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Effect of procurement cost on the drinking of a saccharin-sucrose solution by non-deprived rats
Physiology & Behavior, Vol. 33, pp. 917-921. Copyright©PergamonPress Ltd., 1984. Printedin the U.S.A.
0031-9384/84$3.00 + .00
Effect of Procurement Cost on the Drinking of a Saccharin-Sucrose Solution by Non-Deprived Rats KENNETH
N . G A N N O N , H O W A R D V. S M I T H 1 A N D K E V I N J. T I E R N E Y
D e p a r t m e n t o f P s y c h o l o g y , Trinity College, D u b l i n 2, I r e l a n d R e c e i v e d 31 O c t o b e r 1983 GANNON, K. N., H. V. SMITH AND K. J. TIERNEY. Effect of procurement cost on the drinking of a saccharinsucrose solution by non-deprived rats. PHYSIOL BEHAV 33(6) 917-921, 1984.--Four non-deprived female rats were required to run in a wheel to obtain 20 min unconstrained access to a saccharin and sucrose solution. Each was run in a series of conditions in which the requirement was a proportion (0.25, 0.5, 1.0, 1.25 and 1.5) of the amount of running performed in a condition in which the wheel alone was available. A condition in which no running was required to gain access to the solution and one in which the subject was locked in the stationary wheel for the time taken to complete the highest requirement before being allowed access to the solution were also included. The results showed that as the requirement increased the amount of solution consumed also increased, and this relationship did not depend on the time taken to perform the requirement. Procurement cost Adaptation
Saccharin-sucrose solution
Wheel-running
IN a number of experiments Collier and his associates have found that when animals live in a situation in which they must perform an instrumental requirement in order to gain the opportunity to eat they increase both the size of the meals they take, and the intervals between the meals, as the procurement cost is increased (e.g., [4, 5, 6, 10, 13, 14, 15, 16]). They respond similarly with respect to both the size of drinking bouts, and the intervals between bouts, when an increase is made in the procurement cost that must be paid to gain access to water [11,23]. These changes appear to depend upon the amount of energy, or "effort," rather than time, expended in performing the procurement requirement [6], and it may be inferred that they contribute to an optimal strategy, but little is known about the processes that are responsible for them. The nature of the situation makes it impossible to distinguish between, for example, any direct effect of the procurement cost on meal size, and any possible indirect effect on this variable mediated by the period of self-imposed deprivation which precedes the initiation of the meal, because the latter also varies with the cost. In an attempt to disentangle some of these factors Gannon, Smith and Tierney [7] studied the effect of increasing the number of lever-presses that had to be performed to gain access to food on the amount that was eaten by rats tested under constant conditions of deprivation. A control subject, that did not have to perform the procurement requirement, was yoked to each experimental subject and received access to food only when the latter had completed the requirement. The results showed that both the amount eaten by the experimental subjects, and the time for which they delayed before commencing the procurement requirement, increased with
Non-deprived subjects
Foraging
the size of the procurement cost. In contrast, the amount eaten by the control subjects did not vary with the procurement cost paid by their experimental partners. Thus Gannon et al. were able to conclude that both the size of the meals taken and the latency to initiate procurement responding were independently affected by the procurement cost. It was necessary for Gannon et al. to impose a constant level of deprivation on their subjects in order to isolate the effect of the procurement cost on meal size from the effect of the duration of fasting. However, this procedure inevitably created a difference between the conditions under which their subjects were tested and those which prevail in the situation usually employed by Collier. No experimenterimposed deprivation is employed in the latter situation. It would be necessary to eliminate this difference in order to discover whether there is an effect of the procurement cost on meal size in Collier's situation. The present experiment was designed to achieve this objective, in an indirect way, by studying the effect of the procurement cost on the amount of consummatory responding in non-deprived subjects. The experiment also had the subsidiary aim of discovering whether the effect observed by Gannon et al. [7] occurs when different instrumental and consummatory responses are employed. In the present experiment the subjects were required to run in a wheel in order to get the opportunity to drink a saccharin and sucrose solution. The times for which they were required to run were based on the times they each spent running in a baseline condition, rather than being constant across subjects. Baseline rates of lever-pressing by naive rats are generally minimal, and it may be assumed that a
~Requests for reprints should be addressed to H. Smith, Department of Psychology, Trinity College, Dublin 2, Ireland.
