the cost of changing from one activity to another

Anim. Behav., 1978,26,1237-1246

THE COST OF CHANGING FROM ONE ACTIVITY TO ANOTHER

BY STEPHEN LARKIN & DAVID McFARLANDAnimal Behaviour Research Group, Department of Zoology, University of Oxford

Abstract. When there is a cost of changing from one activity to another it affects the temporal organiza-tion of behaviour. It is shown theoretically that the cost of changing should be allocated by an animalto the cost of the behaviour that is about to be performed . This hypothesis is confirmed for the Barbarydove (Streptopelia risoria) changing between feeding and drinking in an experimental room . The doubleinterruption method is used to determine the position of dominance boundaries in food and waterdeficit space. Boundary rotation is used to determine how a cost of changing affects motivational stateand therefore how the cost is allocated . Results obtained in the room experiments are shown to becomparable to results obtained in Skinner boxes in which the double interruption and dominanceboundary methods have been previously used, supporting their validity .

When an animal changes from one activity toanother, there may be a cost involved in theactivity of changing itself. For example, agranivorous bird, feeding in a dry field, willbuild up a considerable thirst (McFarland 1965),but may have to fly half a mile to obtain water.In addition to the physiological cost of thejourney, the bird is losing time in making thetransition from feeding to drinking ; it will not bereducing its food and water deficits during thetransition . It may also be exposing itself topredators, etc . The cost of changing from oneactivity to another, therefore, should be seen asdecrement in fitness, of which there may becomponents of work, time and risk (McFarland1976) .

It has been suggested that the cost of changingmay influence the timing of decisions, and maythus affect the structure of behaviour sequences(McFarland 1971). In this paper we explore aparticular hypothesis concerning the way inwhich the cost of changing is taken into accountby animals in deciding to change from oneactivity to another.

Theoretical ConsiderationsAn animal engaged in activity A will incur bothcosts and benefits. For example, if it is feeding itis benefiting by reducing its food deficit but ithas the costs of the work involved and the cost ofnot reducing its water deficit. The animal maybe expected to change to activity B when the netcost of B is lower than that of A (McFarland1976 ; Sibly & McFarland 1976) . However, asillustrated in Fig. 1, the transition behaviour Cwill inevitably incur a net cost that is higher thanthat of A or B, because the benefits that accrueto normal activities do not apply to activitiesthat are purely transitional. In other words,

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while the costs of A and B are offset by benefitsgained, behaviour C incurs only costs . Whenwe look at the cumulative costs in the region of atransition (Fig . 2), we can see that in order forthe transition to be worthwhile, the animalmust pursue activity B for at least the period T .Figure 2 is a first approximation to what isactually happening, as the instantaneous costs ofA and B are continuously changing while thetwo activities are being pursued . For examplewhile an animal is feeding the instantaneous costof feeding is being reduced because the fooddeficit is being reduced . The changing instan-taneous costs will produce curved cumulativecost lines. The period T in Fig . 2 must be shortenough for the animal to be reasonably certainthat it will be able to pursue activity B until theend of that period of time . It may be the casethat feeding and drinking are often interruptedby the presence of predators and this wouldinfluence the decision to change by reducing T .

0 A , C i B

Time

Fig. 1 . The instantaneous net costs of activities A and Band of changing between them, C .

1 23 8

ANIMAL BEHAVIOUR, 26, 4

If we extrapolate the cumulative cost of continu-ing activity A (dotted line), we can find the pointat which it crosses the cumulative cost afterchanging to B, and at this point in time, thechange to B has just become worthwhile . Clearly,the greater the cost of C, and the less thedifference between the slopes of A and B, thegreater the shaded area in Fig . 2. In decidingwhether or not to change from A to B, theanimal should take into account the differencein the net instantaneous costs of A and B, andthe total cost of C . The problem for the animalis to decide whether, given particular valuesfor each of these three variables, it is worthwhileto change from behaviour A to behaviour B .It will be worthwhile if the cumulative cost afterchanging to B is less than the extrapolated costof continuing A within the period of time T(Fig. 2) . On this basis it is a simple exercise tocalculate at what relative values of the instan-taneous costs of A and B the transition isworthwhile, for any given combination of valuesof the cost of C, and of the time spent at C .The results of such a calculation are showngraphically in Fig . 3, in which the instantaneouscosts of A and B are plotted against each other,and the equicost lines join points at which itjust becomes worthwhile to change from A to B .These lines change their position according tothe cost of C. We can imagine that an animal isinitially in a situation (marked X) in which theinstantaneous cost of A is less than that of B,but that the former is increasing while the latteris decreasing, as shown by the trajectory inFig. 3 . If changing takes zero time then the costof C is zero (if no work is involved), and itbecomes worthwhile to change to activity Bwhen the cost of B is just less than that of A

