BEHAVIORAL BIOLOGY, 14, 109-113 (1975), Abstract No. 4261
B R I E F REPORT Limitations o f " C o n t a i n e r N e o p h o b i a " as an Explanation of Rats' Responding for F o o d in the Presence o f Free F o o d 1
BROOKS CARDER and GARY C. BECKMAN
Department of Psychology, University of California, Los Angeles, California 90024
The hypothesis that responding for food in the presence of free food results from "container neophobia" was discusse& An alternative explanation of the data cited in support of this hypothesis was presented: that habits established during training largely determine the performance of subjects tested for responding in the presence of free food. A study in which container neophobia was not confounded with habits established in training was described. Rats pre-exposed to the free food container before being tested for responding in the presence of free food performed no differently from rats which did not receive this pre-exposure. Thus the data failed to confirm the hypothesis that container neophobia is the principal determinant of responding in this situation.
There have been by this time a number of demonstrations that rats and other organisms will respond for food in the presence of free food (Jensen, 1963; Neuringer, 1969; Carder and Berkowitz, 1970; Carder, 1972; Davidson, 1971; Singh, 1970). Very recently, Mitchell and his co-workers (1973) argued that much of this data could be explained on the basis of "container neophobia." They contended that the principle determinant of the rat's preference for earned food is the relative familiarity of the earned source and the free source. If the free food source is novel, the rat is frightened by it and does not eat from it. They pointed out that many of the previous studies which demonstrated high proportions of response-produced food (Jensen, 1963; Carder and Berkowitz, 1970; Neuringer, 1969; Davidson, 1971) employed procedures that gave the subjects more experience with response produced food than with free food. Mitchell, Scott and Williams (1973) went on to present a series of three experiments, the results of which supported their hypothesis. 1This research was supported in part by Grant No. DA00288 from the Public Health Service to the first author. The authors are grateful to Stuart M. Deikel and Professor Eric Holman for their helpful comments on earlier drafts of this manuscript.
109 Copyright Q 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
110
CARDER AND BECKMAN
The purpose of the present paper is twofold: 1) to present an alternative to the neophobia hypothesis that can also account for the data of Mitchell et al., namely that habits of food source utilization developed in training rather than neophobia determine behavior in the free food test, and 2) to describe an experiment in which the effect of container novelty could be examined independent of the effect of habits of food source utilization. In their first experiment, Mitchell e t al. gave rats six days of lever press training and three days of training to eat free food, followed by choice testing with both lever press and free food available. The subjects that received lever press training on the day before the test pressed more than did subjects that received free food training on the day before the test. The authors proposed that this resulted from the fact that subjects were neophobic and avoided food sources to which they had not been recently exposed. The data are also consistent, however, with the interpretation that the most recently practiced habit predominates in the choice test. In the second experiment, rats received lever press training in which pellets were delivered into a left or right magazine on alternate days. In a choice test, a large number of pellets was freely available in one magazine while pressing delivered single pellets to the other. Subjects preferred the freely available food. The authors argued that the absence of neophobia led to a preference for free food, since subjects were equally familiar with both food sources. The data are also consistent with a habit interpretation. During training, an empty magazine was an S+ for pressing, while a magazine with pellets in it was an S+ for eating. Once subjects reached the free food magazine they only received the S+ for eating and thus pressed very little. In a third experiment subjects preferred to press for food delivered into the magazine used in training rather than to freeload from a novel glass dish. The same subjects preferred to freeload from the familiar magazine rather than earn pellets delivered into the novel dish. While the authors argue that this is further support for the neophobia hypothesis, the data are also consistent with the habit interpretation. Subjects chose to get food by the route with which they had the most practice. In order to examine the effects of container neophobia in a situation in which this was not confounded with prior feeding habits we chose a very simple experimental approach to the problem. If container neophobia is the determinant of whether a rat will prefer free or response produced food, then rats given free food in a novel container should respond more and eat less free food than rats given free food in a familiar container. Twelve, male Sprague-Dawley rats over 90 days old were obtained from the Simonsen Breeding Laboratory, Gilroy, CA. They were housed in individual cages, given free access to water, and deprived to 80% of their free feeding weight. All subjects were trained to lever press in a standard Gerbrands operant chamber. Each press produced a 45 mg food pellet. The
CONTAINER NEOPHOBIA AND FREE FOOD
111
