- Email: [email protected]
Imprinting and the “law of effort”
IMPRINTING AND THE "LAW OF EFFORT" By WILLIAM R . THOMPSON AND R . A . DUBANOSKIt Wesleyan University
Experiments I and II Subjects Forty-two commercially-hatched Vantress chicks were used, 24 in Experiment 1, 18 in Experiment II . They were transferred to the laboratory in light-proof boxes within 3 to 6 hours of hatching . In the laboratory, they were housed in individual cages 9 x 8 x 51 inches covered with fine mesh nylon screening that allowed heat and light to enter but prevented any effective perception of objects outside the cages . Cage temperature was maintained at approximately 88° F . Chicks in each experiment were randomly divided into three equal groups to be described below .
One important set of problems relating to the understanding of imprinting concerns the specification of those conditions that maximize the attachment of the young animal to the parent or parent-surrogate . Both input and output variables have been implicated . Thus, it appears to be true that a number of sensory parameters are important (Hess, 1959), one of the most crucial of these being retinal flicker (James, 1959) . Likewise, there is some evidence that, on the motor side, the amount of energy expended by the young animal during initial exposure to an imprinting object is directly related to strength of later following (Hess, 1958) . This general relationship has been designated by Hess (1958, 1959) as the "law of effort" . It is on this that we wish to focus attention in this paper . Assuming that the empirical data hold trueand this is by no means conclusively established (cf. Moltz, et al ., 1960 ; Rice, 1962)-we may postulate a number of possible explanatory mechanisms . In the first place, it may be, as Hess seems to imply by his term "effort", that the purely efferent components of motor activity are crucial . Secondly, it is possible that an increase in arousal level or general drive resulting from afferent feedback is the important factor . Thirdly, we might suppose that some role is played by an increase in perceived figure-ground differentiation consequent on following an object that is itself moving across a stationary field or environment . Any or all of these factors may be responsible for the data that define the law of effort . It is the purpose of the three experiments reported here to explore the relative contribution to strength of later following made by the motor and sensory components of movement during initial training . More specifically, comparisons are made between groups of chicks under several conditions : (a) Chick in active motion, and surrogate in motion ; (b) Surrogate in motion only, chick in passive motion (confined in moving box), or stationary ; (c) Chick and surrogate both stationary .
Apparatus A circular runway was used after Hess (1957) . The outside circumference was 13 feet, the inside was 6 feet . The walls of the runway were 3 feet high and were made of white cardboard painted with heavy irregular black lines so as to provide highly inhomogeneous ground conditions . Lighting was supplied by a single overhead 200 watt bulb . The imprinting object in both experiments was a coloured (blue and yellow) toy rubber doll . The non-imprinting object in Experiment I was a surrogate made of cheesecloth stuffed with rags to resemble an adult hen . In Experiment II, the non-imprinting surrogate was a coloured (blue and yellow) toy cellulose duck . Surrogates were suspended a few inches above the floor of the apparatus and driven around by a variable speed motor . For the purposes of testing strength of imprinting later in life, the same runway was used . In Experiment I only, a discrimination test was also employed . This consisted of a rectangular enclosure 4 x 21 feet with white cardboard walls 20 inches high . In it were placed, 2 feet apart against one wall, the imprinting and nonimprinting surrogates . Procedure (a) Training for Ss in Experiment I was as follows : Chicks in Group A were taken from their living cages to the experimental room in an enclosed box and then placed in the training alley, 1 foot from the model . One minute
'The research reported here was supported by a grantin-aid from the National Science Foundation, No . G15522 . tNow at the University of Minnesota . 213
214
ANIMAL BEHAVIOUR, XII, 2-3
habituation was given to each S, following which the motor was turned on and the model moved around the alley at a speed accommodated by E to the speed of S . A following experience lasting through six revolutions of the surrogate was given to each chick with a maximum time set at half an hour . Ss in Group B were given the same treatment except that each was placed for 1 minute in the plastic box, which was then rotated with the surrogate (1 foot away from it) through 6 revolutions . Whenever the S in the box turned away from the surrogate, the box was turned by E until the chick was again facing toward the surrogate . Each S in Group C was carried in an enclosed box to the training alley and, after the I minute habituation period, was exposed for a 6 minute period to a stationary surrogate placed 1 foot away . This distance was maintained by moving the surrogate whenever S moved farther than 1 foot away from it . It may be noted that the time of exposure to the surrogate was, on the average, longer for Ss in Group A than for Ss in the other two groups . However, Hess (1958) has reported that this variable is not crucial in determining strength of imprinting . In Experiment II, movement of the surrogate was accommodated to the following of the chicks of Group A during a 10 minute period . Distance traversed by each chick was recorded . Ss in Group B were again confined in the plastic box . Each chick was then rotated with the surrogate over a distance matched to that traversed by a chick in Group A . Rate of movement was adjusted so that each S in Group B was exposed to the surrogate