a.a8W7E7U

C l B I

T

(point Y). However, changing will normallytake a finite time and the cost of C will be greaterthan zero. The animal should then persist withA for longer and change to B when the cost ofB is n units smaller than the cost of A (point Z) .In other words, in deciding whether to change,the animal should allocate the cost of C to theinstantaneous cost of B, and compare this withthe instantaneous cost of A. The fact that theequicost lines are parallel in Fig . 3, shows thatthis allocation consists of addition, at leastfor this first approximation when the cumulativecost curves are assumed to be straight lines .

So far we have discussed what animals shoulddo to minimize the cost of behavioural transi-tion, and we have come to the conclusion that theanimal should allocate (its estimate of) the costof changing to the cost of the activity which it ischanging to. When this sum is less than the costof the current activity, the animal should changeits behaviour. In order to put this hypothesisto experimental test, it is necessary to make anexplicit theoretical connection between theconcept of cost and variables relevant to themotivation of behaviour . Following Sibly &McFarland (1976), we define cost per unit time

C(t) = µ,(t) - A,(t)

(1)where X I (t), is the chance of the individualgiving birth at time t, and g i(t) is the chance of itsdying. A behaviour sequence is optimal if it

Instantaneous cost of ATimeFig. 2 . The cumulative costs of activities A and B and Fig. 3 . The equicost lines linking points where it justthe change between them, C . After time T the cumulative becomes worthwhile to change from A to B. Co is withcost after changing to B is less than the extrapolated no cost of changing, C n with n units of cost and Cz„cost of continuing with A (dotted line).

with 2n units of cost of changing (see text) .

LARKIN & McFARLAND : COST OF CHANGING ACTIVITY

1239

maximizes fitness, and Sibly & McFarland (1976)show that this is equivalent to minimizing

(' TC(x, u)dt,

0where the vector x represents the motivationalstate of the animal and the vector u representsits behaviour. For the case of feeding anddrinking in the Barbary dove (Streptopeliarisoria), with which we will be concerned in thispaper, Sibly & McFarland (1976) suggest thatthe cost function is quadratic, viz . :

C(x, u) = K1xi + K2xQ + K3ui + K4u2' (2)where the Ki are scaling constants, x 1 is the fooddeficit, x 2 the water deficit, ul the rate of feedingand u2 the rate of drinking. When an animalhas to spend some of its time changing betweenfeeding and drinking, during the changes thefood and water deficits will remain at the levelsat which they were before the change began .In addition to the costs associated with the con-stant deficits there will be a cost of the changingbehaviour itself. The cost function would there-fore beC(x, U)K,X 2 + K24 + K3ui + K4u2 + Ksus

(3)where u 3 is the rate of changing . Sibly &McFarland (1976) show how the cost functionwith no cost of changing (equation (2)) is, undercertain specified conditions, equivalent to thefollowing set of decision rules :

ifx1r1k1 > x2r2k2 i eatifx 1 r 1k 1 < x2r2k 2i drink

where x 1 is the state of hunger, r 1 is the avail-ability of food, k 1 is the maximum rate at whichthe bird can obtain food, x 2 is the state of thirst,r2 the availability of water, and k2 the limitingrate for drinking. The conditions under whichthese rules operate are that the animal cannot eatand drink at the same time, and there are limitsto the rates at which it can obtain food andwater. In other words, for a hungry and thirstydove in a situation in which it can either workfor food at a certain maximum rate, or work forwater at another limited rate, the cost function(equation 2) can be minimized if the animalchooses to eat or drink according to whetherthe product (deficit x availability x the limitingattempt rate) is greater for food or water . Usingthe dominance boundary method, Sibly (1975)

obtained evidence that doves, working for foodand water in a Skinner box, do follow these rules .In this paper, we draw upon this work indesigning experiments to test the hypothesis thathungry and thirsty doves, in changing fromfeeding to drinking or vice versa, allocate thecost of changing to the cost of the behaviourthey are about to change to .