first day on which subject made 100 or more responses was counted as the first training day and all subjects were given a total of six training sessions, each lasting 40 min. As soon as subject had mastered the response, and had made over 100 responses in a session, the only food that subject received was that which it earned during the 40 min training session. The 12 subjects were divided into two groups of six for the manipulation of exposure to the free food container during training. Whenever subjects in group A (pre-exposed) were placed in the operant chamber, there was an empty free food container in the corner of the wall opposite the lever, near the door of the chamber. The free food container was a Plexiglas trough about 7 cm long, 3 cm wide and 3 cm deep. During training sessions it was not fastened to the floor of the cage but could be moved around the cage by the subject. It was evident that subjects engaged in considerable exploration of this empty free food container since at the end of every training session it was found to have been moved from its original location. Subjects in group B were never exposed to the free food container during training. On the day following the sixth training day, all 12 subjects received identical tests for responding in the presence of free food. The free food container, to which group A had been previously exposed, was filled with pellets (over 600 pellets), placed in its usual location and fastened to the floor of the cage so that subjects could not tip it over. Subjects were placed in the box and given a 40 min test session. Responses on the lever were counted and the reduction in weight of the free food container was used to measure the quantity of free food consumed. For each subject we calculated a proportion of food earned to the amount of total food consumed in the test situation. The proportion for each individual subject in group A and group B is presented in Table 1. Two of six
TABLE 1 Proportion of Earned Food to Total Food Consumed as a Function of Container Pre-Exposure
mean
A-pre-exposed
B-not exposed
0.91 0.67 O.26 0.24 0.08 0.00
0.72 0.62 0.46 0.27 0.01 0.00
0.35
0.36
112
CARDER AND BECKMAN
subjects in each group preferred earned food to free food. The mean proportions of food earned for the two groups is virtually identical, with group A at 0.35 and group B at 0.36. Thus, if prior exposure to the container influences the amount of responding for food in the presence of free food, the effect is tiny and would require the running of many subjects to detect it. In the present study subjects were exposed to free food either in a novel container or in a container to which they had been exposed previously on at least six occasions. Prior exposure to the free food container had no influence on responding for food in the presence of free food. Thus the data failed to support a notion that "container neophobia" provides a complete explanation of why rats respond for food in the presence of free food. In the present study, exposure to the free food container was not confounded with training to eat from it or extinction training on the bar press. Under this circumstance, exposure to the container had no significant effect on earned food preference. Future experiments on this problem should include a careful analysis of the relationship between exposure to food sources and the type of explicit training of feeding patterns that the exposure may provide. Probably the most powerful effect that has been demonstrated in free food studies is that increased training to lever press increases the amount of responding in the presence of free food, while increased training to eat from the free food source decreases the amount of responding for food in the presence of free food (Jensen, 1963; Mitchell et al., 1974). It would appear that the data of all the studies carried out by Mitchell et al. are consistent with the interpretation that behavior in the free food test situation is largely determined by habits established during training. It must be pointed out, however, that prior feeding habit is not the only variable which influences responding in the presence of free food, nor does it provide a complete account of the phenomenon. Neuringer (1969) demonstrated substantial responding for food in the presence of free food in pigeons which had never received any training to perform the operant response. Thus, while prior feeding habits are a powerful determinant of performance in the free food situation, it remains to be explained why some subjects even in the absence of appropriate training, will perform an effortful response for food in the presence of free food.
REFERENCES Carder, B., and Berkowitz, K. (1970). Rat's preference for earned in comparison with free food. Science 167, 1273-1274. Carder, B. (1972). Rats' preference for earned in comparison with free liquid reinforcers. Psychonom. Sci. 26, 25-26.
CONTAINER NEOPHOBIA AND FREE FOOD
1 13
Davidson, A. B. (1971). Factors affecting keypress responding by rats in the presence of free food. Psychonom. Sci. 24, 135-137. Jensen, G. D. (1963). Preference for bar pressing over "free-loading" as a function of number of rewarded presses. J. Exp. Psychol. 65, 451-454. Mitchell, D., Scott, D. W., and Williams, K. D. (1973). Container neophobia and the rat's preference for earned food. Behav. Biol. 9, 613-624. Neuringer, A. J. (1969). Animals respond for food in the presence of free food. Science 166, 399-401. Singh, D. (1970). Preference for bar pressing to obtain reward over freeloading in rats and children, jr. Comp. Physiol. Psychol. 73, 320-327. Tarte, R. D., and Snyder, R. L. (1972). Barpressing in the presence of free food as a function of food deprivation. Psychonom. Sci. 26, 169-170.