for a 10 minute period . Ss in Group C were exposed to the stationary surrogate for 10 minutes as in Experiment I . Their proximity to it was enforced this time, however, by use of the plastic shield behind the surrogate and another shield 6 inches further away from it . For all Ss, training was carried out at 9, 12 and 16 hours of age . (b) Testing . Experiment I chicks were tested for strength of imprinting at 24 and 48 hours of age in the circular apparatus . Following an initial 30 second adaptation period, the imprinting surrogate was moved through 3 complete revolutions (I per minute) . This was repeated subquently using the non-imprinting surrogate . Following was scored whenever the chick was moving within 1 foot of the surrogate . The same tests were given to chicks of Experiment II at 38 and 62 hours of age . In addition, the discrimination test was given to chicks of Experiment I approximately 10 minutes after the following tests . The chick was placed at a point equidistant (1 foot) from each surrogate . A record was made of the time taken (in seconds) by an S to approach one of the two objects, a maximum time of 3 minutes being allowed . Results The data are summarized in Tables I and II which show median following scores and ranges for the different groups in both experiments . Since distributions were skewed and variances rather uneven, non-parametric analyses were computed, specifically, Kruskal-Wallis and Friedman variance analyses on ranks . In Experiment
Table I . Median and Ranges (seconds) of Following Scores in Experiment I of Imprinting and Non-imprinting Surrogates by Chicks in the Three Groups at Two Ages of Testing . Age
Imprinting Median
168
24 hours
48 hours
Surrogate
Surrogate
Non-imprinting
Imprinting
Non-imprinting
108
155
17
Group A Range
28-171
2-130
109-171
Median
15
12
44
Range
0-80
0-83
56
6
0-162 23
Group B Median
2-119 36
1-164 11
Group C Range
0-145
0-114
0-140
0-171
215
THOMPSON & DUBANOSKI : IMPRINTING AND THE "LAW OF EFFORT"
Table II . Median and Ranges (seconds) of Following Scores in Experiment II of Imprinting and Non-imprinting Surrogates by Chicks in the Three Groups at Two Ages of Testing . Age
Median
38 hours
62 hours
Surrogate
Surrogate
-
Imprinting
Non-imprinting
Imprinting
Non-imprinting
180
85
159
104
Group A Range
74-180
58-162
49-180
Median
12
29
82
12-137
24-179
90
42
25-174
-
55 -
Group B Range
0-174
Median
1
Range
0-71
I, significant differences between groups were found for following of imprinting surrogate (H==9 . 48, p < -01) and non-imprinting surrogate (H=6 . 76, p < -05) at the 24 hour testing and for imprinting surrogate only (H=10 .60, p<-O1) at the 48 hour testing . Chicks of Group A followed much more than those of Groups B and C . Differences between Groups B and C were not clear-cut . There was a clear preference for the imprinting surrogate shown by chicks in Group A (Chi Square=14 . 20, p< •0 1). Chicks in Group B followed more at 48 hours than at 24 hours (Chi Square=10 . 21, p< •02) . Group C chicks showed uniform performance . In Experiment II, chicks of Group A again showed more following than chicks of Groups B and C in respect of imprinting surrogate at both 38 (H= 10 . 43, p<, •0 1) and 62 hours (H=7 . 03, p< •05) but not in respect to the non-imprinting surrogate at either age . Groups B and C were not very different from each other . Again, chicks of Group A showed distinctly more following of the imprinting than of the non-imprinting surrogate (Chi Square=13 . 03, p<-01) . No significant differences were found from the other two groups . Data from the discrimination test in Experiment I are shown in Table III . Results were as follows : all chicks of Group A chose the imprinting surrogate at both testing ages . Of Group B, three Ss at 24, and one S at 48 hours made no choice within the 3 minute test period . Of Group C, two Ss at 24 hours, and 5 Ss at 48 hours made no choice . All chicks in these groups making a choice went to the imprinting surro-
0-180
0-163 42
0-82
2-175
Table III. Experiment I . Median Latencies and Ranges (seconds) of Approach to Imprinting Surrogate in Discrimination Test by Chicks in the Three Groups at Two Ages of Testing. Age 24 hour 48 hour Group A
Median
35 . 0
17 . 5
Range
20-99
8-50
Median
155 . 5
89 . 5
Group B Range
72-180 -Median -- 76-5
30-180 - 100-0
Range
45-180
Group C i
36-180
gate . A Kruskal-Wallis test applied to these data showed that Group A latencies were much lower than those of chicks in the other two groups, both at 24 hours (H=11 . 30, p<-01), and at 48 hours (H=14 . 06, p< . 01). Discussion The major conclusion to be drawn from Experiments I and II is that actual locomotion by the young animal towards the surrogate (A Groups) resulted in much stronger imprinting at a later testing . Motion of the surrogate coupled with passive motion of the chick (B Groups) produced weaker following ; and absence of opportunity for following by chicks (C Groups) produced the weakest imprinting . The efferent components of motion in the young