Experiment 1The aim of this experiment was to investigatethe effect of increasing the cost of changingbetween feeding and drinking, by increasingthe length of a partition between the two placesin which food and water could be obtained in aSkinner box .

MethodsThe subjects were 24 Barbary doves (S .

risoria) bred in the department's colony andpreviously trained in Skinner boxes. The birdswere kept separately, visually isolated underartificial `winter' conditions of short day-length(8-h light) and constant temperature (20 C) toprevent them from coming into breedingcondition .The birds were trained in Skinner boxes

(dimensions 0 .3 x 0 .3 x 0.3 m) to obtain foodand water rewards by pecking at red and greenkeys respectively . The rewards were 3 s accessto grain, allowing the bird to obtain an averageof 0 . 1 g of food, or 3 s access to 0 . 1 g of water.The key-illumination was turned off duringreward delivery. Pecks at the keys during the un-illuminated periods were ineffective, and thebirds learned not to peck during these times .During the training period the birds were

deprived of food and water for 24 h. Trainingbegan with no partition. A partition was intro-duced between the food and water keys (Fig. 4)and this was increased by 50 mm increments inlength in subsequent training sessions, from 50mm to 250 mm . Half of the birds were given atransparent partition and half a black partition .

During the experimental session the birdswere given the different partition lengths in arandom order including no partition . Each birdwas deprived for 48 h of both food and waterand always tested at the same time of day . Asatiation criterion of 5 min without any peckswas used and the test terminated when this wasreached. Food and water rewards as a functionof time were recorded automatically on magnetictape for later computer analysis .

1 24 0


The effect of partition length on tendencyto change between feeding and drinking can beexamined by finding the lock-on index describedby McFarland & Lloyd (1973). The indexindicates the extent to which the animal lockson to an activity and its readiness to change tothe alternative activity. It was obtained byplotting food and water intake as in Fig. 5,and summing the shaded area expressed as apercentage of the total area . If the bird took allits food in one bout followed by all its water theindex would be 100 but if it alternated veryfrequently between feeding and drinking theindex would be low.

ResultsThe effect on lock-on index of partition length

is shown in Fig. 6 for both transparent partitionsand black partitions. The slopes of both regres-sion lines are significantly greater than zero(transparent P < 0 .05, black P < 0 .001, 1tailed test). Black partitions have a greatereffect than transparent. This is probablybecause the stimulus of the key light on theother side of the partition influences the birdwhen there is a transparent partition, makingit more likely to switch, counteracting to someextent the effect of the partition .

The results show that increasing the partitionlength causes a bird to prolong the ongoingactivity before changing, suggesting that the costof changing is an important factor in the

Foodmagazine

food and water keys.

Watermagazine

temporal organization of feeding and drinkingbehaviour, confirming the preliminary results ofMcFarland (1971).

Experiment 2The aim of this experiment was to examine theeffects of manipulating the cost of changingbetween feeding and drinking on the positionof the dominance boundary .

MethodsEight of the Barbary doves used in experiment

1 were used for this experiment. The experimentswere carried out in a room (Fig . 7) (2.86 x 1 .20m) with the observer behind a window of one-way glass at one end . On one side of the room'sfloor food was provided in the form of grains ofwheat buried under sand on a tray. On the otherside droplets of water are provided in metal foilmilk bottle tops, fixed in a grid pattern on aboard, to prevent the droplets from spreadingout and becoming unavailable to the subjects .Initial training was carried out with 200 g ofwheat just covered with sand and 1 g of waterin each milk bottle top . The birds soon learnedto feed, by flicking the sand with their beaks,and to drink from the milk bottle tops. Duringtraining the amounts of food and water werereduced and the depth of sand increased untilthere were 20 g of food at a depth of 10 mm ofsand and 0 . 1 g of water in each of 144 milkbottle tops ; these quantities were calculated to

100

80

60a,Y

3 20

0

IIIi0

20

40

60

80 100

Food intake (%)

Fig. 5. Percentage of total water intake during a singleFig. 4. Plan of Skinner box showing partition between

session, plotted against that of food intake. A lock-onindex is obtained by summing the shaded areas.

produce rates of feeding and drinking com-parable to those found in experiment 1 .

