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Effect of procurement wheel running on the rate of drinking in rats
Physiology & Behavior, Vol. 33, pp. 927-930. Copyright©Pergamon Press Ltd., 1984. Printedin the U.S.A.
0031-9384/84$3.00 + .00
Effect of Procurement Wheel Running on the Rate of Drinking in Rats H O W A R D V. S M I T H / K E N N E T H
N. GANNON
e A N D K E V I N J. T I E R N E Y
Trinity College, D u b l i n R e c e i v e d 13 D e c e m b e r 1983 SMITH, H. V., K. N. GANNON AND K. J. TIERNEY. Effect of procurement wheel running on the rate of drinking in rats. PHYSIOL BEHAV 33(6) 927-930, 1984.--Four rats, 22.5 hr deprived of water, were tested in 3 conditions, in which they were required to run zero, 5 or 300 one-sixth revolutions in a wheel to gain 30 rain access to water in a drinking tube. The number of licks performed in the 30 min increased monotonically with the procurement cost, and was 20% greater following the larger cost than when there was no cost. However, examination of the rates of drinking throughout the 30 rain revealed that differences occurred between the conditions only at the beginning of the period. In the first 3 rain there was a monotonic relationship between the proportion of time spent drinking and the procurement cost, but no effect of the cost on the rate could be detected after the first 6 rain. The results are consistent with the hypothesis that the effect is mediated by a transient elevation of the subject's arousal level. Procurement-eos~
Drinking
Wheel-running
Arousal
C O L L I E R and his associates have reported that when the procurement cost that must be paid to gain access to a commodity, such as food (e.g., [3, 4, 6, 8] ) or water [7,9], is increased, animals reduce the frequency with which they seek access to it, but increase the amount they take once access has been gained. In an attempt to elucidate some of the processes involved in these changes Gannon, Smith and Tierney [5] compared the amounts of food eaten in a 60 min period by 2 groups of rats tested at a constant level of deprivation. Those in the experimental group had to perform a range of lever press requirements to gain access to the food, while those in a yoked control group received access when their experimental partners had completed the requirements. In the experimental group both the latency before procurement responding was initiated and the amount eaten when food became available increased monotonically as the procurement cost was increased, but the amounts eaten by the control subjects did not depend on the procurement costs paid by their experimental partners. Gannon et al. were able to conclude, therefore, that there is a direct effect of performing a procurement requirement on the amount of consummatory responding that will occur in a fixed period following its completion. The main aim of the present experiment was to study the way in which the performance of a procurement requirement affects the rate of consummatory responding during such a period of access, to discover whether the rate is elevated equally throughout the entire period or is only elevated during certain parts of it, such as the beginning or the end. Drinking was selected as the consummatory response to be studied, because it is much easier to record individual ticks
during drinking than it is to record any equivalent response during eating. The rate measure employed was the proportion of time spent licking within specified blocks of time throughout the period of access. This measure was chosen because rats drink by emitting bursts of ticking separated by pauses, and the rate of ticking (i.e., the frequency per unit time) within the bursts is thought to be relatively resistant to change (cf, [2]). Thus any change in the rate would be expected to manifest itself in terms of a change in the relative amounts of time spent in bursts of licking and pauses. The choice of drinking as the consummatory response also provided the opportunity to discover whether the basic procurement-cost effect reported by Gannon et al. [5] occurs when a different consummatory response is involved, and the opportunity was taken to change the procurement response, wheel running being substituted for lever pressing, to test also the generality of the effect in respect of the response on the other side of the relationship. METHOD Subjects The subjects were 4 experimentally naive male Wistar rats, approximately 120 days old at the start of the experiment. They were bred in the University animal house, and supplied to the laboratory at about 60 days of age. Apparatus The subjects were tested in a wooden chamber (27×35×35 cm), one of the long sides of which consisted
tRequest for reprints should be addressed to Dr. H. V. Smith, Department of Psychology, Trinity College, Dublin 2, Ireland. 2This research was conducted while K. N. Gannon was in receipt of maintenance allowance number CO3A21/05/80 from the Department of Education.