21 6
ANIMAL BEHAVIOUR, XII, 2-3
animal do seem to be of great importance . The part played by the afferent or perceptual outcomes of motion, however, is not fully settled by the experimental data . The chicks in the B Groups did imprint somewhat more strongly than those in the C Groups, but the difference was not great. It is possible, of course, that this lack of difference between Groups B and C may have been due to possible emotional effects produced in the chicks by confinement in the moving box . It should be noted that in these two experiments, the imprinting surrogate was always presented first in the test trials . This might be expected to produce higher scores for the imprinting surrogate. In actual fact, this was consistently so, only in the case of chicks of Group A. Consequently, priority of presentation does not appear to be of first importance, though, of course, this is not firmly established . To obtain greater control over some of these ambiguities, as well as to try to replicate the results of the first two experiments, a third study was undertaken . This also replicates, in part, the procedure used by Moltz et al. (1960) in their Experiment I, in that it involves a comparison between the effects on imprinting of active following, of experience with a moving surrogate, and of experience with a stationery surrogate . Details of prodedure are described below. Experiment III Subjects Twenty-four commercially-hatched Vantress chicks were used . They were transported to and housed in the laboratory in the same manner as described in Experiment I . Transfer was carried out within three hours of hatching . Again, Ss were divided into three groups : Group A, a locomotor group ; Group B, surrogate moving, chick stationary ; Group C, both surrogate and chick stationary . Apparatus Training was carried out in a rectangular runway (after Jaynes, 1956) 10 x 2 x 1 feet) . The floor was covered with white cardboard and the walls were painted white with heavy, irregular black lines . Illumination was supplied by a single 200-watt bulb hung over the centre of the runway . A rope belt was held by two 8 inch pulleys positioned on cross bars 9 feet apart, at either end of the runway. These pulleys could also be located closer together by moving the
cross bars. The pulleys and cable were driven by a motor so as to rotate a surrogate hung on the cable at the rate of 3 .5 feet per minute . Surrogates were as in Experiment 11 . For use mostly with Group B and Group C, a semicircular, wire-mesh restraining enclosure was placed against one wall of the alley midway between the two pulleys . The height of this unit was 12 inches, the diameter, 9 inches . Procedure (a) Training. Procedure for Ss in the three groups was as follows : Chicks in Group A were given a 1 minute adaptation period in the restraining unit in the alley . The imprinting object was then hung in front of S . The unit was shielded during this time by an opaque screen . Following this, the restraining unit was removed, and the imprinting object then rotated around the alley for a period of 10 minutes . The speed of the object was varied so as to accommodate to the following speed of each chick. Ss in Group B were first given a 1 minute adaptation period in the same manner as Ss in Group A . Following this, the surrogate was moved in front of the chick for a 10 minute period . To keep the surrogate always within reasonable distance of S's vision, the pulleys were located 4 feet apart equidistant from the midpoint of the alley . Minimum distance between chick and the surrogate was 6 inches. Ss in Group C were first given a 1 minute adaptation period in the restraining unit, following which the opaque screen was removed . The surrogate was then placed on the floor directly in front of the restraining unit and 6 inches distant from it . It was kept in this position for 10 minutes . All chicks were carried to and from the experimental room in an enclosed metal can so as to prevent any visual contact with the environment outside the alley . Each S was given training sessions at 9, 12 and 16 hours after hatching . (b) Testing. All Ss were tested for following responses at 30 and 54 hours posthatch . Both imprinting and non-imprinting surrogates were used, each in successive tests . Half of the S were given the imprinting surrogate first, half were given the non-imprinting surrogate first . Each test was commenced with a 30 second adaptation period during which the S was confined in the shielded restraining unit . Following this, the unit was removed and the
217
THOMPSON & DUBANOSKI : IMPRINTING AND THE "LAW OF EFFORT"
Table IV. Median and Ranges (seconds) of Following Scores of the Imprinting and Non-imprinting Surrogates by Chicks in the Three Groups at Two Ages of Testing . Age
Median
30 hours
54 hours
Surrogate
Surrogate
Imprinting
Non-imprinting
Imprinting
Non-imprinting
163
174
170
164
0-177
0-174
Group A Range
64-179
48-180
Median
5
74
11
66
Group B Range
0-101
0-158
0-113
0-176
Median
0
0
2
0
Range
0-20
0-31
0-5
0-17
Group C