In the control sessions food and water wereprovided, as described, on the floor of the roomwith no partition between them. In the experi-mental sessions a partition 0 . 3 m high wasplaced between the food and the water, thusintroducing an additional cost of changing inboth directions between feeding and drinking .To make the cost apply to changing in onedirection only, in other sessions the food orwater was raised 0 .3 m above the floor to becomelevel with the top of the partition . It was assumedthat the energetic cost of jumping down 0 .3 mwould be approximately the same as that ofstepping between the feeding and drinking areas,

x 50mME 40C0

•

300

•

•20

02 10

60

y 50CcC 400

iYU0•

20001•

10

30

LARKIN & McFARLAND: COST OF CHANGING ACTIVITY

1241

0 IIIIII0

50

100

150

200

250Partition length (mm)

Fig. 6 (a)

00

50

100 150 200

250Partition length (mm)

Fig. 6 (b)

Fig. 6. Mean lock-on index as a function of partitionlength . Each point represents the mean result from 12birds. Standard errors calculated from results for in-dividual birds. (a) Transparent partition. y = 0 .71x +28 .56 with standard error in the slope ± 0 . 37. (b) Opaquepartition . y = 1 .56x + 16 . 15 with standard error in theslope ± 0 . 34.

and it would certainly be considerably less thanthe cost of flying up to make a change in theopposite direction . The interruption procedureused to determine the position of the dominanceboundary was as follows : whenever the subjectflew up to the top of the partition or stoodright on the edge of the raised area ready to flydown (or walked between the two areas whenthere was no partition), the room lights wereswitched off for 1 min making the room com-pletely dark. The activity that was observedimmediately the lights came on was counted asthe dominant activity. To obtain further dataon which behaviour was dominant the lightswere switched off for 1 min each time 50 grainsof wheat had been eaten since the last interrup-tion. They were switched off for 2 min after

Drinking area

One way glassFig. 7 . Plan of the experimental room showing feedingand drinking areas on the floor .

000000000000000000000000000000000000000000000000000000000000000000

Feeding area 00000000000000000000000000000000000000000000000000 0000000000000000000000

1 242

visits to 30 milk bottle tops . It could not bedetermined by observation during the experimentwhether or not water had already been con-sumed from a particular milk bottle top but aseries of visits to 30 was on average equivalentto 25 visits to full milk bottle tops . Water waseasier to obtain than food so the interruptionsto drinking were made twice as long and twice asfrequent as the interruptions to feeding, makingfood and water reward rates more equal .

The subjects were pretrained with each of thefour experimental conditions, and were deprivedof food for 48 h and of water for 24 h beforeeach of the test sessions, which were carriedout at the same time of day for each subject .The order of presentation of the four tests wasdetermined on a Latin square basis.

The method used for determining the positionof the dominance boundary from the resultsfollows that of previous workers (McFarland1974 ; Sibly 1975 ; Sibly & McCleery 1976) .The feeding and drinking trajectory was plottedin a space with axes food deficit and waterdeficit (both measured in grams) . The dominancetest points were marked on the trajectory . A

0

m

0iiI0

5

10

Food deficit )g to satiation)

Fig. 8 . The boundary fitting procedure showing a splitboundary (see text) . F denotes a feeding dominant point,D a drinking dominant point and S the satiation point .The line SB is the initially drawn unsplit boundary. STthe thirst dominance boundary and SH the hungerdominance boundary.


straight line was drawn through the satiationpoint such that : (1) When it was possible todraw more than one line, leaving all the feedingdominant points below it and all the drinkingdominant points above, the line chosen was thatbisecting the possible extreme lines ; (2) Whenthis was not possible, the line was drawn sothat the sum of the distances of the inconsistentfeeding dominant points from the line was madeequal to the sum of the distances of inconsistentdrinking dominant points from it. This pro-cedure was initially followed for finding thedominance boundaries in the three experimentaland one control condition. If the sum of thedistances of inconsistent dominance points fromthe line, in the experimental cases, was greaterthan the mean sum for the controls, it wasassumed that the boundary had been split intoseparate hunger and thirst dominance boun-daries. The hunger dominance boundary (drin-king to feeding switching line) was drawn from

Fig. 9 . The dominance boundaries obtained under thefour conditions of experiment 2 for bird 713 . The relativepositions of food (F) and water (W) are shown on eachdiagram.