927
928 only of a frame 3 cm wide, behind which was the open face of a running wheel, 31 cm in diameter and 7 cm deep. The wheel revolved in only one direction and could be prevented from revolving by means of a solenoid-operated brake. In the opposite side of the chamber was a 2 cm diameter hole, 11.5 cm above the floor. A drinking tube, containing tap water, could be brought by means of a solenoid to a position 0.5 cm behind this hole, or withdrawn sideways to a position in which it could not be reached by a rat in the chamber. This side of the chamber was movable, and during the training phase of the experiment it was moved to enclose the subjects in the running wheel. During all other phases of the experiment the subjects had to cross the chamber (27 cm) to travel from the wheel to the drinking tube. On these occasions a food tray was attached to a third wall of the chamber. Electromechanical equipment recorded each lick of the drinking tube and each one-sixth revolution of the wheel. A recycling clock defined 1 sec intervals, and the pen of a cumulative recorder was stepped each time there was a 1 sec interval during which there had been at least one lick of the drinking tube. When a running requirement had to be performed before the drinking tube became available, a predetermined counter caused the solenoids to operate to make the drinking tube available and to prevent the wheel revolving, as soon as the requirement was completed.
SMITH, GANNON AND TIERNEY
70-
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.
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3 min Blocks FIG. I. Mean time spent drinking in 3 conditions within each 3 min block in the 30 rain period of access to water. Time spent drinking was measured from the increase in height of the cumulative record ( 1 mm=2.54 sec). Five and 300 represent the distance (in one-sixth revolutions) that had to be run in the wheel before access was gained.
Procedure
When not being tested the subjects were housed individually in standard laboratory cages, and had food available to them at all times. They were tested once each day, at the same time every day, 7 days a week, for a total of 20 weeks. From the start of the adaptation phase through to the end of the experiment they received access to water for only one hour during each day. There was an adaptation phase lasting 7 weeks, during which the subjects adjusted to this drinking schedule, and learned to drink from the drinking tube in the chamber. There followed a training phase of 3 weeks during which the subjects were given one 30 min session each day enclosed in the running wheel. During these sessions they were repeatedly required to make a running response in order to receive 10 sec access to the drinking tube. The running requirement for which these rewards were given was progressively increased from 1 to 100 one-sixth revolutions of the wheel. When these sessions were over the subjects were returned to their home cages and 30 rain later were given their water bottles in those cages for a further 30 rain. Thus the subjects were always 22.5 hours deprived at the start of each session. The training phase was concluded when the subjects were all performing well in the highest requirement condition. During the experimental phase each subject was required to perform in 3 conditions, with 10 successive sessions in each condition. These were a Free Access condition, in which no running was required and 30 min access to the drinking tube was available as soon as the subject was placed in the chamber, and 2 Running Requirement conditions in which the subject had to run a specified distance (5 or 300 one-sixth revolutions) before 30 rain access to the drinking tube was gained. As in the training phase each subject also received 30 min access to water in its home cage, 30 min after the end of the session. Two subjects performed the conditions in the order Free Access, 300, 5, and two performed them in the opposite order. When changing from one condition to another the