surrogate rotated around the full length of the alley for a 3 minute period. Following time was scored in seconds on a Kodak timer whenever S was in motion and was within 1 foot of the object, as indicated by markers on the rope cable . Results Data are summarized in Table IV . Group A chicks showed more following of the imprinting surrogate than Ss of Groups B and C both at 30 hours (H=13 . 34, p<-01) and 54 hours (H= 6 . 68, p< .05) . They also followed the nonimprinting surrogate more at 30 hours (H= 11 . 20, p<-01), and at 54 hours (H=7 . 11, p<-05) . All chicks showed a slight perference for the non-imprinting surrogate, though this was a significant effect only in the case of Group B chicks (Chi Square =8 . 10, p<-05) . Discussion The above results agree with those obtained in Experiments I and II . Some minor differences may be mentioned . The conditions imposed on Group C chicks in Experiment II, that is, confinement in a restraining cage with a stationary surrogate, sharply reduced strength of following later on . This effect is in line with the results of Moltz, et al. (1960, cf. Table I) for their AStationary and R-Stationary groups . On the other hand, Ss of Group B in Experiment III showed stronger imprinting than Ss of Group B in the two previous experiments . It is likely that the procedure of restraining the chicks in a moving cage had a deleterious effect on attention
to the surrogate, thus resulting in weaker imprinting . As before, Ss in Group A again showed superior following, the median score of the whole group being very close to that obtained for Group A of Experiment II . This result contrasts with those obtained by Moltz et al. (1960) for his A-moving and R-moving groups of ducklings . This discrepancy may relate to differences between ducks and chicks, or to differences in procedure, in particular the distance relations between subject and surrogate (6 inch minimum distance for our Group B) . The differences between the three experiments in respect to discrimination of imprinting versus non-imprinting surrogates are not easily explained . Presumably, the objects used in Experiment I were more discriminable, this producing more following in the test trials of the object used in training. Those used in the other two experiments, however, yielded much smaller differences . In Experiment III, in fact, chicks appeared to show a preference for the nonimprinting object (the toy man) . This fact suggests that the procedure used for Group A may have produced a strong locomotor response to objects in general, rather than a preference for a particular discriminated object . In general, the data from all three experiments together do not contradict the position taken by Hess . Under several conditions of training and with following tests given at a variety of ages, ranging from 24 to 62 hours, chicks followed to locomote freely after a moving object were superior to chicks trained under conditions designed to preserve the afferent or perceptual
21 8
ANIMAL BEHAVIOUR,
consequence of actual locomotion without the efferent components . These, in turn, were superior, in two of the three experiments, to chicks trained with a stationary surrogate, particularly if training involved restraint . While the results do indicate that efferent activity has importance in the control of imprinting, they still do not clearly show whether it is important in itself or only insofar as it is a secondary symptom of strong attention . Certainly, in the first two experiments, the moving cage situation could reliably produce the perceptual conditions of real locomotion only if a chick's attention was focused on the surrogate moving in front of it. This was clearly not always the case . Consequently, we may not definitely conclude that locomotion per se is a necessary condition for imprinting . However, it is likely that it reflects a physiological situation in the organism that is most favourable for imprinting to occur. Summary Three experiments were designed to test Hess's notion that strength of imprinting is related to effort expended by the chick during initial following . Comparisons were made between imprinting scores by chicks trained under different conditions as follows : (a) chick following
XII, 2-3
moving object ; (b) chick confined in plastic box moving behind a moving surrogate ; (c) chick confined behind screen and exposed to moving surrogate ; (d) chick and surrogate both stationary . Data from all three experiments indicated strongly that chicks trained under the first set of conditions showed superior imprinting . Those trained with stationary surrogates showed lowest imprinting scores . The results were taken as agreeing with, though not confirming Hess's "Law of Effort", since they do not demonstrate conclusively the greater importance of efferent as opposed to afferent variables . REFERENCES Hess, E . H . (1958) . Imprinting in animals . Sci. Amer ., 198,81-90 . Hess, E . H . (1959) . The relationship between imprinting and motivation . In Jones, M . R . (Ed .) . Nebraska Symposium on Motivation, 44-77 . James, H . Flicker (1959) . An unconditioned stimulus for imprinting. Canad. J . Psycho! ., 13, 59-67 . Jaynes, J . (1956). The interaction of learned and innate behavior : I . Development and generalization . J. comp . physiol. Psychol., 49, 201-206 . Moltz, H ., Rosenblum, L . A . & Stettner, L . J . (1960) . Some parameters of imprinting effectiveness . J. comp . physiol . Psycho!., 53, 297-301 . Rice, C . E . (1962) . Imprinting by force . Science, 138, 680-681 . (Accepted for publication 16th January, 1964 ; Ms . number : 364) .
