THE MEASUREMENT OF IMPRINTING

BY KEVIN CONNOLLY* AND NEVILLE MORAYDepartment of Psychology, University of Sheffield .

From the time when Lorenz (1937) describedthe phenomenon which he called "imprinting"and attributed to it certain characteristics, agreat deal of interest has been shown in it byethologists and psychologists . "Imprinting"is the process whereby a young bird or otheranimal becomes attached to an object duringthe early period of its life, and subsequently be-haves towards that object as if it were a mem-ber of the animal's own species, often as if itwere the animal's parent, mate, (etc .). Imprintinghas (often) been operationally defined in termsof the following response of the animal towardsthe stimulus object ; and much of the work onimprinting has utilized the following response asa measure .

Following has been used mainly in two ways,in terms of the distance for which the animalfollows the model Hess (1959) ; Salzen &Sluckin (1959) ; or in terms of the amount oftime which the animal spends following themodel during the trial, usually expressed inseconds, Jaynes (1956) ; Guiton (1959) ; Gott-lieb (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 suchas these. (1) The animal should be orientatedtowards the model. (2) The animal should bemoving in the same direction as the model. (3)The animal should be within a given distance ofthe model . For example, in the experiments ofJaynes (1956, 1957) the animal had to be withinan area extending 12 inches behind, and 4 inchesin front of, or on either side of, the model .

This paper is intended to deal with two prob-lems which arise in such experiments : first,the score which an animal may accumulate bychance (which appears never to have been calcu-lated in any imprinting experiment with whichwe are familiar) : and secondly, a way of trans-forming raw scores into a form which will allowfor animals which show different basic rates ofmovement .

A search through the literature much widerthan the references quoted has ended in failureto find a single case in which chance scores have*Now at the Department of Psychology, BirkbeckCollege .

209

been measured . We are ready to agree that any-one who has watched animals in the usual situ-ations used in imprinting experiments will beright when they say that the animals are fol-lowing, and that many cases of following underdifferent values of experimental parametershave been studied and the differential scorescompared. But if we are interested in the timecourse of imprinting, for example, we must beable to say when it rises and falls about chance .The animal for example, may be moving in therunway in the same direction, in front of, butmore slowly than the model ; and it will then beovertaken, and for a few seconds will satisfythe criteria for "following", by chance.

It is in fact, quite easy to calculate the chancescore under this condition, and to generalizeit for any shape of apparatus and any duration ofexperiment. We proceed as follows: Consider arunway of the type used by Hess (1959) andConnolly, Norman & Moray (1962) . This isshown 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 = radiusof criterion circle . Broken line indicates path of model .

shaped area in which a thick central pillarinterrupts vision across the apparatus and hidesthe machinery propelling the model. The criter-ion may be considered as a small circle round themodel. Providing the animal is within this circleand 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,

2 1 0

ANIMAL BEHAVIOUR, XII, 2-3

Let the radii of the various parts of the appar-atus be as shown in Fig . 1 .

Let the duration of the experimental sessionbe T seconds.

Let the time in which the animal moves betm seconds .Then, the chance of the animal being within thecriterion area at any instant is given by

a/A (1)assuming the animal is small with respect tothese areas . This provides a close estimate interms of random walk theory . Determining therandom walk more precisely would be difficultand would provide no futher information otherthan some idea of the shape of the distributionwithin certain specified parameters and this isnot relevant for our purposes here .

For a circular runway of the type describeda/A is equivalent to

r2/(R2-R'2) .

(2)Similarly, the chance of the animal moving

at any instant is given bytm/T. (3)

Hence the chance of the animal both being withinthe criterion area and of moving at the sametime, is given by :

r 2(4)

R2-R' 2

TThen the expected time during which followingwill occur by chance multiplied by the durationof the experiment, is :

r 2

tillT

(5)R 2-R' 2

TIt is clear that the shape of the apparatus isirrelevant to this calculation, since the calcu-lation as performed in terms of the radii of thecircular apparatus can be generalized to

atill

(6)A

Where a is the same as above, and A is now thetotal area in which the animal is free to move,regardless of its shape . Note also that by suitablydefining a the direction of movement can betaken into account. If a is a circle round themodel some workers might feel that this is odd,for a bird could be "following" when runningbackwards in front of the model! Such workerswill define a perhaps as the semicircle behind themodel. But whatever particular usage is adopted,the general form of Equation 6 can be used .

In terms of this more general equation we mayexpress the relation of the animal's score to thechance score thus :

Let tF be the number of seconds which theanimal satisfies the criterion of the following :Then, where

atF>- - till

(7)A

the animal is following above chance. In certaintypes of apparatus, an artefact may be intro-duced by the bird being attracted to some partof the apparatus-for example to the walls of asmall circular apparatus. This would reduce theexpected following time . A correction could beinserted in Equation 6 and elsewhere as appro-priate, but such corrections must always beempirical and will have to be calculated wherean experimenter thinks it necessary . Ideallyapparatus should be designed to reduce sucheffects to insignificant proportions .