procurement cost was adjusted in steps over 20 sessions. The first 2 sessions in each condition were used to familiarize the subjects with the new procurement cost, and the data subsequently analyzed were only those from the last 8 sessions in each condition. RESULTS The data analyzed were twofold: (a) The means of the subjects over the 8 sessions of the numbers of licks in the 30 min period of access; and (b) The means of the subjects over the 8 sessions of the increase in height (in mm), in each 3 min period throughout the 30 min, of the cumulative record of time spent drinking. (Note: 1 mm=2.54 sec of drinking.) An analysis of variance, for a 2 × 3 (orders x conditions) factorial experiment with repeated measures on the second factor [ 13], of the number of licks revealed a significant main effect of conditions, F(2,4)=8.63, p<0.05. Comparisons of the means by the Newman-Keuls procedure [13] revealed that there was significantly more licking, p<0.05, in the 5 and 300 conditions (means 2678 and 2723 respectively) than there was in the Free Access condition (mean=2255). There was no significant difference between the means of the 5 and 300 conditions, but Page's trend test [10] revealed that when the 3 conditions were considered there was a significant monotonic increase in the number of licks as the procurement cost increased, L=54, p =0.05. An analysis of variance, for a 2 x 3 × 10 (ordersxconditions×3 min blocks) factorial experiment with repeated measures on the last two factors [13], was performed on the data from the cumulative records. This revealed a highly significant main effect of 3 min blocks, F(9,18)=665.4, p<0.001, and a significant interaction between the blocks and conditions, F(18,36)=2.46, p<0.05. The former of these merely reflected the fact that the proportion of the time spent drinking in all the conditions declined progressively
P R O C U R E M E N T COST A N D D R I N K I N G RATE
929
throughout the period of access, but the latter reflected the fact that there were differences between conditions in the proportion of time spent drinking within the blocks. These differences can be seen in Figure 1, which shows the mean increase in height o f the cumulative record in each 3 min block for each of the 3 conditions. Analyses of the simple main effect of conditions within each of the 3 rain blocks [13] revealed that the conditions produced significant differences only within Blocks l and 2, F(2,39)=10.28, and F(2,39)=6.66 respectively, p ' s < 0 . 0 1 . Comparisons between the means within these two blocks revealed that in Block 1 there was a significantly higher rate of drinking in both the 300 condition, p<0.01, and in the 5 condition, p<0.05, than there was in the Free Access condition. The rates did hot differ significantly between the 5 and 300 conditions, but Page's trend test revealed a significant monotonic trend across the 3 conditions, L=56, p=0.001. In Block 2 the rate in the 300 condition was significantly higher than both that in the Free Access condition, p <0.01, and that in the 5 condition, p<0.05, but there was no significant difference between the rates in the Free Access and 5 conditions. DISCUSSION The results of this experiment, with respect to the number of licks performed in the 30 min period of access, are clearly very similar to the findings obtained by Gannon et al. [5] when they required rats to press a lever to gain access to food. F e w e r procurement cost conditions were included in the present experiment, and it was not designed to discover whether the effect could be attributed directly to the performance of the procurement requirement, but in all other respects the results of the 2 studies were so consistent that it seems likely that a common process was responsible for them both, and that the procurement-cost effect is not restricted to specific categories of responding. The magnitude of the increase was also of a similar order in the two experiments. The largest procurement costs employed by Gannon et al. [5] were followed by the intake of nearly 30% more food in a 60 min period than occurred when there was no procurement cost. In the present experiment the numbers of licks increased by just over 20%, although the period of access was only half as long. The results of the analyses of the rates of drinking at different times throughout the period of access provide interesting information concerning the way in which this procurement-cost effect is produced. The initial rate of consummatory responding is elevated by a proportion that increases as the procurement cost is increased, and the duration for which this elevation persists depends upon its initial