Experiments I and II Subjects Forty-two commercially-hatched Vantress chicks were used, 24 in Experiment 1, 18 in Experiment II . They were transferred to the laboratory in light-proof boxes within 3 to 6 hours of hatching . In the laboratory, they were housed in individual cages 9 x 8 x 51 inches covered with fine mesh nylon screening that allowed heat and light to enter but prevented any effective perception of objects outside the cages . Cage temperature was maintained at approximately 88° F . Chicks in each experiment were randomly divided into three equal groups to be described below .
One important set of problems relating to the understanding of imprinting concerns the specification of those conditions that maximize the attachment of the young animal to the parent or parent-surrogate . Both input and output variables have been implicated . Thus, it appears to be true that a number of sensory parameters are important (Hess, 1959), one of the most crucial of these being retinal flicker (James, 1959) . Likewise, there is some evidence that, on the motor side, the amount of energy expended by the young animal during initial exposure to an imprinting object is directly related to strength of later following (Hess, 1958) . This general relationship has been designated by Hess (1958, 1959) as the "law of effort" . It is on this that we wish to focus attention in this paper . Assuming that the empirical data hold trueand this is by no means conclusively established (cf. Moltz, et al ., 1960 ; Rice, 1962)-we may postulate a number of possible explanatory mechanisms . In the first place, it may be, as Hess seems to imply by his term "effort", that the purely efferent components of motor activity are crucial . Secondly, it is possible that an increase in arousal level or general drive resulting from afferent feedback is the important factor . Thirdly, we might suppose that some role is played by an increase in perceived figure-ground differentiation consequent on following an object that is itself moving across a stationary field or environment . Any or all of these factors may be responsible for the data that define the law of effort . It is the purpose of the three experiments reported here to explore the relative contribution to strength of later following made by the motor and sensory components of movement during initial training . More specifically, comparisons are made between groups of chicks under several conditions : (a) Chick in active motion, and surrogate in motion ; (b) Surrogate in motion only, chick in passive motion (confined in moving box), or stationary ; (c) Chick and surrogate both stationary .
Apparatus A circular runway was used after Hess (1957) . The outside circumference was 13 feet, the inside was 6 feet . The walls of the runway were 3 feet high and were made of white cardboard painted with heavy irregular black lines so as to provide highly inhomogeneous ground conditions . Lighting was supplied by a single overhead 200 watt bulb . The imprinting object in both experiments was a coloured (blue and yellow) toy rubber doll . The non-imprinting object in Experiment I was a surrogate made of cheesecloth stuffed with rags to resemble an adult hen . In Experiment II, the non-imprinting surrogate was a coloured (blue and yellow) toy cellulose duck . Surrogates were suspended a few inches above the floor of the apparatus and driven around by a variable speed motor . For the purposes of testing strength of imprinting later in life, the same runway was used . In Experiment I only, a discrimination test was also employed . This consisted of a rectangular enclosure 4 x 21 feet with white cardboard walls 20 inches high . In it were placed, 2 feet apart against one wall, the imprinting and nonimprinting surrogates . Procedure (a) Training for Ss in Experiment I was as follows : Chicks in Group A were taken from their living cages to the experimental room in an enclosed box and then placed in the training alley, 1 foot from the model . One minute
'The research reported here was supported by a grantin-aid from the National Science Foundation, No . G15522 . tNow at the University of Minnesota . 213
214
ANIMAL BEHAVIOUR, XII, 2-3
habituation was given to each S, following which the motor was turned on and the model moved around the alley at a speed accommodated by E to the speed of S . A following experience lasting through six revolutions of the surrogate was given to each chick with a maximum time set at half an hour . Ss in Group B were given the same treatment except that each was placed for 1 minute in the plastic box, which was then rotated with the surrogate (1 foot away from it) through 6 revolutions . Whenever the S in the box turned away from the surrogate, the box was turned by E until the chick was again facing toward the surrogate . Each S in Group C was carried in an enclosed box to the training alley and, after the I minute habituation period, was exposed for a 6 minute period to a stationary surrogate placed 1 foot away . This distance was maintained by moving the surrogate whenever S moved farther than 1 foot away from it . It may be noted that the time of exposure to the surrogate was, on the average, longer for Ss in Group A than for Ss in the other two groups . However, Hess (1958) has reported that this variable is not crucial in determining strength of imprinting . In Experiment II, movement of the surrogate was accommodated to the following of the chicks of Group A during a 10 minute period . Distance traversed by each chick was recorded . Ss in Group B were again confined in the plastic box . Each chick was then rotated with the surrogate over a distance matched to that traversed by a chick in Group A . Rate of movement was adjusted so that each S in Group B was exposed to the surrogate