B R I E F REPORT Limitations o f " C o n t a i n e r N e o p h o b i a " as an Explanation of Rats' Responding for F o o d in the Presence o f Free F o o d 1
BROOKS CARDER and GARY C. BECKMAN
Department of Psychology, University of California, Los Angeles, California 90024
The hypothesis that responding for food in the presence of free food results from "container neophobia" was discusse& An alternative explanation of the data cited in support of this hypothesis was presented: that habits established during training largely determine the performance of subjects tested for responding in the presence of free food. A study in which container neophobia was not confounded with habits established in training was described. Rats pre-exposed to the free food container before being tested for responding in the presence of free food performed no differently from rats which did not receive this pre-exposure. Thus the data failed to confirm the hypothesis that container neophobia is the principal determinant of responding in this situation.
There have been by this time a number of demonstrations that rats and other organisms will respond for food in the presence of free food (Jensen, 1963; Neuringer, 1969; Carder and Berkowitz, 1970; Carder, 1972; Davidson, 1971; Singh, 1970). Very recently, Mitchell and his co-workers (1973) argued that much of this data could be explained on the basis of "container neophobia." They contended that the principle determinant of the rat's preference for earned food is the relative familiarity of the earned source and the free source. If the free food source is novel, the rat is frightened by it and does not eat from it. They pointed out that many of the previous studies which demonstrated high proportions of response-produced food (Jensen, 1963; Carder and Berkowitz, 1970; Neuringer, 1969; Davidson, 1971) employed procedures that gave the subjects more experience with response produced food than with free food. Mitchell, Scott and Williams (1973) went on to present a series of three experiments, the results of which supported their hypothesis. 1This research was supported in part by Grant No. DA00288 from the Public Health Service to the first author. The authors are grateful to Stuart M. Deikel and Professor Eric Holman for their helpful comments on earlier drafts of this manuscript.
109 Copyright Q 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.
110
CARDER AND BECKMAN
The purpose of the present paper is twofold: 1) to present an alternative to the neophobia hypothesis that can also account for the data of Mitchell et al., namely that habits of food source utilization developed in training rather than neophobia determine behavior in the free food test, and 2) to describe an experiment in which the effect of container novelty could be examined independent of the effect of habits of food source utilization. In their first experiment, Mitchell e t al. gave rats six days of lever press training and three days of training to eat free food, followed by choice testing with both lever press and free food available. The subjects that received lever press training on the day before the test pressed more than did subjects that received free food training on the day before the test. The authors proposed that this resulted from the fact that subjects were neophobic and avoided food sources to which they had not been recently exposed. The data are also consistent, however, with the interpretation that the most recently practiced habit predominates in the choice test. In the second experiment, rats received lever press training in which pellets were delivered into a left or right magazine on alternate days. In a choice test, a large number of pellets was freely available in one magazine while pressing delivered single pellets to the other. Subjects preferred the freely available food. The authors argued that the absence of neophobia led to a preference for free food, since subjects were equally familiar with both food sources. The data are also consistent with a habit interpretation. During training, an empty magazine was an S+ for pressing, while a magazine with pellets in it was an S+ for eating. Once subjects reached the free food magazine they only received the S+ for eating and thus pressed very little. In a third experiment subjects preferred to press for food delivered into the magazine used in training rather than to freeload from a novel glass dish. The same subjects preferred to freeload from the familiar magazine rather than earn pellets delivered into the novel dish. While the authors argue that this is further support for the neophobia hypothesis, the data are also consistent with the habit interpretation. Subjects chose to get food by the route with which they had the most practice. In order to examine the effects of container neophobia in a situation in which this was not confounded with prior feeding habits we chose a very simple experimental approach to the problem. If container neophobia is the determinant of whether a rat will prefer free or response produced food, then rats given free food in a novel container should respond more and eat less free food than rats given free food in a familiar container. Twelve, male Sprague-Dawley rats over 90 days old were obtained from the Simonsen Breeding Laboratory, Gilroy, CA. They were housed in individual cages, given free access to water, and deprived to 80% of their free feeding weight. All subjects were trained to lever press in a standard Gerbrands operant chamber. Each press produced a 45 mg food pellet. The
CONTAINER NEOPHOBIA AND FREE FOOD
111