level. The effect does not persist for very long even when the cost is large, but it occurs during the period of most intensive consummatory responding. The largest cost in the present experiment had an effect that was detectable statistically for the first 6 min of access to water, a period during which about 60% of the total drinking for the entire 30 min took place. These findings suggest that the effect would not be lost even if the duration of the period of access was increased, or if the subjects determine it for themselves as happens in the situation employed by Collier [3, 4, 6, 7, 8, 9]. The criterion of drinking employed in this experiment probably slightly overestimated the time spent drinking, because part seconds of licking at the beginning and end of bursts were counted as whole seconds. However, there do not seem to have been differences between the conditions in the within-burst rates of licking, which suggests that similar numbers of bursts were involved. From the total time spent drinking and the total number of licks it can be calculated that across the subjects the average rate of licking within bursts was much the same for the Free Access, 5 and 300 conditions (5.6, 5.8 and 5.7 licks/sec respectively). These rates do not differ greatly from the range of about 6--7 licks/ sec commonly reported for rats in a variety of conditions [2]. It is likely, therefore, that the increase in the rate of drinking produced by the procurement cost reflects an increase in the proportion of time spent in bursts of licking, consequent on a reduction in the duration of pauses between bursts. The purpose of this experiment was to obtain more detailed information than had been available previously concerning the way the procurement-cost effect is manifest rather than to permit the identification of the mechanisms that mediate it. Nevertheless, it may be inferred from the fact that the effect is both immediate and transient that a mechanism with similar properties is involved. A possible candidate f o r this role might be the arousal level of the subject. It has been pointed out [14] that it would be adaptive if intake was managed more efficiently when disruption or danger threatens, and this could be brought about if the rate of intake was elevated when arousal level was increased. This suggestion is consistent with the fact that manipulations which might be expected to increase arousal level have been found to increase eating in rats. F o r example, the induction of stress [I 1] and prior tail-pinching [1] have been found to have this effect on rats, and brief electric shock immediately prior to the regular 60 min period of access to food in 23 hr deprived rats has been shown to increase the initial eating rate [12] in a way similar to the effect reported here. However, so little information is available at present concerning the procurement-cost effect that the hypothesis that arousal is involved can only be advanced tentatively at this time.
REFERENCES 1. Antelman, S. M., H. Szechtman, P. Chin and R. E. Fisher. Tailpinch-induced eating, gnawing and licking behavior in rats: dependence on nigrostriatal dopamine system. Brain Res 99: 319-337, 1975. 2. Bolles, R. C. Theory of Motivation, 2nd edition. New York: Harper and Row, 1975. 3. Collier, G. H., E. Hirsch and P. Hamlin. The ecological determinants of reinforcement in the rat. Physiol Behav 9: 705-716, 1972.
4. Collier, G. H., L. W. Kaufman, R. Kanarek and J. Fagen. Optimization of time and energy constraints in the feeding behavior of cats. Carnivore 1: 34--41, 1978. 5. Gannon, K. N., H. V. Smith and K. J. Tierney. Effects of procurement cost on food consumption in rats. Physiol Behav 31: 331-338, 1983. 6. Hirsch, E. and G. Collier. The ecological determinants of reinforcement in the guinea pig. Physiol Behav 12: 239-249, 1974.
930
7. Hirsch, E. and G. Collier. Effort as a determinant of intake and patterns of drinking in the guinea pig. Physiol Behav 12: 647655, 1974. 8. Kaufman, L. W., G. H. Collier, W. L. Hill and K. Collins. Meal cost and meal patterns in an uncaged domestic cat. Physiol Behav 25: 135-137, 1980. 9. Marwine, A. and G. Collier. The rat at the waterhole. J Comp Physiol Psychol 93: 391-402, 1979. 10. Page, E. B. Ordered hypotheses for multiple treatments: a significance test for linear ranks. J Am Stat Assoc 58: 216-230, 1963.
SMITH, GANNON AND TIERNEY
11. 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. 12. Strongman, K. T. The effect of anxiety on food intake in the rat. Q J Exp Psychol 17: 255-260, 1965. 13. Winer, B. J. Statistical Principles in Experimental Design. London: McGraw-Hill Kogakusha, 1971. 14. Wooley, S. C. (Rapporteur) Psychological aspects of feeding: Group report. In: Appetite and Food Intake, edited by T. Silverstone. Berlin: Dahlem Konferenzen/Abakon, 1976.