for a 10 minute period . Ss in Group C were exposed to the stationary surrogate for 10 minutes as in Experiment I . Their proximity to it was enforced this time, however, by use of the plastic shield behind the surrogate and another shield 6 inches further away from it . For all Ss, training was carried out at 9, 12 and 16 hours of age . (b) Testing . Experiment I chicks were tested for strength of imprinting at 24 and 48 hours of age in the circular apparatus . Following an initial 30 second adaptation period, the imprinting surrogate was moved through 3 complete revolutions (I per minute) . This was repeated subquently using the non-imprinting surrogate . Following was scored whenever the chick was moving within 1 foot of the surrogate . The same tests were given to chicks of Experiment II at 38 and 62 hours of age . In addition, the discrimination test was given to chicks of Experiment I approximately 10 minutes after the following tests . The chick was placed at a point equidistant (1 foot) from each surrogate . A record was made of the time taken (in seconds) by an S to approach one of the two objects, a maximum time of 3 minutes being allowed . Results The data are summarized in Tables I and II which show median following scores and ranges for the different groups in both experiments . Since distributions were skewed and variances rather uneven, non-parametric analyses were computed, specifically, Kruskal-Wallis and Friedman variance analyses on ranks . In Experiment
Table I . Median and Ranges (seconds) of Following Scores in Experiment I of Imprinting and Non-imprinting Surrogates by Chicks in the Three Groups at Two Ages of Testing . Age
Imprinting Median
168
24 hours
48 hours
Surrogate
Surrogate
Non-imprinting
Imprinting
Non-imprinting
108
155
17
Group A Range
28-171
2-130
109-171
Median
15
12
44
Range
0-80
0-83
56
6
0-162 23
Group B Median
2-119 36
1-164 11
Group C Range
0-145
0-114
0-140
0-171
215
THOMPSON & DUBANOSKI : IMPRINTING AND THE "LAW OF EFFORT"
Table II . Median and Ranges (seconds) of Following Scores in Experiment II of Imprinting and Non-imprinting Surrogates by Chicks in the Three Groups at Two Ages of Testing . Age
Median
38 hours
62 hours
Surrogate
Surrogate
-
Imprinting
Non-imprinting
Imprinting
Non-imprinting
180
85
159
104
Group A Range
74-180
58-162
49-180
Median
12
29
82
12-137
24-179
90
42
25-174
-
55 -
Group B Range
0-174
Median
1
Range
0-71
I, significant differences between groups were found for following of imprinting surrogate (H==9 . 48, p < -01) and non-imprinting surrogate (H=6 . 76, p < -05) at the 24 hour testing and for imprinting surrogate only (H=10 .60, p<-O1) at the 48 hour testing . Chicks of Group A followed much more than those of Groups B and C . Differences between Groups B and C were not clear-cut . There was a clear preference for the imprinting surrogate shown by chicks in Group A (Chi Square=14 . 20, p< •0 1). Chicks in Group B followed more at 48 hours than at 24 hours (Chi Square=10 . 21, p< •02) . Group C chicks showed uniform performance . In Experiment II, chicks of Group A again showed more following than chicks of Groups B and C in respect of imprinting surrogate at both 38 (H= 10 . 43, p<, •0 1) and 62 hours (H=7 . 03, p< •05) but not in respect to the non-imprinting surrogate at either age . Groups B and C were not very different from each other . Again, chicks of Group A showed distinctly more following of the imprinting than of the non-imprinting surrogate (Chi Square=13 . 03, p<-01) . No significant differences were found from the other two groups . Data from the discrimination test in Experiment I are shown in Table III . Results were as follows : all chicks of Group A chose the imprinting surrogate at both testing ages . Of Group B, three Ss at 24, and one S at 48 hours made no choice within the 3 minute test period . Of Group C, two Ss at 24 hours, and 5 Ss at 48 hours made no choice . All chicks in these groups making a choice went to the imprinting surro-
0-180
0-163 42
0-82
2-175
Table III. Experiment I . Median Latencies and Ranges (seconds) of Approach to Imprinting Surrogate in Discrimination Test by Chicks in the Three Groups at Two Ages of Testing. Age 24 hour 48 hour Group A
Median
35 . 0
17 . 5
Range
20-99
8-50
Median
155 . 5
89 . 5
Group B Range
72-180 -Median -- 76-5
30-180 - 100-0
Range
45-180
Group C i
36-180
gate . A Kruskal-Wallis test applied to these data showed that Group A latencies were much lower than those of chicks in the other two groups, both at 24 hours (H=11 . 30, p<-01), and at 48 hours (H=14 . 06, p< . 01). Discussion The major conclusion to be drawn from Experiments I and II is that actual locomotion by the young animal towards the surrogate (A Groups) resulted in much stronger imprinting at a later testing . Motion of the surrogate coupled with passive motion of the chick (B Groups) produced weaker following ; and absence of opportunity for following by chicks (C Groups) produced the weakest imprinting . The efferent components of motion in the young
