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The measurement of imprinting
THE MEASUREMENT OF IMPRINTING BY
KEVIN CONNOLLY*
AND
NEVILLE MORAY
Department of Psychology, University of Sheffield .
From the time when Lorenz (1937) described the phenomenon which he called "imprinting" and attributed to it certain characteristics, a great deal of interest has been shown in it by ethologists and psychologists . "Imprinting" is the process whereby a young bird or other animal becomes attached to an object during the early period of its life, and subsequently behaves towards that object as if it were a member of the animal's own species, often as if it were the animal's parent, mate, (etc .). Imprinting has (often) been operationally defined in terms of the following response of the animal towards the stimulus object ; and much of the work on imprinting has utilized the following response as a measure. Following has been used mainly in two ways, in terms of the distance for which the animal follows the model Hess (1959) ; Salzen & Sluckin (1959) ; or in terms of the amount of time which the animal spends following the model during the trial, usually expressed in seconds, Jaynes (1956) ; Guiton (1959) ; Gottlieb (1961) ; Pitz & Ross (1961) . The criterion used to decide whether the animal is following varies from study to study, but basically it depends on considerations such as these . (1) The animal should be orientated towards the model . (2) The animal should be moving in the same direction as the model. (3) The animal should be within a given distance of the model . For example, in the experiments of Jaynes (1956, 1957) the animal had to be within an area extending 12 inches behind, and 4 inches in front of, or on either side of, the model . This paper is intended to deal with two problems which arise in such experiments : first, the score which an animal may accumulate by chance (which appears never to have been calculated in any imprinting experiment with which we are familiar) : and secondly, a way of transforming raw scores into a form which will allow for animals which show different basic rates of movement . A search through the literature much wider than the references quoted has ended in failure to find a single case in which chance scores have
been measured . We are ready to agree that anyone who has watched animals in the usual situations used in imprinting experiments will be right when they say that the animals are following, and that many cases of following under different values of experimental parameters have been studied and the differential scores compared. But if we are interested in the time course of imprinting, for example, we must be able to say when it rises and falls about chance . The animal for example, may be moving in the runway in the same direction, in front of, but more slowly than the model ; and it will then be overtaken, and for a few seconds will satisfy the criteria for "following", by chance . It is in fact, quite easy to calculate the chance score under this condition, and to generalize it for any shape of apparatus and any duration of experiment . We proceed as follows : Consider a runway of the type used by Hess (1959) and Connolly, Norman & Moray (1962) . This is shown in Fig. 1 . The animal moves round a ring
Fig . 1 . Diagrammatic representation of runway . R = radius of apparatus ; R1 = radius of centre core . r = radius of criterion circle . Broken line indicates path of model .
shaped area in which a thick central pillar interrupts vision across the apparatus and hides the machinery propelling the model . The criterion may be considered as a small circle round the model. Providing the animal is within this circle and moving it, it scores as "following" . Now : Let the area in which the animal moves be A . Let the area in which the animals scores be a,
*Now at the Department of Psychology, Birkbeck College . 209
210
ANIMAL BEHAVIOUR, XII,
Let the radii of the various parts of the apparatus be as shown in Fig . 1 . Let the duration of the experimental session be T seconds. Let the time in which the animal moves be tm seconds . Then, the chance of the animal being within the criterion area at any instant is given by a/A (1) assuming the animal is small with respect to these areas . This provides a close estimate in terms of random walk theory . Determining the random walk more precisely would be difficult and would provide no futher information other than some idea of the shape of the distribution within certain specified parameters and this is not relevant for our purposes here . For a circular runway of the type described a/A is equivalent to r2/(R2-R'2) . (2) Similarly, the chance of the animal moving at any instant is given by tm/T. (3) Hence the chance of the animal both being within the criterion area and of moving at the same time, is given by : r2 (4) R2-R' 2 T Then the expected time during which following will occur by chance multiplied by the duration of the experiment, is : r2 till T (5) R 2-R' 2 T It is clear that the shape of the apparatus is irrelevant to this calculation, since the calculation as performed in terms of the radii of the circular apparatus can be generalized to a till (6) A Where a is the same as above, and A is now the total area in which the animal is free to move, regardless of its shape . Note also that by suitably defining a the direction of movement can be taken into account . If a is a circle round the model some workers might feel that this is odd, for a bird could be "following" when running backwards in front of the model! Such workers will define a perhaps as the semicircle behind the model. But whatever particular usage is adopted, the general form of Equation 6 can be used . In terms of this more general equation we may express the relation of the animal's score to the chance score thus :