0031-9384/84$3.00 + .00
Effect of Procurement Wheel Running on the Rate of Drinking in Rats H O W A R D V. S M I T H / K E N N E T H
N. GANNON
e A N D K E V I N J. T I E R N E Y
Trinity College, D u b l i n R e c e i v e d 13 D e c e m b e r 1983 SMITH, H. V., K. N. GANNON AND K. J. TIERNEY. Effect of procurement wheel running on the rate of drinking in rats. PHYSIOL BEHAV 33(6) 927-930, 1984.--Four rats, 22.5 hr deprived of water, were tested in 3 conditions, in which they were required to run zero, 5 or 300 one-sixth revolutions in a wheel to gain 30 rain access to water in a drinking tube. The number of licks performed in the 30 min increased monotonically with the procurement cost, and was 20% greater following the larger cost than when there was no cost. However, examination of the rates of drinking throughout the 30 rain revealed that differences occurred between the conditions only at the beginning of the period. In the first 3 rain there was a monotonic relationship between the proportion of time spent drinking and the procurement cost, but no effect of the cost on the rate could be detected after the first 6 rain. The results are consistent with the hypothesis that the effect is mediated by a transient elevation of the subject's arousal level. Procurement-eos~
Drinking
Wheel-running
Arousal
C O L L I E R and his associates have reported that when the procurement cost that must be paid to gain access to a commodity, such as food (e.g., [3, 4, 6, 8] ) or water [7,9], is increased, animals reduce the frequency with which they seek access to it, but increase the amount they take once access has been gained. In an attempt to elucidate some of the processes involved in these changes Gannon, Smith and Tierney [5] compared the amounts of food eaten in a 60 min period by 2 groups of rats tested at a constant level of deprivation. Those in the experimental group had to perform a range of lever press requirements to gain access to the food, while those in a yoked control group received access when their experimental partners had completed the requirements. In the experimental group both the latency before procurement responding was initiated and the amount eaten when food became available increased monotonically as the procurement cost was increased, but the amounts eaten by the control subjects did not depend on the procurement costs paid by their experimental partners. Gannon et al. were able to conclude, therefore, that there is a direct effect of performing a procurement requirement on the amount of consummatory responding that will occur in a fixed period following its completion. The main aim of the present experiment was to study the way in which the performance of a procurement requirement affects the rate of consummatory responding during such a period of access, to discover whether the rate is elevated equally throughout the entire period or is only elevated during certain parts of it, such as the beginning or the end. Drinking was selected as the consummatory response to be studied, because it is much easier to record individual ticks
during drinking than it is to record any equivalent response during eating. The rate measure employed was the proportion of time spent licking within specified blocks of time throughout the period of access. This measure was chosen because rats drink by emitting bursts of ticking separated by pauses, and the rate of ticking (i.e., the frequency per unit time) within the bursts is thought to be relatively resistant to change (cf, [2]). Thus any change in the rate would be expected to manifest itself in terms of a change in the relative amounts of time spent in bursts of licking and pauses. The choice of drinking as the consummatory response also provided the opportunity to discover whether the basic procurement-cost effect reported by Gannon et al. [5] occurs when a different consummatory response is involved, and the opportunity was taken to change the procurement response, wheel running being substituted for lever pressing, to test also the generality of the effect in respect of the response on the other side of the relationship. METHOD Subjects The subjects were 4 experimentally naive male Wistar rats, approximately 120 days old at the start of the experiment. They were bred in the University animal house, and supplied to the laboratory at about 60 days of age. Apparatus The subjects were tested in a wooden chamber (27×35×35 cm), one of the long sides of which consisted
tRequest for reprints should be addressed to Dr. H. V. Smith, Department of Psychology, Trinity College, Dublin 2, Ireland. 2This research was conducted while K. N. Gannon was in receipt of maintenance allowance number CO3A21/05/80 from the Department of Education.