917
918
G A N N O N , SMITH AND TIERNEY TABLE 1 MEANTIMESPENTRUNNINGIN THE BASELINECONDITION,AND MEANTIME TAKEN TO GAIN ACCESSTO THE DRINKINGTUBE, BY EACH SUBJECT
Subject 1 2 3 4
Baseline running time (sec) 354.3 205.9 224.0 248.3
Time taken to gain access to drinking tube (see) Running Requirement Conditions 0.25 124.8 130.2 115.8 144.6
particular number of lever-presses is about equally "effortfui" for all subjects. However, rats differ considerably in their baseline levels of running, and what is a large procurement cost for one may be relatively small for another. Thus each subject was required to perform in a number of conditions involving a series of proportions of its own baseline running level as the procurement costs. In this experiment the yoked control procedure was not employed. It had already been demonstrated that the delay between introduction into the apparatus and completion of the procurement requirement did not produce any effect on consummatory responding, even when the subjects were deprived of the consummatory response [7], thus it seemed unlikely that it would produce any effect in non-deprived subjects. Nevertheless, to ensure that this was the case, the subjects were run in a condition in which each had to wait in the apparatus, for the length of time it had taken to perform the largest procurement requirement, but without the procurement response being available, before access to the consummatory response was granted. Only an ascending series of procurement cost conditions was run in this experiment, and there were no control subjects. Thus it was necessary to check that any progressive change in the amount of consummatory responding across the series was an effect of the increasing procurement cost and not an effect of some other time dependent variable. This was done by including a condition at the end of the series, in which no procurement cost was imposed, and in which consummatory responding would be expected to decline again to a low level. METHOD
Subjects The subjects were four female albino rats of the Carworth Europe strain, approximately 180 days old at the start of the experiment. They had been bred in the University animal house, and supplied to the laboratory at 120 days of age. Prior to this experiment they had been used as subjects by undergraduates in a practical involving the shaping of a chain of responses. They were always housed individually in standard laboratory cages, and throughout this experiment food and water were always available in their home cages.
Apparatus The subjects were tested in an enclosed running-wheel 31 cm in diameter and 7 cm deep which could be prevented
0.5 249.6 259.6 196.8 352.2
1.0 623.4 854.4 975.6 1232.4
1.25
1.5
1558.8 2057.4 1699.8 2841.6
3116.4 5141).8 4939.2 4299.6
from revolving by means of a solenoid-operated brake. A 2 cm diameter hole in the non-revolving front panel of the wheel was positioned in such a way that a rat in the wheel could conveniently drink from a tube made available behind the hole. The drinking tube could be brought by means of a solenoid to a position 0.5 cm behind the hole, or withdrawn sideways to a position in which it could not be reached by a rat in the wheel. The drinking tube contained a solution made by dissolving two 17 mg tablets of saccharin sodium (Hermesetas) and 5 g of sucrose in 150 ml of water. Electromechanical counters recorded each lick of the tube and each one-sixth revolution of the wheel. A C.B.M. 3008 micro-computer controlled the operation of the solenoids and recorded the number of 0.25 sec intervals in which at least one lick of the tube occurred and the number of 0.25 sec intervals in which at least one-sixth of a revolution of the wheel occurred. In those conditions in which a running requirement had to be performed before water became available, it also recorded the time taken to complete the running requirement.
Procedure Throughout the experiment the subjects were run once a day, seven days a week, in sessions commencing at the same time each day. The subjects were initially habituated to the apparatus by running them in sessions of 20 rain duration in which the drinking tube was constantly available and the wheel was free to revolve. This condition was terminated when both the time spent drinking and the time spent running had stabilized according to two criteria: (1) Daniel's test for trend [3] indicated that there was no statistically significant 09<0.05) increasing or decreasing trend over 8 successive sessions, and (2) the averages of the 5 overlapping 4---sessions blocks in the same 8 sessions did not differ by more than 10% from the mean of five averages. Following this habituation phase the subjects were run in a condition in which only running was possible in the 20 min sessions. This single baseline condition was terminated when the time spent running met the criteria described above. The mean time per session spent running by each subject in the last 8 sessions of this condition can be seen in Table 1. These times were used to determine the time each would have to spend running in a subsequent series of conditions in which the drinking tube was made available only after a running requirement had been completed. The first of these conditions required the subjects to run for a quarter of this time; once they had done so the wheel was locked and the drinking
P R O C U R E M E N T COST A N D D R I N K I N G
919 TABLE 2
MEAN (AND S.D.) OF TIME SPENT DRINKING AND NUMBER OF LICKS IN 7 CONDITIONS, AND RESULTS OF COMPARISONS BETWEEN MEANS BY DUNCAN'S NEW MULTIPLE RANGE TEST
Time spent drinking (sec)
mean (S.D.) Duncan's test*
Number of licks
mean (S.D.) Duncan's test*
Delay Condition
Free Access Condition
0.25
0.5
367.2 (57.8)
385.7 (43.6)
391.9 (55.4)
409.4 (63.8)
2697 (267)
3007 (359)
3060 (445)
Running Requirement Conditions 1.0 1.25
3219 (504)
469.3(36.7)
3638 (331)
1.5
489.5 (30.4)
3791 (294)
494.8 (52.5)
3804 (426)
*Means not underscored by a common line differ significantly. p<0.01.