2-3
Let t F be the number of seconds which the animal satisfies the criterion of the following : Then, where a tF> - - till (7) A the animal is following above chance . In certain types of apparatus, an artefact may be introduced by the bird being attracted to some part of the apparatus-for example to the walls of a small circular apparatus. This would reduce the expected following time . A correction could be inserted in Equation 6 and elsewhere as appropriate, but such corrections must always be empirical and will have to be calculated where an experimenter thinks it necessary . Ideally apparatus should be designed to reduce such effects to insignificant proportions . One of the troubles which arise in interpreting the results of these experiments is that there are enormous individual differences between birds on any given day when the experiment is run . For example, in one experiment recently carried out by the authors, two birds treated identically gained scores on the same day of 562 seconds and 82 seconds in a trial time of 15 minutes . Does this mean that the second was not following, or was following less than the other? Would not a shorter period of its total activity time spent following count for as much as a longer period in a more active bird? The raw data show such a scatter that any comparison seems unjustified and consequently one feels that following times should be weighted according to the general activity levels of the animals . If we make the not unreasonable assumption that the time a bird is going to follow in a given period is related to its general level of activity, we may proceed as follows, and derive a standard score in terms of the proportion of the total movement time (t) which is spent in following (t F). An active bird must therefore, follow for longer than a less active bird to get the same score . This seems plausible, and the standard score is given by tF S = (8) t,,, Now whereas the original raw scores could (and frequently do) take an extremely wide range of values under apparently identical conditions, due apparently to individual differences between animals, this statistic can vary only between 0 and 1 . And even species differences in activity will be catered for by such standardization . It is not just that we have
21 1
CONNOLLY & MORAY : MEASUREMENT OF IMPRINTING
scaled down the numerical scores but that it has been standardized in a way which allows a .oa 70 greater degree of comparison between birds than the raw scores allow. We have argued that more active birds will 4 60 necessarily achieve higher raw score following Y times . But if we have two strains, one of which ° is more active a strain than the other, we may 5 well be able to say that when allowance is made for this, they follow equally-although in raw c score terms all the birds in one group might X40 follow twice as long, this will now be seen to indicate only a following response equal in ° strength to the less active strain . Such standard- o 30 ization we feel allows a more coherent picture c to emerge when comparisons are made . n An example of this gain in clarity is seen when Figs . 2a and 2b (taken from a recent experiment b 20 of the authors) are examined . It will be seen that 0V while in the raw data there seems to be no well w defined peak of following (50 per cent . of birds ° 10 on day 3 and 10 per cent . on day 4) when allow- p ance is made for activity level, the peak on day 4 Z 0 becomes much more apparent . There is a further advantage conferred by the 1 2 use of the standardization score which appears Fig. 2a . when we consider its relation to the chance level as calculated previously . It will be recalled that the bird can be said to be following above chance when it follows for a time greater than 0.7 a . t(by Equ . 6 above) A where a is the size of the "criterion region' and t,,, is the total duration of its movement time. Let us now transform this value into a standard score according to the equation given above. The chance level for following in a period t n, is given by a . t,,, (by Equ . 6 above) A So to get the standard score corresponding to this duration of following, we must divide this by the total movement time, which is equal to t,,, (by Equ . 8 above) which reduces simply to a (9) S =A That is, in terms of a standard score, the chance level is given by a line drawn horizontally across the graph at a level given by the ratio of the criterion area to the total floor area of the apparatus . When the following score is standard-
4 DAYS
0.6 0 .5
0 .1 -
1 Fig. 2b,
2
4 PAYS
5
6
ANIMAL
212
BEHAVIOUR, XII, 2-3
ized and plotted day by day in an experiment, it can be seen to be significant as soon as it rises above this line . An example taken from some work of the authors (op . cit .) is shown in Figs . 3a and 3b and shows both the graphical representation of the chance level, and the more orderly array of data, after conversion to standard scores, which allows more meaningful comparison to be made between behaviour under this condition and under other conditions .