first day on which subject made 100 or more responses was counted as the first training day and all subjects were given a total of six training sessions, each lasting 40 min. As soon as subject had mastered the response, and had made over 100 responses in a session, the only food that subject received was that which it earned during the 40 min training session. The 12 subjects were divided into two groups of six for the manipulation of exposure to the free food container during training. Whenever subjects in group A (pre-exposed) were placed in the operant chamber, there was an empty free food container in the corner of the wall opposite the lever, near the door of the chamber. The free food container was a Plexiglas trough about 7 cm long, 3 cm wide and 3 cm deep. During training sessions it was not fastened to the floor of the cage but could be moved around the cage by the subject. It was evident that subjects engaged in considerable exploration of this empty free food container since at the end of every training session it was found to have been moved from its original location. Subjects in group B were never exposed to the free food container during training. On the day following the sixth training day, all 12 subjects received identical tests for responding in the presence of free food. The free food container, to which group A had been previously exposed, was filled with pellets (over 600 pellets), placed in its usual location and fastened to the floor of the cage so that subjects could not tip it over. Subjects were placed in the box and given a 40 min test session. Responses on the lever were counted and the reduction in weight of the free food container was used to measure the quantity of free food consumed. For each subject we calculated a proportion of food earned to the amount of total food consumed in the test situation. The proportion for each individual subject in group A and group B is presented in Table 1. Two of six
TABLE 1 Proportion of Earned Food to Total Food Consumed as a Function of Container Pre-Exposure
mean
A-pre-exposed
B-not exposed
0.91 0.67 O.26 0.24 0.08 0.00
0.72 0.62 0.46 0.27 0.01 0.00
0.35
0.36
112
CARDER AND BECKMAN
subjects in each group preferred earned food to free food. The mean proportions of food earned for the two groups is virtually identical, with group A at 0.35 and group B at 0.36. Thus, if prior exposure to the container influences the amount of responding for food in the presence of free food, the effect is tiny and would require the running of many subjects to detect it. In the present study subjects were exposed to free food either in a novel container or in a container to which they had been exposed previously on at least six occasions. Prior exposure to the free food container had no influence on responding for food in the presence of free food. Thus the data failed to support a notion that "container neophobia" provides a complete explanation of why rats respond for food in the presence of free food. In the present study, exposure to the free food container was not confounded with training to eat from it or extinction training on the bar press. Under this circumstance, exposure to the container had no significant effect on earned food preference. Future experiments on this problem should include a careful analysis of the relationship between exposure to food sources and the type of explicit training of feeding patterns that the exposure may provide. Probably the most powerful effect that has been demonstrated in free food studies is that increased training to lever press increases the amount of responding in the presence of free food, while increased training to eat from the free food source decreases the amount of responding for food in the presence of free food (Jensen, 1963; Mitchell et al., 1974). It would appear that the data of all the studies carried out by Mitchell et al. are consistent with the interpretation that behavior in the free food test situation is largely determined by habits established during training. It must be pointed out, however, that prior feeding habit is not the only variable which influences responding in the presence of free food, nor does it provide a complete account of the phenomenon. Neuringer (1969) demonstrated substantial responding for food in the presence of free food in pigeons which had never received any training to perform the operant response. Thus, while prior feeding habits are a powerful determinant of performance in the free food situation, it remains to be explained why some subjects even in the absence of appropriate training, will perform an effortful response for food in the presence of free food.
REFERENCES Carder, B., and Berkowitz, K. (1970). Rat's preference for earned in comparison with free food. Science 167, 1273-1274. Carder, B. (1972). Rats' preference for earned in comparison with free liquid reinforcers. Psychonom. Sci. 26, 25-26.
CONTAINER NEOPHOBIA AND FREE FOOD
1 13
Davidson, A. B. (1971). Factors affecting keypress responding by rats in the presence of free food. Psychonom. Sci. 24, 135-137. Jensen, G. D. (1963). Preference for bar pressing over "free-loading" as a function of number of rewarded presses. J. Exp. Psychol. 65, 451-454. Mitchell, D., Scott, D. W., and Williams, K. D. (1973). Container neophobia and the rat's preference for earned food. Behav. Biol. 9, 613-624. Neuringer, A. J. (1969). Animals respond for food in the presence of free food. Science 166, 399-401. Singh, D. (1970). Preference for bar pressing to obtain reward over freeloading in rats and children, jr. Comp. Physiol. Psychol. 73, 320-327. Tarte, R. D., and Snyder, R. L. (1972). Barpressing in the presence of free food as a function of food deprivation. Psychonom. Sci. 26, 169-170.
