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The development of activity differences in isolated and aggregated mice
Anhn. Behav., 1966, 14, 406--409
THE DEVELOPMENT OF ACTIVITY DIFFERENCES IN ISOLATED AND AGGREGATED MICE BY W A L T E R B. ESSMAN
Queens College of the City University of New York which did develop following weaning could then be attributed to the introduction of differential housing conditions.
The effect of differential housing conditions upon the activity level of rodents has been the subject of studies from which contradictory findings have emerged. When group-housed rats were compared with individually-housed rats, in some instances (Stern et al., 1960; Myers & Fox, 1963) grouping led to greater locomotor behaviour or greater initial exploratory behaviour than was observed for individuallyhoused animals. In contrast to these findings, greater exploratory behaviour has been reported for isolation-reared than for group-reared rats (Zimbardo & Montgomery, 1957; Woods, 1959). In mice the question of motor activity as a function of either isolation or aggregation in the housing conditions has similarly been controversial. Several studies (Thiessen, 1963, 1964) have suggested that adult mice which are aggregated show greater locomotor activity than isolated animals. The observation of an activity difference between isolated or grouped adult mice has also been confirmed in pharmacological studies (Gunn & Gurd, 1940; Chance, 1946, 1947), in which differential sensitivity in isolated or aggregated mice to drug toxicity was demonstrated. A major factor in the discrepant findings regarding the differential effects of isolation and aggregation upon activity levels in rodents is that the differential housing has been introduced at different ages ranging from several weeks post- weaning to adulthood, and different methods have been utilized to measure motor activity. It is therefore reasonable to expect that other variables, such as post-weaning population density, social stimulation, fighting, competitive behaviour etc., may interact with subsequent modification in the housing conditions to account for differences in activity. It was the purpose of the present investigation to study the development of activity differences in mice raised in differentially housed conditions immediately after weaning. Since animals were chosen from litters of equal size, it could be reasonably assumed that the pre-weaning environmental conditions were relatively equivalent for all animals, and therefore any activity differences
Method
Litters of new-born mice of the SwissWebster strain (Mus musculis) were sexed and redistributed among the mothers so that each of five litters consisted of eight male mice born on the same day. All mice were weaned at 21 days of age and four animals from each cage were randomly assigned to isolated housing conditions (n == 20) and four animals assigned to aggregated housing conditions (n = 20). The isolated mice were individually housed in metal cages (18 • 25 cm) with Sanicel bedding and ad libitum food and water. The aggregated mice were housed in groups of five in cages identical with those in which the isolated animals were housed. At 22 days of age activity testing was begun. The apparatus consisted of individual boxes (16 • 26 • 12.5 cm) with clear lucite walls and a floor consisting of four 7 • 11 cm aluminium plates, mounted on lucite rods, separated by a space of 5 mm. The plates were wired through an external power supply requiring approximately 35 t~A to activate the input to an external six-digit counter panel. Contact between any two adjacent plates by a mouse completed the circuit and registered a single count on the panel. The activity boxes were located on individual shelves in an insulated chamber, maintained at 26 ~ with a low ambient level of illumination. Isolation- and aggregation-housed mice were individually given daily testing sessions in the activity boxes for 15 rain over twenty-two consecutive days. A total daily cumulative activity record was recorded for the 15 min testing session. All testing was done between 9.30 and 11.30 a.m., and on each day the sequence in which individual animals were tested was randomly varied. After removal from the activity boxes mice were returned to their respective isolated or aggregated housing conditions. Cage bedding was changed daily and, 406
ESSMAN: ACTIVITY DIFFERENCES IN MICE
except for the activity testing, the animals were not otherwise handled during the experiment. Results
The mean activity record for the isolated and aggregated mice is shown for each of the twentytwo daily testing days in Fig. 1. A comparison 24C 22C 20C
18C 16C 140
~20 I00
407
decrements in activity for the twenty-two testing days was significant (p = 0.446, P < 0.05) when linearly transformed difference scores were correlated between groups. The transformed increment or decrement levels between successive testing days differed significantly as a function of the housing condition, when these differences were evaluated with a Mann-Whitney test (U = 20, P < 0.002). Although the incidence of increments (ten) and decrements (eleven) was equal for both the groups, the magnitude of the activity decrements among the aggregated animals was significantly greater. Discussion
go 6O 40 I 2
I
I 4
I
I 6
l
I 8
I
I IO
I
1 12 0Au
I
I 14
I
I 16
~
l 18
I
I 20
I
J 22
Fig. 1. Mean daily activity level for isolated (0) and aggregated (O) mice.