21 6
ANIMAL BEHAVIOUR, XII, 2-3
animal do seem to be of great importance . The part played by the afferent or perceptual outcomes of motion, however, is not fully settled by the experimental data . The chicks in the B Groups did imprint somewhat more strongly than those in the C Groups, but the difference was not great. It is possible, of course, that this lack of difference between Groups B and C may have been due to possible emotional effects produced in the chicks by confinement in the moving box . It should be noted that in these two experiments, the imprinting surrogate was always presented first in the test trials . This might be expected to produce higher scores for the imprinting surrogate. In actual fact, this was consistently so, only in the case of chicks of Group A. Consequently, priority of presentation does not appear to be of first importance, though, of course, this is not firmly established . To obtain greater control over some of these ambiguities, as well as to try to replicate the results of the first two experiments, a third study was undertaken . This also replicates, in part, the procedure used by Moltz et al. (1960) in their Experiment I, in that it involves a comparison between the effects on imprinting of active following, of experience with a moving surrogate, and of experience with a stationery surrogate . Details of prodedure are described below. Experiment III Subjects Twenty-four commercially-hatched Vantress chicks were used . They were transported to and housed in the laboratory in the same manner as described in Experiment I . Transfer was carried out within three hours of hatching . Again, Ss were divided into three groups : Group A, a locomotor group ; Group B, surrogate moving, chick stationary ; Group C, both surrogate and chick stationary . Apparatus Training was carried out in a rectangular runway (after Jaynes, 1956) 10 x 2 x 1 feet) . The floor was covered with white cardboard and the walls were painted white with heavy, irregular black lines . Illumination was supplied by a single 200-watt bulb hung over the centre of the runway . A rope belt was held by two 8 inch pulleys positioned on cross bars 9 feet apart, at either end of the runway. These pulleys could also be located closer together by moving the
cross bars. The pulleys and cable were driven by a motor so as to rotate a surrogate hung on the cable at the rate of 3 .5 feet per minute . Surrogates were as in Experiment 11 . For use mostly with Group B and Group C, a semicircular, wire-mesh restraining enclosure was placed against one wall of the alley midway between the two pulleys . The height of this unit was 12 inches, the diameter, 9 inches . Procedure (a) Training. Procedure for Ss in the three groups was as follows : Chicks in Group A were given a 1 minute adaptation period in the restraining unit in the alley . The imprinting object was then hung in front of S . The unit was shielded during this time by an opaque screen . Following this, the restraining unit was removed, and the imprinting object then rotated around the alley for a period of 10 minutes . The speed of the object was varied so as to accommodate to the following speed of each chick. Ss in Group B were first given a 1 minute adaptation period in the same manner as Ss in Group A . Following this, the surrogate was moved in front of the chick for a 10 minute period . To keep the surrogate always within reasonable distance of S's vision, the pulleys were located 4 feet apart equidistant from the midpoint of the alley . Minimum distance between chick and the surrogate was 6 inches. Ss in Group C were first given a 1 minute adaptation period in the restraining unit, following which the opaque screen was removed . The surrogate was then placed on the floor directly in front of the restraining unit and 6 inches distant from it . It was kept in this position for 10 minutes . All chicks were carried to and from the experimental room in an enclosed metal can so as to prevent any visual contact with the environment outside the alley . Each S was given training sessions at 9, 12 and 16 hours after hatching . (b) Testing. All Ss were tested for following responses at 30 and 54 hours posthatch . Both imprinting and non-imprinting surrogates were used, each in successive tests . Half of the S were given the imprinting surrogate first, half were given the non-imprinting surrogate first . Each test was commenced with a 30 second adaptation period during which the S was confined in the shielded restraining unit . Following this, the unit was removed and the
217
THOMPSON & DUBANOSKI : IMPRINTING AND THE "LAW OF EFFORT"
Table IV. Median and Ranges (seconds) of Following Scores of the Imprinting and Non-imprinting Surrogates by Chicks in the Three Groups at Two Ages of Testing . Age
Median
30 hours
54 hours
Surrogate
Surrogate
Imprinting
Non-imprinting
Imprinting
Non-imprinting
163
174
170
164
0-177
0-174
Group A Range
64-179
48-180
Median
5
74
11
66
Group B Range
0-101
0-158
0-113
0-176
Median
0
0
2
0
Range
0-20
0-31
0-5
0-17
Group C