200 190 180 170 160 150 N
5140 -
Summary 1 . A method is described for calculating the chance level of performance to be expected if the following response is used to measure imprinting in birds . 2 . A method is described for standardizing the scores of more or less active birds .
a 130 E-' 120 110 ° 100
80
Acknowledgments The authors take pleasure in thanking Dr . J . Annett and Mr. J . Clarkson for much helpful discussion and assistance in preparing this paper . Particular thanks are due to Mr . D . F . Kerridge and Professor D . R . Cox The work was carried out while K .J .C . was in receipt of a grant from the Medical Research Council .
70 60 50 40 30 20
REFERENCES 10 Connolly, K . J ., Norman, J . & Moray, N. P . (1962). To be submitted for publication . 4 5 6 7 Gottlieb, G. (1961). The following response and imprint1 2 3 Fig . 3a . DAYS ing in wild and domestic ducklings of the same Following seconds (raw score) plotted against days . species (Anas platyrhynchos) . Behaviour, 18, 205-228 . Guiton, P . (1959) . Socialisation and imprinting in Brown Leghorn chicks. Anim. Behav ., 7, 26-34.
4
5
6
7
DAYS
Fig . 3b . Standard score plotted against days . Broken line indicates chance level .
Hess, E . H . (1959) . Imprinting . Science, 130, 133-141 . Jaynes, J . (1956) . Imprinting : The interaction of learned and innate behaviour. I . Development and Generalisation . J . comp . physiol. Psycho! ., 49, 201-206 . Jaynes, J. (1957) . Imprinting : The interacton of learned and innate behaviour . 11 . The critical period . J. comp . physiol . Psycho! ., 50, 6-10 . Lorenz, K . (1937) . The companion in the bird's world . Auk, 54, 245-273 . Pitz & Ross (1961) . Imprinting as a function of arousal . J. comp . physiol . Psycho!., 54, 602-604. Salzen, E. A . & Sluckin, W . (1959) . The incidence of the following response and the duration of responsiveness in domestic fowl . Anim . Behav., 7,172-179 . (Accepted for publication 16th March, 1964 ; Ms, number : 296) .
KEVIN CONNOLLY*
AND
NEVILLE MORAY
Department of Psychology, University of Sheffield .
From the time when Lorenz (1937) described the phenomenon which he called "imprinting" and attributed to it certain characteristics, a great deal of interest has been shown in it by ethologists and psychologists . "Imprinting" is the process whereby a young bird or other animal becomes attached to an object during the early period of its life, and subsequently behaves towards that object as if it were a member of the animal's own species, often as if it were the animal's parent, mate, (etc .). Imprinting has (often) been operationally defined in terms of the following response of the animal towards the stimulus object ; and much of the work on imprinting has utilized the following response as a measure. Following has been used mainly in two ways, in terms of the distance for which the animal follows the model Hess (1959) ; Salzen & Sluckin (1959) ; or in terms of the amount of time which the animal spends following the model during the trial, usually expressed in seconds, Jaynes (1956) ; Guiton (1959) ; Gottlieb (1961) ; Pitz & Ross (1961) . The criterion used to decide whether the animal is following varies from study to study, but basically it depends on considerations such as these . (1) The animal should be orientated towards the model . (2) The animal should be moving in the same direction as the model. (3) The animal should be within a given distance of the model . For example, in the experiments of Jaynes (1956, 1957) the animal had to be within an area extending 12 inches behind, and 4 inches in front of, or on either side of, the model . This paper is intended to deal with two problems which arise in such experiments : first, the score which an animal may accumulate by chance (which appears never to have been calculated in any imprinting experiment with which we are familiar) : and secondly, a way of transforming raw scores into a form which will allow for animals which show different basic rates of movement . A search through the literature much wider than the references quoted has ended in failure to find a single case in which chance scores have
been measured . We are ready to agree that anyone who has watched animals in the usual situations used in imprinting experiments will be right when they say that the animals are following, and that many cases of following under different values of experimental parameters have been studied and the differential scores compared. But if we are interested in the time course of imprinting, for example, we must be able to say when it rises and falls about chance . The animal for example, may be moving in the runway in the same direction, in front of, but more slowly than the model ; and it will then be overtaken, and for a few seconds will satisfy the criteria for "following", by chance . It is in fact, quite easy to calculate the chance score under this condition, and to generalize it for any shape of apparatus and any duration of experiment . We proceed as follows : Consider a runway of the type used by Hess (1959) and Connolly, Norman & Moray (1962) . This is shown in Fig. 1 . The animal moves round a ring
Fig . 1 . Diagrammatic representation of runway . R = radius of apparatus ; R1 = radius of centre core . r = radius of criterion circle . Broken line indicates path of model .