1 243

the satiation point through a drinking dominancepoint, such that all others were above the line .Similarly the thirst dominance boundary (feedingto drinking switching line) was drawn from thesatiation point through a feeding dominantpoint, such that all others were below the line .An example of a split dominance boundary isshown in Fig . 8. The angles subtended by theboundaries with the horizontal axis were usedas a measure of boundary position .

ResultsThe switching lines obtained in the four

conditions for bird 713 are shown in Fig . 9 .Where food was raised above floor level theangle subtended by the hunger dominanceboundary was reduced with a small reductionin the angle of the thirst dominance boundary .When the water was raised there was an increasein the angle of the thirst dominance boundaryand a decrease in the angle of the hungerdominance boundary. Where the bird had to flyover a partition in both directions there was anincrease in the angles subtended by bothboundaries in this particular case but a greaterincrease in the angle of the thirst dominanceboundary .

The angles subtended by the single dominanceboundaries of the controls and the rotationsfrom these boundaries of the split boundaries inthe experiments are shown for all eight birds in

Table L The Rotations of the Dominance Boundaries Away from the BoundariesFound in the Controls

Experiment

Directionof

change

Food

Water

Table I. In two cases no split occurred and intwo cases with food and water on the floor witha partition between, it was not possible to drawa hunger dominance boundary, because thebirds did not switch from drinking to feedingduring the course of the experiment .The significance of the observed rotations

was tested with the Wilcoxon signed rank test(Siegel 1956) which showed that the anglesubtended by the hunger dominance boundarywhen food was raised was significantly decreased(one-tailed test, N = 8, P < 0 .025). Similarly,the angle subtended by the thirst dominanceboundary when water was raised was significantlyincreased (one-tailed test, N = 8, P < 0 .005) .Conversely, when the food was raised the thirstdominance boundary showed a significantpositive rotation (one-tailed test, N = 8,P < 0 .025), but there was no significant rotationof the hunger dominance boundary when thewater was raised. When food and water were onthe floor with a partition between them there wasno significant rotation of the thirst dominanceboundary. The two birds which did not changefrom drinking to feeding could be regarded ashaving not reached the hunger dominanceboundary, which therefore presumably had beenrotated to subtend an angle of zero degrees . Ifthis is assumed then overall the hunger domi-nance boundary showed a significant negativerotation (one-tailed test, N = 8, P < 0 . 01).

FWater

oodFoodIWater

BirdAngle ofboundaryin control

Rotations

731 43 + 18 + 18 +45 + 6 +42603 67 - 3 - 59 + 18 - 17 - 6 - 44713 60 - 1 - 59 + 15 - 10 + 10 + 1

4 55 + 19 - 37 +18 - 5 - 7 - 7675 56 + 26 - 19 + 27 - 10 - 6 - 34

7 64 + 4 - 7 +21 0 - 15 - 44732 57 + 20 - 48 +18 - 3 + 5 -52712 52 + 37 + 1 + 28 + 8 + 23 -

1244

The rotations of the boundary observedwhen the birds had to fly up to a higher level tochange their activity show that flying up is acost greater than that of walking from oneactivity area to another. Only once in all theexperiments was a bird observed to fly up andthen fly down again after the interruption toresume its former activity . The cost of changingpresumably therefore in all other cases preventedany changing to the sub-dominant behaviour .

The lack of a significant rotation of thehunger dominance boundary when the waterwas raised suggests that the cost of jumping downfrom the water to, the food is about the same asstepping from one to the other, but there appearsto be some cost of changing when the birds flewdown from the food tray to the water. Thiscould be a time cost as the birds often walkedup and down the tray, hesitating before step-ping up onto its edge ready to fly down . The lackof a significant rotation of the thirst dominanceboundary when the birds had to fly up to switchin both directions is probably due to the greatercomplexity of this experiment. The mean rota-tion was positive although the result was notsignificant .

Experiment 3The aim of this experiment was to comparedominance boundaries found in the roomexperiments with dominance boundaries foundin Skinner box experiments, as all previous workof this type has been carried out in Skinnerboxes .

MethodsThis experiment was in two parts, both con-

ducted in Skinner boxes with the eight birds usedin experiment 2. In the first the food and waterreward rates were equal, and the same as thoseused by previous workers (Sibly & McCleery1976). After a 3 s access period to 0 . 1 g ofgrain or 0 . 1 g of water the keys remainedunilluminated for a 3 s time-out period . Thisgave a reward rate of 0 . 17 rewards s - 1 for foodand water.