One of the troubles which arise in interpretingthe results of these experiments is that there areenormous individual differences between birdson any given day when the experiment is run .For example, in one experiment recently carriedout by the authors, two birds treated identicallygained scores on the same day of 562 secondsand 82 seconds in a trial time of 15 minutes .Does this mean that the second was not follow-ing, or was following less than the other?Would not a shorter period of its total activitytime spent following count for as much as alonger period in a more active bird? The rawdata show such a scatter that any comparisonseems unjustified and consequently one feelsthat following times should be weighted accord-ing to the general activity levels of the animals .

If we make the not unreasonable assumptionthat the time a bird is going to follow in a givenperiod is related to its general level of activity,we may proceed as follows, and derive a standardscore in terms of the proportion of the totalmovement time (t) which is spent in following(t F) . An active bird must therefore, follow forlonger than a less active bird to get the samescore. This seems plausible, and the standardscore is given by

S =tF

(8)t,,,

Now whereas the original raw scores could(and frequently do) take an extremely widerange of values under apparently identicalconditions, due apparently to individual differ-ences between animals, this statistic can varyonly between 0 and 1 . And even species differ-ences in activity will be catered for by suchstandardization . It is not just that we have

CONNOLLY & MORAY: MEASUREMENT OF IMPRINTING

scaled down the numerical scores but that it hasbeen standardized in a way which allows a o 70greater degree of comparison between birds

.athan the raw scores allow .

We have argued that more active birds will 4 60necessarily achieve higher raw score following Ytimes. But if we have two strains, one of which °is more active a strain than the other, we may 5well be able to say that when allowance is madefor this, they follow equally-although in raw cscore terms all the birds in one group might X40follow twice as long, this will now be seen toindicate only a following response equal in °strength to the less active strain . Such standard- o 30ization we feel allows a more coherent picture cto emerge when comparisons are made .

nAn 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 0Vwhile in the raw data there seems to be no well wdefined peak of following (50 per cent . of birds ° 10on day 3 and 10 per cent . on day 4) when allow- pance is made for activity level, the peak on day 4becomes much more apparent .

Z 0There is a further advantage conferred by the

use of the standardization score which appearswhen we consider its relation to the chance levelas calculated previously. It will be recalled thatthe bird can be said to be following above chancewhen it follows for a time greater than

a . t(by Equ. 6 above)

0 .7 -

Awhere a is the size of the "criterion region' and

0.6t,,, is the total duration of its movement time.Let us now transform this value into a standardscore according to the equation given above.

0.5The chance level for following in a period t n,is given by

a . t,,, (by Equ. 6 above)

ASo to get the standard score corresponding tothis duration of following, we must divide thisby the total movement time, which is equal to

t,,, (by Equ . 8 above)which reduces simply to

a

(9)S =-

1

2Fig. 2a .

4DAYS

A

0 .1 -That is, in terms of a standard score, the chancelevel is given by a line drawn horizontally acrossthe graph at a level given by the ratio of thecriterion area to the total floor area of the 1 2 4apparatus. When the following score is standard-

Fig. 2b,

PAYS

5 6

2 1 1

212

200

190

180

170

160

150N

5140 -

a 130

E-' 120 -

110

° 100

80

70

60

50

40

30

20

10

ANIMAL

1

2

3

4

5

6

7Fig. 3a .

DAYSFollowing seconds (raw score) plotted against days .

4

5DAYS

Fig . 3b . Standard score plotted against days . Broken lineindicates chance level .

BEHAVIOUR, XII, 2-3

6 7

ized and plotted day by day in an experiment,it can be seen to be significant as soon as itrises above this line . An example taken fromsome work of the authors (op . cit .) is shown inFigs. 3a and 3b and shows both the graphicalrepresentation of the chance level, and the moreorderly array of data, after conversion tostandard scores, which allows more meaningfulcomparison to be made between behaviour underthis condition and under other conditions .

Summary1. A method is described for calculating the

chance level of performance to be expectedif the following response is used to measure im-printing in birds .2. A method is described for standardizing

the scores of more or less active birds .

AcknowledgmentsThe authors take pleasure in thanking Dr . J .

Annett and Mr. J. Clarkson for much helpfuldiscussion and assistance in preparing this paper .Particular thanks are due to Mr. D. F. Kerridgeand Professor D . R. Cox The work was carriedout while K .J .C. was in receipt of a grant fromthe Medical Research Council .

REFERENCES

Connolly, K . J ., Norman, J. & Moray, N. P . (1962).To be submitted for publication .

Gottlieb, G. (1961). The following response and imprint-ing in wild and domestic ducklings of the samespecies (Anas platyrhynchos) . Behaviour, 18,205-228 .

Guiton, P . (1959) . Socialisation and imprinting in BrownLeghorn chicks. Anim. Behav ., 7, 26-34.

Hess, E . H. (1959) . Imprinting . Science, 130, 133-141 .Jaynes, J . (1956) . Imprinting : The interaction of learned

and innate behaviour. I . Development andGeneralisation . J. comp . physiol. Psycho! ., 49,201-206 .

Jaynes, J. (1957) . Imprinting : The interacton of learnedand 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 thefollowing response and the duration of respons-iveness in domestic fowl . Anim. Behav., 7,172-179 .

(Accepted for publication 16th March, 1964 ;Ms, number : 296) .