shaped area in which a thick central pillar interrupts vision across the apparatus and hides the machinery propelling the model . The criterion may be considered as a small circle round the model. Providing the animal is within this circle and moving it, it scores as "following" . Now : Let the area in which the animal moves be A . Let the area in which the animals scores be a,
*Now at the Department of Psychology, Birkbeck College . 209
210
ANIMAL BEHAVIOUR, XII,
Let the radii of the various parts of the apparatus be as shown in Fig . 1 . Let the duration of the experimental session be T seconds. Let the time in which the animal moves be tm seconds . Then, the chance of the animal being within the criterion area at any instant is given by a/A (1) assuming the animal is small with respect to these areas . This provides a close estimate in terms of random walk theory . Determining the random walk more precisely would be difficult and would provide no futher information other than some idea of the shape of the distribution within certain specified parameters and this is not relevant for our purposes here . For a circular runway of the type described a/A is equivalent to r2/(R2-R'2) . (2) Similarly, the chance of the animal moving at any instant is given by tm/T. (3) Hence the chance of the animal both being within the criterion area and of moving at the same time, is given by : r2 (4) R2-R' 2 T Then the expected time during which following will occur by chance multiplied by the duration of the experiment, is : r2 till T (5) R 2-R' 2 T It is clear that the shape of the apparatus is irrelevant to this calculation, since the calculation as performed in terms of the radii of the circular apparatus can be generalized to a till (6) A Where a is the same as above, and A is now the total area in which the animal is free to move, regardless of its shape . Note also that by suitably defining a the direction of movement can be taken into account . If a is a circle round the model some workers might feel that this is odd, for a bird could be "following" when running backwards in front of the model! Such workers will define a perhaps as the semicircle behind the model. But whatever particular usage is adopted, the general form of Equation 6 can be used . In terms of this more general equation we may express the relation of the animal's score to the chance score thus :
2-3
Let t F be the number of seconds which the animal satisfies the criterion of the following : Then, where a tF> - - till (7) A the animal is following above chance . In certain types of apparatus, an artefact may be introduced by the bird being attracted to some part of the apparatus-for example to the walls of a small circular apparatus. This would reduce the expected following time . A correction could be inserted in Equation 6 and elsewhere as appropriate, but such corrections must always be empirical and will have to be calculated where an experimenter thinks it necessary . Ideally apparatus should be designed to reduce such effects to insignificant proportions . One of the troubles which arise in interpreting the results of these experiments is that there are enormous individual differences between birds on any given day when the experiment is run . For example, in one experiment recently carried out by the authors, two birds treated identically gained scores on the same day of 562 seconds and 82 seconds in a trial time of 15 minutes . Does this mean that the second was not following, or was following less than the other? Would not a shorter period of its total activity time spent following count for as much as a longer period in a more active bird? The raw data show such a scatter that any comparison seems unjustified and consequently one feels that following times should be weighted according to the general activity levels of the animals . If we make the not unreasonable assumption that the time a bird is going to follow in a given period is related to its general level of activity, we may proceed as follows, and derive a standard score in terms of the proportion of the total movement time (t) which is spent in following (t F). An active bird must therefore, follow for longer than a less active bird to get the same score . This seems plausible, and the standard score is given by tF S = (8) t,,, Now whereas the original raw scores could (and frequently do) take an extremely wide range of values under apparently identical conditions, due apparently to individual differences between animals, this statistic can vary only between 0 and 1 . And even species differences in activity will be catered for by such standardization . It is not just that we have
21 1
CONNOLLY & MORAY : MEASUREMENT OF IMPRINTING