In the second part the time-out periods wereincreased so that the birds could obtain foodand water reward rates equal to the mean rewardrates that each individual achieved in the controlsession of experiment 2 in the room. Thesereward rates are shown in Table II .

To determine whether feeding or drinkingwas dominant during the experiments a doubleinterruption schedule was used as in experiment


2. When the birds changed from pecking onekey to pecking the other after the reward hadbeen given the illumination behind both keysremained off for 1 min . During this period pecksat the unilluminated keys were ineffective, andthe birds did not peck them . In addition tothese interruptions the birds were also inter-rupted after every 25 rewards of the same type .

The boundary fitting procedure described forexperiment 2 was used to determine the positionof the dominance boundaries in this experiment .ResultsThe dominance boundary angles obtained

in the Skinner box experiments with the eightbirds are shown in Table II with the anglesobtained in the control sessions of experiment 2for comparison .

McFarland & Sibly (1975) and Sibly (1975)have shown that an increase in food rewardrate with no change in the water reward rateincreases the angle of the dominance boundary .If the food reward rate were to be decreased andthe water reward rate held constant a decreasein the angle of the dominance boundary wouldbe expected.

The dominance boundary angles and rewardrates for the room experiment (experiment 2)and the room reward rate Skinner box experi-ment (experiment 3, part 2) can be comparedwith the boundary angles and reward rates givenin the first part of experiment 3, where food andwater reward rates were equal. It can be seenfrom Table II that water reward rates weresimilar in the three experiments, but the foodreward rates in the room experiment and experi-ment 3, part 2, were lower than the rate given in

Table II. The Angle Subtended by the Dominance BoundaryUnder the Following Conditions: (1) Equal Food and WaterReward Rates (0 . 17 rewards s- 1) In a S

BoxThe Control Session In the

Room (Experiment 2 ; (3) Room Reward Rates in a SkinnerBox (Experiment 3, Part 2). The Room Reward Rates

are also Sbown

Boundary angle(degrees)

Reward rates(rewards s - 1)

Bird Cond. 1 Cond . 2 Cond . 3 Food Water

731 60 43 32 0 . 06 0 . 14603 71 67 82 0 . 08 0 . 10675 81 56 53 0 . 06 0 . 21

7 49 64 54 0 . 09 0 . 12713 76 60 - 0 .07 0 . 17

4 65 55 57 0 .04 0 . 10732 78 57 62 0 .05 0 . 21712 76 52 72 0 .05 0 . 18


the equal reward rate experiment (experiment 3,part 1). Reduced dominance boundary angleswere therefore expected and were found forboth the room experiment (Wilcoxon signed ranktest, P < 0 .025, 1 tailed test) and for the roomreward rate Skinner box experiment (P < 0 . 05,1 tailed test) . No significant difference wasfound between the angles of the dominanceboundaries in the room experiment and theSkinner box experiment with the room rewardrates.

From these results we conclude that theroom and Skinner box experiments are com-parable, so it is possible to compare the domi-nance boundaries found quantitatively .

Sibly (1975) and Sibly & McFarland (1976)showed quantitatively how a change in foodreward rate (incentive) affects the slope of thedominance boundary for doves working forfood and water in a Skinner box. Their formula-tion can be extended to include a change inwater reward rate and to make predictions aboutthe relationship between the slopes of thedominance boundaries in the two Skinner boxexperiments (experiment 3) and between theslopes in the room experiment and the equalreward rate Skinner box experiment (experi-ment 3, part 1).

Using the notation of the introduction, forpoints on the dominance boundary x 1r1k 1 =x2r2k2 , and the slope of the boundary is x 2/x 1 .If the food reward rate is changed from r 1k 1 toriki, between experiments, and the waterreward rate is changed from r2k2 to rzkj theslope of the dominance boundary will change tox}/xl . The ratio of the slopes of the boundaries is

x2x1-1 r 1k1 r21k2 1x

x1xi-1

riki

r2lkj-1

But in the first Skinner box experiment foodand water incentives were equal so r 1k 1 = r2k 2

x2x1-1

riki

(4)

(5)xjxl -1

rikiThe left hand side of this equation correspondstoy in Fig. 10 and the right hand side correspondsto z.