tube was made available for 20 min. This condition was terminated when the time spent drinking in these 20 rain sessions met the criteria described earlier. The running requirement was progressively increased in the subsequent conditions, ascending through the following series of proportions of the single baseline running time: 0.5, 1.00, 1.25 and 1.5. A further condition had been planned, in which twice the baseline running time was to have been required. However, this condition was dropped when it was found that the subjects did not complete the requirement in a reasonable time. The penultimate condition was one in which the drinking tube was made available, with the wheel locked, as soon as the session began (Free Access condition), and the final condition (Delay condition) was one in which the subject was placed in the locked wheel and required to wait for the mean length of time it had taken that subject to complete the largest running requirement (i.e., 1.5 baseline running time) in the earlier series of conditions. These times can be seen in the final column of Table 1. As soon as this time had elapsed the drinking tube was made available, with no running required or possible. In both of these last 2 conditions the drinking tube was available for 20 min before the session ended, and the conditions were terminated when the time spent drinking met the criteria described earlier. RESULTS
The data analyzed were the mean time spent drinking and the mean number of licks, per session, in the stable 8 session block in each of the 7 conditions in which a 20 rain period of access to drinking was permitted after the running wheel had been locked. These were the 5 conditions in which a proportion (0.25, 0.5, 1.0, 1.25 and 1.5) of the baseline running time had to be spent running before the drinking tube became available, the Free Access condition, and the Delay condition. Two one-way repeated measures analyses of variance of these data revealed that the conditions were a significant source of variance for both the time spent drinking, F(6,18)=11.92, p<0.01, and the number of licks, F(6.18)=12.08, p<0.01. The group means of both the time spent drinking, and the number o f licks, in the 7 conditions, are shown in Table 2. Also in Table 2 can be seen the results of Duncan's new multiple range test [30] applied to each of
6 40'
|ao. 20.
(JD
C
¢. m >
lO. J.
0.25
0.5
1.0
1.25
1.5
Running Requirement FIG. 1. Mean % change (-+S.E.M.) relative to the Pree Access condition, of the number of licks, in five running requirement conditions.
the sets of means; the means not underscored by a common line differ significantly, p<0.01. In the case of both sets o f data the means of the two largest running requirement conditions (1.25 and 1.5) did not differ significantly, but these two were both significantly larger, p < 0 . 0 1 , than the means of the Delay condition, the Free Access condition and the two smallest running requirement conditions (0.25 and 0.5), which did not differ significantly among themselves. When the drinking that occurred in the Free Access condition was taken as a basis against which to assess the effects of the running requirement conditions, and the percentage increase in the time spent drinking and the number of licks of each subject in these 5 conditions were used as the data in two further analyses o f variance, the results confirmed those already described. The mean percentage increase in the number of licks performed in each of the conditions can be seen in Fig. 1.
920
GANNON. SMITH AND TIERNEY
Page's trend test [26] was applied to these percentage increase data. It revealed that there was a significant monotonically increasing trend, as the running requirement increased, in the percentage increase in drinking over the amount in the Free Access condition, both in the case of the time spent drinking, L=218, p<0.001, and in the case of the number of licks, L =217, p<0.001. DISCUSSION
The results clearly demonstrate that the amount of the saccharin and sucrose solution consumed by the subjects increased monotonically as the running requirement was increased. The fact that consumption declined again to a low level when subjects were given free access to the solution, after the series of running requirement conditions, indicates that this relationship can not be attributed to an order effect or to any progressive time-related process such as the aging of the subjects. The fact that consumption did not increase above the free access level when the subjects were required to wait in the apparatus, without responding, for as long as it took them to perform the largest running requirement, indicates that it was not the time which it took to perform the running requirement that was responsible for the increase in consumption. These results are consistent with those obtained previously by Gannon et al. [7] when the procurement response was lever-pressing and the consummatory response was eating. Thus the effect appears to be a general one, not confined to specific kinds of procurement or consummatory responding. Also, the fact that the subjects in the present experiment worked to gain access to a saccharin and sucrose solution, and had food and water constantly available to them in their home cages, shows that the effect is not restricted to deprived subjects. The control procedure employed by Gannon et al. [7], and that employed in the present experiment, make it possible to conclude that the procurement cost effect does not depend upon the delay in gaining access to the consummatory response, although the delay also increases with the cost. However, it is not possible to conclude from these experiments that the effect depends upon the contingency between consummatory and procurement responding. It is possible, for example, that the probability of eating, or drinking, in rats is elevated following a period of running, or lever pressing, more than it is following whatever rats do in the experimental chamber when they are merely waiting for the consummatory response to become available. Further experimentation will be necessary to resolve this issue, but its outcome will not affect the relevance of the present results in helping to explain the relationship between meal size and procurement cost observed by Collier and his associates (e.g., [4,5]), as similar types of procurement and consummatory responding, in the same temporal sequence, were involved in both cases. These results add support to the argument [7] that the effect observed by Collier is at least partly the consequence of an effect of performing the procurement requirement on the subsequent meal size. The latter may also depend on the interval since the previous meal, but this has yet to be demonstrated. The fact that the subjects in the present experiment were