scaled down the numerical scores but that it has been standardized in a way which allows a .oa 70 greater degree of comparison between birds than the raw scores allow. We have argued that more active birds will 4 60 necessarily achieve higher raw score following Y times . But if we have two strains, one of which ° is more active a strain than the other, we may 5 well be able to say that when allowance is made for this, they follow equally-although in raw c score terms all the birds in one group might X40 follow twice as long, this will now be seen to indicate only a following response equal in ° strength to the less active strain . Such standard- o 30 ization we feel allows a more coherent picture c to emerge when comparisons are made . n An example of this gain in clarity is seen when Figs . 2a and 2b (taken from a recent experiment b 20 of the authors) are examined . It will be seen that 0V while in the raw data there seems to be no well w defined peak of following (50 per cent . of birds ° 10 on day 3 and 10 per cent . on day 4) when allow- p ance is made for activity level, the peak on day 4 Z 0 becomes much more apparent . There is a further advantage conferred by the 1 2 use of the standardization score which appears Fig. 2a . when we consider its relation to the chance level as calculated previously . It will be recalled that the bird can be said to be following above chance when it follows for a time greater than 0.7 a . t(by Equ . 6 above) A where a is the size of the "criterion region' and t,,, is the total duration of its movement time. Let us now transform this value into a standard score according to the equation given above. The chance level for following in a period t n, is given by a . t,,, (by Equ . 6 above) A So to get the standard score corresponding to this duration of following, we must divide this by the total movement time, which is equal to t,,, (by Equ . 8 above) which reduces simply to a (9) S =A That is, in terms of a standard score, the chance level is given by a line drawn horizontally across the graph at a level given by the ratio of the criterion area to the total floor area of the apparatus . When the following score is standard-
4 DAYS
0.6 0 .5
0 .1 -
1 Fig. 2b,
2
4 PAYS
5
6
ANIMAL
212
BEHAVIOUR, XII, 2-3
ized and plotted day by day in an experiment, it can be seen to be significant as soon as it rises above this line . An example taken from some work of the authors (op . cit .) is shown in Figs . 3a and 3b and shows both the graphical representation of the chance level, and the more orderly array of data, after conversion to standard scores, which allows more meaningful comparison to be made between behaviour under this condition and under other conditions .
200 190 180 170 160 150 N
5140 -
Summary 1 . A method is described for calculating the chance level of performance to be expected if the following response is used to measure imprinting in birds . 2 . A method is described for standardizing the scores of more or less active birds .
a 130 E-' 120 110 ° 100
80
Acknowledgments The authors take pleasure in thanking Dr . J . Annett and Mr. J . Clarkson for much helpful discussion and assistance in preparing this paper . Particular thanks are due to Mr . D . F . Kerridge and Professor D . R . Cox The work was carried out while K .J .C . was in receipt of a grant from the Medical Research Council .
70 60 50 40 30 20
REFERENCES 10 Connolly, K . J ., Norman, J . & Moray, N. P . (1962). To be submitted for publication . 4 5 6 7 Gottlieb, G. (1961). The following response and imprint1 2 3 Fig . 3a . DAYS ing in wild and domestic ducklings of the same Following seconds (raw score) plotted against days . species (Anas platyrhynchos) . Behaviour, 18, 205-228 . Guiton, P . (1959) . Socialisation and imprinting in Brown Leghorn chicks. Anim. Behav ., 7, 26-34.
4
5
6
7
DAYS
Fig . 3b . Standard score plotted against days . Broken line indicates chance level .
Hess, E . H . (1959) . Imprinting . Science, 130, 133-141 . Jaynes, J . (1956) . Imprinting : The interaction of learned and innate behaviour. I . Development and Generalisation . J . comp . physiol. Psycho! ., 49, 201-206 . Jaynes, J. (1957) . Imprinting : The interacton of learned and innate behaviour . 11 . The critical period . J. comp . physiol . Psycho! ., 50, 6-10 . Lorenz, K . (1937) . The companion in the bird's world . Auk, 54, 245-273 . Pitz & Ross (1961) . Imprinting as a function of arousal . J. comp . physiol . Psycho!., 54, 602-604. Salzen, E. A . & Sluckin, W . (1959) . The incidence of the following response and the duration of responsiveness in domestic fowl . Anim . Behav., 7,172-179 . (Accepted for publication 16th March, 1964 ; Ms, number : 296) .