927
928 only of a frame 3 cm wide, behind which was the open face of a running wheel, 31 cm in diameter and 7 cm deep. The wheel revolved in only one direction and could be prevented from revolving by means of a solenoid-operated brake. In the opposite side of the chamber was a 2 cm diameter hole, 11.5 cm above the floor. A drinking tube, containing tap water, could be brought by means of a solenoid to a position 0.5 cm behind this hole, or withdrawn sideways to a position in which it could not be reached by a rat in the chamber. This side of the chamber was movable, and during the training phase of the experiment it was moved to enclose the subjects in the running wheel. During all other phases of the experiment the subjects had to cross the chamber (27 cm) to travel from the wheel to the drinking tube. On these occasions a food tray was attached to a third wall of the chamber. Electromechanical equipment recorded each lick of the drinking tube and each one-sixth revolution of the wheel. A recycling clock defined 1 sec intervals, and the pen of a cumulative recorder was stepped each time there was a 1 sec interval during which there had been at least one lick of the drinking tube. When a running requirement had to be performed before the drinking tube became available, a predetermined counter caused the solenoids to operate to make the drinking tube available and to prevent the wheel revolving, as soon as the requirement was completed.
SMITH, GANNON AND TIERNEY
70-
t~.60
~'~
,~,
e "~ 50ii
""
.......
300
--m- --m --
5
: :
,=,...
iloo 90
,,,, . . . . .
"~'~
60 -~
• - E 40~
2, , . , ,,,
\\,
,o1 o
80 ,o 50
"-
i
.
2 3 4 5 6 7 8 9 1'0
,0 -o o
3 min Blocks FIG. I. Mean time spent drinking in 3 conditions within each 3 min block in the 30 rain period of access to water. Time spent drinking was measured from the increase in height of the cumulative record ( 1 mm=2.54 sec). Five and 300 represent the distance (in one-sixth revolutions) that had to be run in the wheel before access was gained.
Procedure
When not being tested the subjects were housed individually in standard laboratory cages, and had food available to them at all times. They were tested once each day, at the same time every day, 7 days a week, for a total of 20 weeks. From the start of the adaptation phase through to the end of the experiment they received access to water for only one hour during each day. There was an adaptation phase lasting 7 weeks, during which the subjects adjusted to this drinking schedule, and learned to drink from the drinking tube in the chamber. There followed a training phase of 3 weeks during which the subjects were given one 30 min session each day enclosed in the running wheel. During these sessions they were repeatedly required to make a running response in order to receive 10 sec access to the drinking tube. The running requirement for which these rewards were given was progressively increased from 1 to 100 one-sixth revolutions of the wheel. When these sessions were over the subjects were returned to their home cages and 30 rain later were given their water bottles in those cages for a further 30 rain. Thus the subjects were always 22.5 hours deprived at the start of each session. The training phase was concluded when the subjects were all performing well in the highest requirement condition. During the experimental phase each subject was required to perform in 3 conditions, with 10 successive sessions in each condition. These were a Free Access condition, in which no running was required and 30 min access to the drinking tube was available as soon as the subject was placed in the chamber, and 2 Running Requirement conditions in which the subject had to run a specified distance (5 or 300 one-sixth revolutions) before 30 rain access to the drinking tube was gained. As in the training phase each subject also received 30 min access to water in its home cage, 30 min after the end of the session. Two subjects performed the conditions in the order Free Access, 300, 5, and two performed them in the opposite order. When changing from one condition to another the