not deprived, and had access to the saccharin and sucrose solution only in the experimental chamber and only following performance of the procurement response adds support to this generalization. Hursh [12] has defined a closed economy as a condition in which the subject's total intake is determined solely by its interaction with the schedule of reinforcement, and he has argued that generalizations fi-om open economy laboratory studies to the behavior of animals living in such closed economies may be invalid. The conditions in which Collier has described the procurement cost effect on meal patterning and meal size fall within this definition of a closed economy, while those employed by Gannon et al. [7] did not. The latter tested their subjects on alternate days, and allowed them free access to food for a limited period in their cages on the other days. This procedure was necessary to prevent any possible deficit (or surplus) of intake produced by the procurement cost from progressively accumulating, but it had the disadvantage that it made the conditions an open economy. The conditions employed in the present experiment more nearly approximated those employed by Collier in this respect, although it would not be strictly appropriate to describe the situation as a closed economy, because some components of the saccharin and sucrose solution (water and the calories in the sucrose) were also available to the subjects outside the experimental sessions. Much of the traditional literature on feeding emphasizes the role of central and peripheral processes in the determination of the initiation and termination of bouts of eating, and also of the quantity consumed (e.g., [2,29]). Collier et u/. [5] have argued that greater recognition should be given to the role of ecological factors, such as the difficulty of obtaining food, and the present results and those of Gannon et al. [7], support this argument. There is evidence that central place foragers will take more, and larger, items of food as the distance they must travel to obtain them increases [9,17]. One of the problems in the study of foraging has been to identify how animals can track and respond to the availability of food [27]. However, if the effort that an animal has to expend in obtaining food is related to its distribution, the present results may help to explain this behaviour. However, at the present time it is not possible to offer a causal account of the phenomenon reported here. The effects of "effort" on subsequent behaviour have been studied in other contexts (e.g., [ 19, 20, 21, 22, 25]), but these studies have not identified the way in which the effects are produced. Manipulations such as prior tail-pinching [1] and the induction of stress [28] have been found to produce effects on eating similar to the performance of an "effortful" procurement requirement, but it would be premature to conclude that these manipulations had anything in common with the performance of such a requirement. When an animal has expended considerable effort in gaining access to a resource it would be a good (i.e.. adaptive) strategy for it to maximize its utilization of that resource. Functional accounts of this kind need to be treated with some caution because of both the difficulties inherent in optimality accounts [ 18,24] and because of the danger of them degenerating into "just so" stories [8], but in combination with a suitable causal account, this may prove to be the most satisfactory explanation of this phenomenon.
PROCUREMENT
COST AND DRINKING
921 REFERENCES
1. Antelman, S. M., H. Szechtman, P. Chin and A. E. Fisher. Tail-pinch-induced eating, gnawing and licking behavior in rats: dependence on the nigrostriatal dopamine system. Brain Res 99" 31%337, 1975. 2. Booth, D. A. (Ed.) Hunger Models: Computable Theory of Feeding Control. London: Academic Press, 1978. 3. Bradley, J. W. Distribution-free Statistical Tests. Englewood Cliffs, NJ: Prentice-Hall, 1968. 4. Collier, G. H., E. Hirsch and P. Hamlin. The ecological determinants of reinforcement in the rat. Physiol Behav 9- 705-716, 1972. 5. Collier, G. H., E. Hirsch and R. Kanarek. The operant revisited. In: Handbook of Operant Behavior, edited by W. K. Honig and J. E. R. Staddon. Englewood Cliffs, NJ: PrenticeHall, 1977. 6. Collier, G., L. W. Kaufman, R. Kanarek and J. Fagen. Optimization of time and energy constraints in the feeding behaviour of cats. Carnivore 1: 34-41, 1978. 7. Gannon, K. N., H. V. Smith and K. J. Tierney. Effects of procurement cost on food consumption in rats. Physiol Behav 31: 331-337, 1983. 8. Gould, S. J. and R. C. Lewontin. The spandrels of San Marco and the Panglassian paradigm: a critique of the adaptationist programme. Proc R Soc Lond (Biol) 205: 581-598, 1979. 9. Hegmer, R. E. Central place foraging in the white-fronted beeeater. Anim Behav 30: 953-963, 1982. 10. Hirsch, E. and G. Collier. The ecological determinants of reinforcement in the guinea pig. Physiol Behav 12- 23%249, 1974. 11. Hirsch, E. and G. Collier. Effort as a determinant of intake and patterns of drinking in the guinea pig. Physiol Behav 12: 647655, 1974. 12. Hursh, S. R. Economic concepts for the analysis of behavior. J Exp Anal Behav 34: 21%238, 1980. 13. Kanarek, R. B. and G. H. Collier. Patterns of eating as a function of the cost of the meal Physiol Behav 23: 141-145, 1979. 14. Kaufman, L. and G. Collier. Meal-taking by domestic chicks t~allu~ Gallur) Anita Behav 31: 397-40q 1983.