of the differences in activity between groups over the 22 day period yielded a significant difference (F = 4-95, P < 0.10). The major differences which contributed to this significance were accounted for by 15 out of 19 days, after the first 3 days of testing, where the isolated animals were significantly more active than the aggregates (P = 0.046). For the first 3 days of testing the differences between the isolated and aggregated animals were insignificant (t = 1.02, 0.89, and 0.27, respectively, for days I, 2 and 3; P > 0.20). Those days, after day 3, for which acceptable levels of statistical significance did not emerge between the groups were days 9, 10, 11 and 19. These represent days on which the activity of the aggregated mice increased from the level of the previous testing day. For all other days after day 3 the activity differences between isolated and aggregated mice were all significant; the isolated mice, after the third day of testing, had mean activity counts which ranged from 7.85 (P > 0.50) to 84.46 (P < 0.001) more than the mean activity count of the aggregated animals. An analysis of the trend in distribution of activity over the 22 days indicated that the aggregated mice showed a significant reduction in motor activity (F = 6.19, df = 1, P < 0.001), whereas the isolated animals did not show any significant alteration in trend (F = 0.46, df = 1, P > 0"50). The relationship between aggregated and isolated animals for daily increments or de-
The data suggest that differences in activity level develop in weanling Swiss-Webster strain mice as a function of the post-weaning housing conditions. The differences in activity were observed after 3 days of isolation or aggregation; and, whereas isolated animals showed a trend toward maintaining activity level within the range shown just following weaning, the aggregated mice showed a significant trend toward decreased activity. Daily fluctuations in activity level were significantly correlated between groups and the aggregated animals tended toward greater activity decrement. Differences between isolated and aggregated animals which manifest themselves behaviourally may be related to at least one hypothesis-that either condition may be behaviourally stressful and that activity differences may then reflect the effect of stress. Isolation may be viewed as a stress insofar as reduced sensory and social stimulation are concerned,; aggregation has also been assigned a stressful role insofar as crowding, competition, and fighting may constitute stressors. A previous study (Essman, 1966) has indicated that isolated mice are more susceptible to starvation-induced gastric ulcers and also tend to show a somewhat greater degree of depletion in adrenal ascorbic acid than do aggregated animals. These findings support the suggestion that isolation is more stressful for mice than aggregation. The relationship between activity level and stress may be posited on both behavioural as well as physiological levels. Motor activity could, in one sense, be interpreted as a response to heightened motivational factors introduced by isolation. On still another level, biochemical changes have been indicated as a function of the housing conditions. Welch & Welch (1965) have indicated that Swiss mice, 2-3 months of age, became more excitable and
408
ANIMAL
BEHAVIOUR,
aggressive with isolation; a higher level of brain norepinepherine was indicated to account for this finding. Geller, Yuwiler & Zolman (1965), in comparing environmentally stimulated grouphoused rats with individually-housed rats, observed higher levels of brain norepinepherine in the latter. The isolated animals also had somewhat heavier adrenals and increased liver transaminase and pyrrolase activity; these latter differences are characteristic of a stress response. Other measures which have been shown to differ in differentially-housed mice are blood glucose and tissue glycogen; these were shown to be reduced in group-housed adult mice, rendering them more susceptible to toxicity from d-amphetamine (Moore, Sawdy & Shaul, 1965). It appears reasonable to assume that hypoglycaemic mice would also be less active. A related consideration is the question of emotionality and differential housing which, while possibly applicable to the present findings, holds some inconsistency as far as several previous studies are concerned. Activity level has sometimes been indicated as a measure of emotionality. Several investigators have suggested that reduced activity or exploratory behaviour in rodents indicates a greater level of 'emotionality' (Thiessen, 1963; Stern et al., 1960). These same investigators have suggested that 'emotionality' reduction is associated with an increase in activity. Whereas Stern et al. (1960) observed a greater level of activity, measured in a relatively larger field than that employed in the present study, in group-housed rats, and interpreted this finding as 'less "fearful" behaviour in this situation', these animals developed more gastric lesions when immobilized; this finding appears somewhat inconsistent in that it suggests that reduced 'emotionality', as measured by increased activity, is associated with a more severe somatic response to stressor stimulation. Most of the previous studies which have suggested that activity levels are higher for either isolated or aggregated animals have utilized adult populations with considerable variation in the duration of the differential housing and the size of the aggregated group. Strain differences may possibly account for some of these differences, although results consistent with those presented in the present study with SwissWebster strain mice have also been obtained in this laboratory with CF-I strain mice. The present findings indicate that differential post-weaning housing conditions in SwissWebster mice lead to differences in the develop-
14, 4
ment of motor activity levels as measured in a field approximately the size of the home cage. These data suggest that group-housed mice tend to decrease their activity with time, from an initially high level of activity at 22 days of age, whereas isolated mice tend to maintain higher activity levels over time. The initially high and equivalent level of motor activity shown by both isolated and aggregated animals on the first testing day may reflect the change in housing conditions just following weaning. It also seems likely that the first 3 days of motor activity, being equivalent under both housing conditions, represent a response to altered housing conditions, in addition to or rather than a response to differential housing p e r se. With a relatively stable level of motor activity developed in differentiaUy-housed mice by 44 days of age, the isolated animals showed a significantly higher level of activity than the aggregated mice. Summary Male Swiss-Webster strain mice, housed under isolated or aggregated conditions from weaning (21 days), were tested daily for activity levels. By the fourth post-weaning day significant differences in motor activity between groups emerged and the activity of the isolated mice was generally greater thhn that of the aggregates; the latter showed a significant trend toward activity decrement over days, whereas the former tended to maintain higher stable activity levels. Daily fluctuations in activity were correlated between groups. The activity differences which developed as a function of post-weaning differential housing are considered in view of the possible contribution of 'stress', biochemical differences, and motivational factors. Acknowledgment This study was supported in part by U.S.P.H.S Grant MH 08698-02. The author wishes to thank D. Glaubinger for his technical assistance. REFERENCES Chance, M. R. A. (1946). Aggregation as a factor influencing the toxicityof sympathomimeticamines in mice. J. PharmacoL, 87, 214-219. Chance, M. R. A. (1947). Factors influencingthe toxicity of sympathomimetic amines to solitary mice. J. Pharmacol., 89, 289-296. Essman, W. B. (1966). Gastric ulceration as a function of food deprivation in isolated vs. aggregated mice. Psychon. Sci., 4, 251-252.