procurement cost was adjusted in steps over 20 sessions. The first 2 sessions in each condition were used to familiarize the subjects with the new procurement cost, and the data subsequently analyzed were only those from the last 8 sessions in each condition. RESULTS The data analyzed were twofold: (a) The means of the subjects over the 8 sessions of the numbers of licks in the 30 min period of access; and (b) The means of the subjects over the 8 sessions of the increase in height (in mm), in each 3 min period throughout the 30 min, of the cumulative record of time spent drinking. (Note: 1 mm=2.54 sec of drinking.) An analysis of variance, for a 2 × 3 (orders x conditions) factorial experiment with repeated measures on the second factor [ 13], of the number of licks revealed a significant main effect of conditions, F(2,4)=8.63, p<0.05. Comparisons of the means by the Newman-Keuls procedure [13] revealed that there was significantly more licking, p<0.05, in the 5 and 300 conditions (means 2678 and 2723 respectively) than there was in the Free Access condition (mean=2255). There was no significant difference between the means of the 5 and 300 conditions, but Page's trend test [10] revealed that when the 3 conditions were considered there was a significant monotonic increase in the number of licks as the procurement cost increased, L=54, p =0.05. An analysis of variance, for a 2 x 3 × 10 (ordersxconditions×3 min blocks) factorial experiment with repeated measures on the last two factors [13], was performed on the data from the cumulative records. This revealed a highly significant main effect of 3 min blocks, F(9,18)=665.4, p<0.001, and a significant interaction between the blocks and conditions, F(18,36)=2.46, p<0.05. The former of these merely reflected the fact that the proportion of the time spent drinking in all the conditions declined progressively
P R O C U R E M E N T COST A N D D R I N K I N G RATE
929
throughout the period of access, but the latter reflected the fact that there were differences between conditions in the proportion of time spent drinking within the blocks. These differences can be seen in Figure 1, which shows the mean increase in height o f the cumulative record in each 3 min block for each of the 3 conditions. Analyses of the simple main effect of conditions within each of the 3 rain blocks [13] revealed that the conditions produced significant differences only within Blocks l and 2, F(2,39)=10.28, and F(2,39)=6.66 respectively, p ' s < 0 . 0 1 . Comparisons between the means within these two blocks revealed that in Block 1 there was a significantly higher rate of drinking in both the 300 condition, p<0.01, and in the 5 condition, p<0.05, than there was in the Free Access condition. The rates did hot differ significantly between the 5 and 300 conditions, but Page's trend test revealed a significant monotonic trend across the 3 conditions, L=56, p=0.001. In Block 2 the rate in the 300 condition was significantly higher than both that in the Free Access condition, p <0.01, and that in the 5 condition, p<0.05, but there was no significant difference between the rates in the Free Access and 5 conditions. DISCUSSION The results of this experiment, with respect to the number of licks performed in the 30 min period of access, are clearly very similar to the findings obtained by Gannon et al. [5] when they required rats to press a lever to gain access to food. F e w e r procurement cost conditions were included in the present experiment, and it was not designed to discover whether the effect could be attributed directly to the performance of the procurement requirement, but in all other respects the results of the 2 studies were so consistent that it seems likely that a common process was responsible for them both, and that the procurement-cost effect is not restricted to specific categories of responding. The magnitude of the increase was also of a similar order in the two experiments. The largest procurement costs employed by Gannon et al. [5] were followed by the intake of nearly 30% more food in a 60 min period than occurred when there was no procurement cost. In the present experiment the numbers of licks increased by just over 20%, although the period of access was only half as long. The results of the analyses of the rates of drinking at different times throughout the period of access provide interesting information concerning the way in which this procurement-cost effect is produced. The initial rate of consummatory responding is elevated by a proportion that increases as the procurement cost is increased, and the duration for which this elevation persists depends upon its initial