15. Kaufman, L. W. and G. Collier. Cost and meal patterns in wild-caught rats. Physiol Behav 30: 445-449, 1983. 16. Kaufman, L. W., G. Collier, W. L. Hill and K. Collins. Meal cost and meal patterns in an uncaged domestic cat. Physiol Behav 25: 135-137, 1980. 17. Killeen, P. R., J. P. Smith and S. J. Hanson. Central place foraging in Rattus Norvegicus. Anita Behav 29: 64-70, 1981. 18. Krebs, J. R. and N. B. Davies. An Introduction to Behavioural Ecology. Oxford: Blackwell Scientific, 1981. 19. Larkin, S. and D. J. McFarland. The cost of changing from one activity to another. Anim Behav 26: 1237-1246, 1978. 20. Lewis, M. Effect of effort on choice: value of a secondary reinforcer. Psychol Rep 16: 557-560, 1965. 21. Lewis, M. Effect of effort on value: an exploratory study of children. Child Dev 35: 1337-1342, 1964. 22. Lewis, M. Some nondecremental effects of effort. J Comp Physiol Psychol 55: 367-372, 1964. 23. Marwine, A. and G. Collier. The rat at the waterhole. J Comp Physiol Psychol 93: 391-402, 1979. 24. Maynard Smith, J. Optimization theory in evolution. Ann Rev Ecol Syst 9: 31-56, 1978. 25. McFarland, D. J. Feedback Mechanisms in Animal Behaviour. London: Academic Press, 1972. 26. Page, E. B. Ordered hypotheses for multiple treatments: a significance test for linear ranks. J Am Stat Assoc 58: 216--230, 1963. 27. Pyke, G. H., H. R. Pulliam and E. L. Charnov. Optimal foraging: a selective review of theory and tests. Q Rev Bio! 52: 137154, 1977. 28. Rowland, N. E. and S. M. Antelman. Stress-induced hyperphagia and obesity in rats: a possible model for understanding human obesity. Science 191: 310-312, 1976. 29. Silverstone, T. (Ed.). Appetite and Food lntake. Berlin: Dahlem Konferenzen/abakon, 1976. 30. Winer, B. J. Statistical Principles in Experimental Design. London: McGraw-Hill Kogakusha, 1971.
0031-9384/84$3.00 + .00
Effect of Procurement Cost on the Drinking of a Saccharin-Sucrose Solution by Non-Deprived Rats KENNETH
N . G A N N O N , H O W A R D V. S M I T H 1 A N D K E V I N J. T I E R N E Y
D e p a r t m e n t o f P s y c h o l o g y , Trinity College, D u b l i n 2, I r e l a n d R e c e i v e d 31 O c t o b e r 1983 GANNON, K. N., H. V. SMITH AND K. J. TIERNEY. Effect of procurement cost on the drinking of a saccharinsucrose solution by non-deprived rats. PHYSIOL BEHAV 33(6) 917-921, 1984.--Four non-deprived female rats were required to run in a wheel to obtain 20 min unconstrained access to a saccharin and sucrose solution. Each was run in a series of conditions in which the requirement was a proportion (0.25, 0.5, 1.0, 1.25 and 1.5) of the amount of running performed in a condition in which the wheel alone was available. A condition in which no running was required to gain access to the solution and one in which the subject was locked in the stationary wheel for the time taken to complete the highest requirement before being allowed access to the solution were also included. The results showed that as the requirement increased the amount of solution consumed also increased, and this relationship did not depend on the time taken to perform the requirement. Procurement cost Adaptation
Saccharin-sucrose solution
Wheel-running
IN a number of experiments Collier and his associates have found that when animals live in a situation in which they must perform an instrumental requirement in order to gain the opportunity to eat they increase both the size of the meals they take, and the intervals between the meals, as the procurement cost is increased (e.g., [4, 5, 6, 10, 13, 14, 15, 16]). They respond similarly with respect to both the size of drinking bouts, and the intervals between bouts, when an increase is made in the procurement cost that must be paid to gain access to water [11,23]. These changes appear to depend upon the amount of energy, or "effort," rather than time, expended in performing the procurement requirement [6], and it may be inferred that they contribute to an optimal strategy, but little is known about the processes that are responsible for them. The nature of the situation makes it impossible to distinguish between, for example, any direct effect of the procurement cost on meal size, and any possible indirect effect on this variable mediated by the period of self-imposed deprivation which precedes the initiation of the meal, because the latter also varies with the cost. In an attempt to disentangle some of these factors Gannon, Smith and Tierney [7] studied the effect of increasing the number of lever-presses that had to be performed to gain access to food on the amount that was eaten by rats tested under constant conditions of deprivation. A control subject, that did not have to perform the procurement requirement, was yoked to each experimental subject and received access to food only when the latter had completed the requirement. The results showed that both the amount eaten by the experimental subjects, and the time for which they delayed before commencing the procurement requirement, increased with
Non-deprived subjects
Foraging