surrogate rotated around the full length of the alley for a 3 minute period. Following time was scored in seconds on a Kodak timer whenever S was in motion and was within 1 foot of the object, as indicated by markers on the rope cable . Results Data are summarized in Table IV . Group A chicks showed more following of the imprinting surrogate than Ss of Groups B and C both at 30 hours (H=13 . 34, p<-01) and 54 hours (H= 6 . 68, p< .05) . They also followed the nonimprinting surrogate more at 30 hours (H= 11 . 20, p<-01), and at 54 hours (H=7 . 11, p<-05) . All chicks showed a slight perference for the non-imprinting surrogate, though this was a significant effect only in the case of Group B chicks (Chi Square =8 . 10, p<-05) . Discussion The above results agree with those obtained in Experiments I and II . Some minor differences may be mentioned . The conditions imposed on Group C chicks in Experiment II, that is, confinement in a restraining cage with a stationary surrogate, sharply reduced strength of following later on . This effect is in line with the results of Moltz, et al. (1960, cf. Table I) for their AStationary and R-Stationary groups . On the other hand, Ss of Group B in Experiment III showed stronger imprinting than Ss of Group B in the two previous experiments . It is likely that the procedure of restraining the chicks in a moving cage had a deleterious effect on attention
to the surrogate, thus resulting in weaker imprinting . As before, Ss in Group A again showed superior following, the median score of the whole group being very close to that obtained for Group A of Experiment II . This result contrasts with those obtained by Moltz et al. (1960) for his A-moving and R-moving groups of ducklings . This discrepancy may relate to differences between ducks and chicks, or to differences in procedure, in particular the distance relations between subject and surrogate (6 inch minimum distance for our Group B) . The differences between the three experiments in respect to discrimination of imprinting versus non-imprinting surrogates are not easily explained . Presumably, the objects used in Experiment I were more discriminable, this producing more following in the test trials of the object used in training. Those used in the other two experiments, however, yielded much smaller differences . In Experiment III, in fact, chicks appeared to show a preference for the nonimprinting object (the toy man) . This fact suggests that the procedure used for Group A may have produced a strong locomotor response to objects in general, rather than a preference for a particular discriminated object . In general, the data from all three experiments together do not contradict the position taken by Hess . Under several conditions of training and with following tests given at a variety of ages, ranging from 24 to 62 hours, chicks followed to locomote freely after a moving object were superior to chicks trained under conditions designed to preserve the afferent or perceptual
21 8
ANIMAL BEHAVIOUR,
consequence of actual locomotion without the efferent components . These, in turn, were superior, in two of the three experiments, to chicks trained with a stationary surrogate, particularly if training involved restraint . While the results do indicate that efferent activity has importance in the control of imprinting, they still do not clearly show whether it is important in itself or only insofar as it is a secondary symptom of strong attention . Certainly, in the first two experiments, the moving cage situation could reliably produce the perceptual conditions of real locomotion only if a chick's attention was focused on the surrogate moving in front of it. This was clearly not always the case . Consequently, we may not definitely conclude that locomotion per se is a necessary condition for imprinting . However, it is likely that it reflects a physiological situation in the organism that is most favourable for imprinting to occur. Summary Three experiments were designed to test Hess's notion that strength of imprinting is related to effort expended by the chick during initial following . Comparisons were made between imprinting scores by chicks trained under different conditions as follows : (a) chick following
XII, 2-3
moving object ; (b) chick confined in plastic box moving behind a moving surrogate ; (c) chick confined behind screen and exposed to moving surrogate ; (d) chick and surrogate both stationary . Data from all three experiments indicated strongly that chicks trained under the first set of conditions showed superior imprinting . Those trained with stationary surrogates showed lowest imprinting scores . The results were taken as agreeing with, though not confirming Hess's "Law of Effort", since they do not demonstrate conclusively the greater importance of efferent as opposed to afferent variables . REFERENCES Hess, E . H . (1958) . Imprinting in animals . Sci. Amer ., 198,81-90 . Hess, E . H . (1959) . The relationship between imprinting and motivation . In Jones, M . R . (Ed .) . Nebraska Symposium on Motivation, 44-77 . James, H . Flicker (1959) . An unconditioned stimulus for imprinting. Canad. J . Psycho! ., 13, 59-67 . Jaynes, J . (1956). The interaction of learned and innate behavior : I . Development and generalization . J. comp . physiol. Psychol., 49, 201-206 . Moltz, H ., Rosenblum, L . A . & Stettner, L . J . (1960) . Some parameters of imprinting effectiveness . J. comp . physiol . Psycho!., 53, 297-301 . Rice, C . E . (1962) . Imprinting by force . Science, 138, 680-681 . (Accepted for publication 16th January, 1964 ; Ms . number : 364) .