Sibly (1975) showed that feeding tendencyequals x 1r 1k1 . Given this and the hypothesisthat drinking tendency equals x 2 r2k2, it can bepredicted that y = z in Fig . 10, which is theratio of the dominance boundary slopes against

the ratio of reward rates for the two Skinnerbox experiments (experiment 3) . The fittedregression line is y = 0 .73z + 0 .04 with stan-dard error in the slope ± 0 . 44 which is not signi-ficantly different from the predicted y = z .

If only the achieved reward rates are importantand not the method of obtaining food and waterthe same result should be obtained when theslopes of the dominance boundaries obtainedin the room control experiments are comparedwith the equal reward rate Skinner box experi-ments. This is shown in Fig . 11 in which thefitted regression line is y = 0 .92z - 0 .23 withstandard error in the slope ± 0 .23. This is alsonot significantly different from the predictedy = z. This result shows that reward rates affectthe position of the dominance boundary in theroom experiment in the same way as in theSkinner box experiments .

ConclusionThe results of the experiments reported in thispaper support the hypothesis, outlined in theintroduction, that in changing from one activityto another, doves allocate the cost of changingto the cost of the activity to which they arechanging. It would appear that there is no shiftin dominance until this sum is exceeded by theinstantaneous cost of the current activity .

1245

Fig . 10. Data from experiment 3 plotted on a graphwith the y axis representing the ratio of the slopes of thedominance boundaries in part 1 and part 2, at differentwater and food reward rates given to the individual birdsin part 2, and the z axis representing the ratios of thosereward rates (see text) . Each point represents the resultsfrom one bird .

1 246

Fig. 11 . A comparison of the results of experiment 2and experiment 3, part 1 plotted as in Fig . 10 . They axisrepresents the ratio of the slopes of the dominanceboundaries in experiment 2 and experiment 3, part 1at different water and food intake rates in experiment 2 .The z axis represents the ratio between those intake rates .

The evidence supporting this conclusioncomes from experiments involving detection ofthe dominance boundary by means of the double-interruption method, on which there has beenmuch previous work (McFarland 1974 ;McFarland & Sibly 1975 ; Sibly 1975 ; Sibly &McCleery 1976 ; Sibly & McFarland 1976).In addition to supporting the hypothesis on thecost of changing from one activity to another,the experiments reported in the paper supportthe validity of the double-interruption methodas an experimental technique, and of dominanceboundary rotation as a measure of motivational


change. In this respect, it is particularly interes-ting that the room experiments seem to be entirelycomparable with the Skinner box experiments .This suggests that ingestion rates per se, ratherthan the method of obtaining food and water iswhat is important for the dove .

AcknowledgmentsThis work was supported by a Medical ResearchCouncil research studentship . We are verygrateful to Paul Chatwell for technical assistance .

REFERENCESMcFarland, D. J. 1965. The effect of hunger on thirst

motivated behaviour in the Barbary dove . Anim.Behav., 13, 286-292 .

McFarland, D. J. 1971 . Feedback Mechanisms in AnimalBehaviour. London : Academic Press .

McFarland, D . J. 1974 . Experimental investigation ofmotivational state . In : Motivational ControlSystems Analysis (Ed. by D. J. McFarland) .London: Academic Press.

McFarland, D. J. 1976. Form and function in thetemporal organisation of behaviour . In : GrowingPoints in Ethology (Ed. by P. P. G. Bateson andR. A. Hinde). Cambridge : The University Press .

McFarland, D. J. & Lloyd, I. H. 1973. Time-sharedfeeding and drinking . Quart. J. exp. Psychol.,25, 48-61 .

McFarland, D . J. & Sibly, R. M . 1975 . The behaviouralfinal common path. Phil. Trans. Roy. Soc. B.,270, 265-293 .

Sibly, R. 1975. How incentive and deficit determinefeeding tendency . Anim. Behav., 23, 437-446 .

Sibly, R . & McFarland, D . 1976. On the fitness ofbehaviour sequences. Am. Nat., 110, 601-617 .

Sibly, R. M. & McCleery, R. H . 1976. The dominanceboundary method of determining motivationalstate . Anim. Behav., 24, 108-124 .

Siegel, S. 1956. Nonparametric Statistics for the Beha-vioural Sciences . New York: McGraw-Hill .

(Received 23 September 1977 ; revised29 December 1977 ;MS. number : 1673)

the cost of changing from one activity to another

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