the size of the procurement cost. In contrast, the amount eaten by the control subjects did not vary with the procurement cost paid by their experimental partners. Thus Gannon et al. were able to conclude that both the size of the meals taken and the latency to initiate procurement responding were independently affected by the procurement cost. It was necessary for Gannon et al. to impose a constant level of deprivation on their subjects in order to isolate the effect of the procurement cost on meal size from the effect of the duration of fasting. However, this procedure inevitably created a difference between the conditions under which their subjects were tested and those which prevail in the situation usually employed by Collier. No experimenterimposed deprivation is employed in the latter situation. It would be necessary to eliminate this difference in order to discover whether there is an effect of the procurement cost on meal size in Collier's situation. The present experiment was designed to achieve this objective, in an indirect way, by studying the effect of the procurement cost on the amount of consummatory responding in non-deprived subjects. The experiment also had the subsidiary aim of discovering whether the effect observed by Gannon et al. [7] occurs when different instrumental and consummatory responses are employed. In the present experiment the subjects were required to run in a wheel in order to get the opportunity to drink a saccharin and sucrose solution. The times for which they were required to run were based on the times they each spent running in a baseline condition, rather than being constant across subjects. Baseline rates of lever-pressing by naive rats are generally minimal, and it may be assumed that a
~Requests for reprints should be addressed to H. Smith, Department of Psychology, Trinity College, Dublin 2, Ireland.
917
918
G A N N O N , SMITH AND TIERNEY TABLE 1 MEANTIMESPENTRUNNINGIN THE BASELINECONDITION,AND MEANTIME TAKEN TO GAIN ACCESSTO THE DRINKINGTUBE, BY EACH SUBJECT
Subject 1 2 3 4
Baseline running time (sec) 354.3 205.9 224.0 248.3
Time taken to gain access to drinking tube (see) Running Requirement Conditions 0.25 124.8 130.2 115.8 144.6
particular number of lever-presses is about equally "effortfui" for all subjects. However, rats differ considerably in their baseline levels of running, and what is a large procurement cost for one may be relatively small for another. Thus each subject was required to perform in a number of conditions involving a series of proportions of its own baseline running level as the procurement costs. In this experiment the yoked control procedure was not employed. It had already been demonstrated that the delay between introduction into the apparatus and completion of the procurement requirement did not produce any effect on consummatory responding, even when the subjects were deprived of the consummatory response [7], thus it seemed unlikely that it would produce any effect in non-deprived subjects. Nevertheless, to ensure that this was the case, the subjects were run in a condition in which each had to wait in the apparatus, for the length of time it had taken to perform the largest procurement requirement, but without the procurement response being available, before access to the consummatory response was granted. Only an ascending series of procurement cost conditions was run in this experiment, and there were no control subjects. Thus it was necessary to check that any progressive change in the amount of consummatory responding across the series was an effect of the increasing procurement cost and not an effect of some other time dependent variable. This was done by including a condition at the end of the series, in which no procurement cost was imposed, and in which consummatory responding would be expected to decline again to a low level. METHOD
Subjects The subjects were four female albino rats of the Carworth Europe strain, approximately 180 days old at the start of the experiment. They had been bred in the University animal house, and supplied to the laboratory at 120 days of age. Prior to this experiment they had been used as subjects by undergraduates in a practical involving the shaping of a chain of responses. They were always housed individually in standard laboratory cages, and throughout this experiment food and water were always available in their home cages.
Apparatus The subjects were tested in an enclosed running-wheel 31 cm in diameter and 7 cm deep which could be prevented
0.5 249.6 259.6 196.8 352.2
1.0 623.4 854.4 975.6 1232.4
1.25
1.5
1558.8 2057.4 1699.8 2841.6
3116.4 5141).8 4939.2 4299.6
from revolving by means of a solenoid-operated brake. A 2 cm diameter hole in the non-revolving front panel of the wheel was positioned in such a way that a rat in the wheel could conveniently drink from a tube made available behind the hole. The drinking tube could be brought by means of a solenoid to a position 0.5 cm behind the hole, or withdrawn sideways to a position in which it could not be reached by a rat in the wheel. The drinking tube contained a solution made by dissolving two 17 mg tablets of saccharin sodium (Hermesetas) and 5 g of sucrose in 150 ml of water. Electromechanical counters recorded each lick of the tube and each one-sixth revolution of the wheel. A C.B.M. 3008 micro-computer controlled the operation of the solenoids and recorded the number of 0.25 sec intervals in which at least one lick of the tube occurred and the number of 0.25 sec intervals in which at least one-sixth of a revolution of the wheel occurred. In those conditions in which a running requirement had to be performed before water became available, it also recorded the time taken to complete the running requirement.