level. The effect does not persist for very long even when the cost is large, but it occurs during the period of most intensive consummatory responding. The largest cost in the present experiment had an effect that was detectable statistically for the first 6 min of access to water, a period during which about 60% of the total drinking for the entire 30 min took place. These findings suggest that the effect would not be lost even if the duration of the period of access was increased, or if the subjects determine it for themselves as happens in the situation employed by Collier [3, 4, 6, 7, 8, 9]. The criterion of drinking employed in this experiment probably slightly overestimated the time spent drinking, because part seconds of licking at the beginning and end of bursts were counted as whole seconds. However, there do not seem to have been differences between the conditions in the within-burst rates of licking, which suggests that similar numbers of bursts were involved. From the total time spent drinking and the total number of licks it can be calculated that across the subjects the average rate of licking within bursts was much the same for the Free Access, 5 and 300 conditions (5.6, 5.8 and 5.7 licks/sec respectively). These rates do not differ greatly from the range of about 6--7 licks/ sec commonly reported for rats in a variety of conditions [2]. It is likely, therefore, that the increase in the rate of drinking produced by the procurement cost reflects an increase in the proportion of time spent in bursts of licking, consequent on a reduction in the duration of pauses between bursts. The purpose of this experiment was to obtain more detailed information than had been available previously concerning the way the procurement-cost effect is manifest rather than to permit the identification of the mechanisms that mediate it. Nevertheless, it may be inferred from the fact that the effect is both immediate and transient that a mechanism with similar properties is involved. A possible candidate f o r this role might be the arousal level of the subject. It has been pointed out [14] that it would be adaptive if intake was managed more efficiently when disruption or danger threatens, and this could be brought about if the rate of intake was elevated when arousal level was increased. This suggestion is consistent with the fact that manipulations which might be expected to increase arousal level have been found to increase eating in rats. F o r example, the induction of stress [I 1] and prior tail-pinching [1] have been found to have this effect on rats, and brief electric shock immediately prior to the regular 60 min period of access to food in 23 hr deprived rats has been shown to increase the initial eating rate [12] in a way similar to the effect reported here. However, so little information is available at present concerning the procurement-cost effect that the hypothesis that arousal is involved can only be advanced tentatively at this time.
REFERENCES 1. Antelman, S. M., H. Szechtman, P. Chin and R. E. Fisher. Tailpinch-induced eating, gnawing and licking behavior in rats: dependence on nigrostriatal dopamine system. Brain Res 99: 319-337, 1975. 2. Bolles, R. C. Theory of Motivation, 2nd edition. New York: Harper and Row, 1975. 3. Collier, G. H., E. Hirsch and P. Hamlin. The ecological determinants of reinforcement in the rat. Physiol Behav 9: 705-716, 1972.
4. Collier, G. H., L. W. Kaufman, R. Kanarek and J. Fagen. Optimization of time and energy constraints in the feeding behavior of cats. Carnivore 1: 34--41, 1978. 5. Gannon, K. N., H. V. Smith and K. J. Tierney. Effects of procurement cost on food consumption in rats. Physiol Behav 31: 331-338, 1983. 6. Hirsch, E. and G. Collier. The ecological determinants of reinforcement in the guinea pig. Physiol Behav 12: 239-249, 1974.
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7. Hirsch, E. and G. Collier. Effort as a determinant of intake and patterns of drinking in the guinea pig. Physiol Behav 12: 647655, 1974. 8. Kaufman, L. W., G. H. Collier, W. L. Hill and K. Collins. Meal cost and meal patterns in an uncaged domestic cat. Physiol Behav 25: 135-137, 1980. 9. Marwine, A. and G. Collier. The rat at the waterhole. J Comp Physiol Psychol 93: 391-402, 1979. 10. Page, E. B. Ordered hypotheses for multiple treatments: a significance test for linear ranks. J Am Stat Assoc 58: 216-230, 1963.
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11. 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. 12. Strongman, K. T. The effect of anxiety on food intake in the rat. Q J Exp Psychol 17: 255-260, 1965. 13. Winer, B. J. Statistical Principles in Experimental Design. London: McGraw-Hill Kogakusha, 1971. 14. Wooley, S. C. (Rapporteur) Psychological aspects of feeding: Group report. In: Appetite and Food Intake, edited by T. Silverstone. Berlin: Dahlem Konferenzen/Abakon, 1976.