchange subjects with the Familiar/Primacy and Novel/Recency objects.
only 5 min of Phase I1 experience (t = .04, df= 26, p > .05).After 5 hr, however, the difference between these scores was marginally significant (t = 1.86, df = 26, p < -10) and the direction of the difference was as predicted. The difference between these scores continued to increase with longer exposure to the Recency stimulus, reaching its biggest difference after 72 hr ( t = 9.74, d f = 26, p < .OOl>. The Interchange animals' hghest level of responsiveness toward the new object (at 72 hr) was lower than the level of responsiveness of the Noninterchange controls a t the same age (t = 6.13, df = 26, p < .OOl) to the Familiar stimulus.
Conclusions The results of this study support the expectation that the longer the animal i s exposed to a novel object, the stronger its preference for that object. This outcome is consistent with the previous findings by Salzen and Meyer (1968), Sluckin and Salzen (1961), Zajonc et al. (1973, 19741, and within our own laboratory. The new information presented here shows that new preferences develop rapidly in the course of an animal's exposure to a new object. Approach and subsequent interactive responses (e.g., pecking) become organized with respect to the new object withm 5 hr after the initial presentation of that object. Although this may not be ilfimediately obvious in the 2-object contrast test situation, it is evident in the home-cage behavior of the animal. This evidence does
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not support the assumption of “irreversibility” in the sense that new preferences cannot be established, and casts doubt upon the idea of a critical period as being an exclusive phase for the development of differential approach in the case of chicks. Although the present data appear to show evidence for “reversibility” as advanced by Salzen and Meyer (1968), i.e., an increase in the preference for the new object and a concomitant decrease in the preference for the old one, whether such would be true for another testing procedure that measures “independent” preferences of the animal is not certain. In a contrast test involving the simultaneous presentation of the test objects such as the one used in this study, the animal may approach and spend some time with one or the other object, with both, or with neither one. If the animal spends time with both objects, it may do so by distributing the time equally between them or, as is more often the case, spend more time with one than with the other. Because of the nature of the contrast test procedure, we do not know just what became of the preference for the original object. The present data merely indicate an apparent loss of preference. Future research might find the use of a noncontrast test appropriate or, alternatively, a 2-object test for the Primacy or Recency preference by using a novel color as the 2nd stimulus. However, complete “reversibility” may nevertheless occur after considerable periods of enforced confmement with a new object, and such a phenomenon may be demonstrated with an appropriate test. The overall conclusion drawn from this research is that the young animal is continuously adapting its responses to the events that are present in its environment. The effects of earlier experience are important but so, too, are the effects of recent ones in the establishment of object preference in domestic chicks.
Notes Request reprints from: Dr. Albert A. Einsiedel, Jr., Psychology Department, Mather Memorial Building, Case Western Reserve University, Cleveland, Ohio 44106, U.S.A.
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