Procedure Throughout the experiment the subjects were run once a day, seven days a week, in sessions commencing at the same time each day. The subjects were initially habituated to the apparatus by running them in sessions of 20 rain duration in which the drinking tube was constantly available and the wheel was free to revolve. This condition was terminated when both the time spent drinking and the time spent running had stabilized according to two criteria: (1) Daniel's test for trend [3] indicated that there was no statistically significant 09<0.05) increasing or decreasing trend over 8 successive sessions, and (2) the averages of the 5 overlapping 4---sessions blocks in the same 8 sessions did not differ by more than 10% from the mean of five averages. Following this habituation phase the subjects were run in a condition in which only running was possible in the 20 min sessions. This single baseline condition was terminated when the time spent running met the criteria described above. The mean time per session spent running by each subject in the last 8 sessions of this condition can be seen in Table 1. These times were used to determine the time each would have to spend running in a subsequent series of conditions in which the drinking tube was made available only after a running requirement had been completed. The first of these conditions required the subjects to run for a quarter of this time; once they had done so the wheel was locked and the drinking
P R O C U R E M E N T COST A N D D R I N K I N G
919 TABLE 2
MEAN (AND S.D.) OF TIME SPENT DRINKING AND NUMBER OF LICKS IN 7 CONDITIONS, AND RESULTS OF COMPARISONS BETWEEN MEANS BY DUNCAN'S NEW MULTIPLE RANGE TEST
Time spent drinking (sec)
mean (S.D.) Duncan's test*
Number of licks
mean (S.D.) Duncan's test*
Delay Condition
Free Access Condition
0.25
0.5
367.2 (57.8)
385.7 (43.6)
391.9 (55.4)
409.4 (63.8)
2697 (267)
3007 (359)
3060 (445)
Running Requirement Conditions 1.0 1.25
3219 (504)
469.3(36.7)
3638 (331)
1.5
489.5 (30.4)
3791 (294)
494.8 (52.5)
3804 (426)
*Means not underscored by a common line differ significantly. p<0.01.
tube was made available for 20 min. This condition was terminated when the time spent drinking in these 20 rain sessions met the criteria described earlier. The running requirement was progressively increased in the subsequent conditions, ascending through the following series of proportions of the single baseline running time: 0.5, 1.00, 1.25 and 1.5. A further condition had been planned, in which twice the baseline running time was to have been required. However, this condition was dropped when it was found that the subjects did not complete the requirement in a reasonable time. The penultimate condition was one in which the drinking tube was made available, with the wheel locked, as soon as the session began (Free Access condition), and the final condition (Delay condition) was one in which the subject was placed in the locked wheel and required to wait for the mean length of time it had taken that subject to complete the largest running requirement (i.e., 1.5 baseline running time) in the earlier series of conditions. These times can be seen in the final column of Table 1. As soon as this time had elapsed the drinking tube was made available, with no running required or possible. In both of these last 2 conditions the drinking tube was available for 20 min before the session ended, and the conditions were terminated when the time spent drinking met the criteria described earlier. RESULTS
The data analyzed were the mean time spent drinking and the mean number of licks, per session, in the stable 8 session block in each of the 7 conditions in which a 20 rain period of access to drinking was permitted after the running wheel had been locked. These were the 5 conditions in which a proportion (0.25, 0.5, 1.0, 1.25 and 1.5) of the baseline running time had to be spent running before the drinking tube became available, the Free Access condition, and the Delay condition. Two one-way repeated measures analyses of variance of these data revealed that the conditions were a significant source of variance for both the time spent drinking, F(6,18)=11.92, p<0.01, and the number of licks, F(6.18)=12.08, p<0.01. The group means of both the time spent drinking, and the number o f licks, in the 7 conditions, are shown in Table 2. Also in Table 2 can be seen the results of Duncan's new multiple range test [30] applied to each of
6 40'
|ao. 20.
(JD
C
¢. m >
lO. J.
0.25
0.5
1.0
1.25
1.5
Running Requirement FIG. 1. Mean % change (-+S.E.M.) relative to the Pree Access condition, of the number of licks, in five running requirement conditions.
the sets of means; the means not underscored by a common line differ significantly, p<0.01. In the case of both sets o f data the means of the two largest running requirement conditions (1.25 and 1.5) did not differ significantly, but these two were both significantly larger, p < 0 . 0 1 , than the means of the Delay condition, the Free Access condition and the two smallest running requirement conditions (0.25 and 0.5), which did not differ significantly among themselves. When the drinking that occurred in the Free Access condition was taken as a basis against which to assess the effects of the running requirement conditions, and the percentage increase in the time spent drinking and the number of licks of each subject in these 5 conditions were used as the data in two further analyses o f variance, the results confirmed those already described. The mean percentage increase in the number of licks performed in each of the conditions can be seen in Fig. 1.
920
GANNON. SMITH AND TIERNEY
Page's trend test [26] was applied to these percentage increase data. It revealed that there was a significant monotonically increasing trend, as the running requirement increased, in the percentage increase in drinking over the amount in the Free Access condition, both in the case of the time spent drinking, L=218, p<0.001, and in the case of the number of licks, L =217, p<0.001. DISCUSSION
The results clearly demonstrate that the amount of the saccharin and sucrose solution consumed by the subjects increased monotonically as the running requirement was increased. The fact that consumption declined again to a low level when subjects were given free access to the solution, after the series of running requirement conditions, indicates that this relationship can not be attributed to an order effect or to any progressive time-related process such as the aging of the subjects. The fact that consumption did not increase above the free access level when the subjects were required to wait in the apparatus, without responding, for as long as it took them to perform the largest running requirement, indicates that it was not the time which it took to perform the running requirement that was responsible for the increase in consumption. These results are consistent with those obtained previously by Gannon et al. [7] when the procurement response was lever-pressing and the consummatory response was eating. Thus the effect appears to be a general one, not confined to specific kinds of procurement or consummatory responding. Also, the fact that the subjects in the present experiment worked to gain access to a saccharin and sucrose solution, and had food and water constantly available to them in their home cages, shows that the effect is not restricted to deprived subjects. The control procedure employed by Gannon et al. [7], and that employed in the present experiment, make it possible to conclude that the procurement cost effect does not depend upon the delay in gaining access to the consummatory response, although the delay also increases with the cost. However, it is not possible to conclude from these experiments that the effect depends upon the contingency between consummatory and procurement responding. It is possible, for example, that the probability of eating, or drinking, in rats is elevated following a period of running, or lever pressing, more than it is following whatever rats do in the experimental chamber when they are merely waiting for the consummatory response to become available. Further experimentation will be necessary to resolve this issue, but its outcome will not affect the relevance of the present results in helping to explain the relationship between meal size and procurement cost observed by Collier and his associates (e.g., [4,5]), as similar types of procurement and consummatory responding, in the same temporal sequence, were involved in both cases. These results add support to the argument [7] that the effect observed by Collier is at least partly the consequence of an effect of performing the procurement requirement on the subsequent meal size. The latter may also depend on the interval since the previous meal, but this has yet to be demonstrated. The fact that the subjects in the present experiment were
not deprived, and had access to the saccharin and sucrose solution only in the experimental chamber and only following performance of the procurement response adds support to this generalization. Hursh [12] has defined a closed economy as a condition in which the subject's total intake is determined solely by its interaction with the schedule of reinforcement, and he has argued that generalizations fi-om open economy laboratory studies to the behavior of animals living in such closed economies may be invalid. The conditions in which Collier has described the procurement cost effect on meal patterning and meal size fall within this definition of a closed economy, while those employed by Gannon et al. [7] did not. The latter tested their subjects on alternate days, and allowed them free access to food for a limited period in their cages on the other days. This procedure was necessary to prevent any possible deficit (or surplus) of intake produced by the procurement cost from progressively accumulating, but it had the disadvantage that it made the conditions an open economy. The conditions employed in the present experiment more nearly approximated those employed by Collier in this respect, although it would not be strictly appropriate to describe the situation as a closed economy, because some components of the saccharin and sucrose solution (water and the calories in the sucrose) were also available to the subjects outside the experimental sessions. Much of the traditional literature on feeding emphasizes the role of central and peripheral processes in the determination of the initiation and termination of bouts of eating, and also of the quantity consumed (e.g., [2,29]). Collier et u/. [5] have argued that greater recognition should be given to the role of ecological factors, such as the difficulty of obtaining food, and the present results and those of Gannon et al. [7], support this argument. There is evidence that central place foragers will take more, and larger, items of food as the distance they must travel to obtain them increases [9,17]. One of the problems in the study of foraging has been to identify how animals can track and respond to the availability of food [27]. However, if the effort that an animal has to expend in obtaining food is related to its distribution, the present results may help to explain this behaviour. However, at the present time it is not possible to offer a causal account of the phenomenon reported here. The effects of "effort" on subsequent behaviour have been studied in other contexts (e.g., [ 19, 20, 21, 22, 25]), but these studies have not identified the way in which the effects are produced. Manipulations such as prior tail-pinching [1] and the induction of stress [28] have been found to produce effects on eating similar to the performance of an "effortful" procurement requirement, but it would be premature to conclude that these manipulations had anything in common with the performance of such a requirement. When an animal has expended considerable effort in gaining access to a resource it would be a good (i.e.. adaptive) strategy for it to maximize its utilization of that resource. Functional accounts of this kind need to be treated with some caution because of both the difficulties inherent in optimality accounts [ 18,24] and because of the danger of them degenerating into "just so" stories [8], but in combination with a suitable causal account, this may prove to be the most satisfactory explanation of this phenomenon.
PROCUREMENT
COST AND DRINKING
921 REFERENCES
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