ESSMAN: ACTIVITY DIFFERENCES IN MICE Essman, W. B. & Frisone, J. (1966). Isolation-induced facilitation of gastric ulcero-genesis in mice. J. psychosom. Res., (in press). Geller, E., Yuwiler, A. & Zolman, J. F. (1965). Effects of environmental complexity on constituents of brain and liver. J. Neurochem., 12, 949-955. Gunn, J. A. & Gurd, J. (1940). The action of some amines related to adrenaline: cyclohexylalkylamines. J. Physiol., Lond., 7, 463-470. Moore, K. E., Sawdy, L. C. & Shaul, S. R. (1965). Effects of d-amphetamine on blood glucose and tissue glycogen levels of isolated and aggregated mice. Biochem. Pharmac., 14, 197-204. Myers, R. D. & Fox, J. (1963). Differences in maze performance of group- vs. isolation-reared rats. Psychol. Rep., 12, 199-202. Stern, J. A., Winokur, G., Eisenstein, A., Taylor, R. & Sly, M. (1960). The effect of group vs. individual housing on behavior and physiological responses to stress in the albino rat. J. psychosom. Res., 4, 187-190.
409
Thiessen, D. D. (1963). Varying sensitivity of C57B1/Crgl mice to grouping. Science, N. Y., 141,825-828. Thiessen, D. D. (1964). Population density mouse genotype, and endocrine function in behavior. J. comp. physiol. PsychoL, 57, 412-416. Welch, B. L. & Welch, A. S. (1965). Effect of grouping on the level of brain norepinepherine in White Swiss Mice. Life Sciences, 4, 1011-1018. Woods, P. J. (1959). The effects of free and restricted environmental experience on problem-solving behavior in the rat. J. comp. physiol. PsychoL, 52, 399-402. Zimbardo, P. & Montgomery, K. C. (1957). Effects of 'free environment' rearing upon exploratory behavior. PsychoL Rep., 3, 589-594.
(Received 4 February 1966, revised 9 April 1966; Ms. number: A417)
THE DEVELOPMENT OF ACTIVITY DIFFERENCES IN ISOLATED AND AGGREGATED MICE BY W A L T E R B. ESSMAN
Queens College of the City University of New York which did develop following weaning could then be attributed to the introduction of differential housing conditions.
The effect of differential housing conditions upon the activity level of rodents has been the subject of studies from which contradictory findings have emerged. When group-housed rats were compared with individually-housed rats, in some instances (Stern et al., 1960; Myers & Fox, 1963) grouping led to greater locomotor behaviour or greater initial exploratory behaviour than was observed for individuallyhoused animals. In contrast to these findings, greater exploratory behaviour has been reported for isolation-reared than for group-reared rats (Zimbardo & Montgomery, 1957; Woods, 1959). In mice the question of motor activity as a function of either isolation or aggregation in the housing conditions has similarly been controversial. Several studies (Thiessen, 1963, 1964) have suggested that adult mice which are aggregated show greater locomotor activity than isolated animals. The observation of an activity difference between isolated or grouped adult mice has also been confirmed in pharmacological studies (Gunn & Gurd, 1940; Chance, 1946, 1947), in which differential sensitivity in isolated or aggregated mice to drug toxicity was demonstrated. A major factor in the discrepant findings regarding the differential effects of isolation and aggregation upon activity levels in rodents is that the differential housing has been introduced at different ages ranging from several weeks post- weaning to adulthood, and different methods have been utilized to measure motor activity. It is therefore reasonable to expect that other variables, such as post-weaning population density, social stimulation, fighting, competitive behaviour etc., may interact with subsequent modification in the housing conditions to account for differences in activity. It was the purpose of the present investigation to study the development of activity differences in mice raised in differentially housed conditions immediately after weaning. Since animals were chosen from litters of equal size, it could be reasonably assumed that the pre-weaning environmental conditions were relatively equivalent for all animals, and therefore any activity differences
Method
Litters of new-born mice of the SwissWebster strain (Mus musculis) were sexed and redistributed among the mothers so that each of five litters consisted of eight male mice born on the same day. All mice were weaned at 21 days of age and four animals from each cage were randomly assigned to isolated housing conditions (n == 20) and four animals assigned to aggregated housing conditions (n = 20). The isolated mice were individually housed in metal cages (18 • 25 cm) with Sanicel bedding and ad libitum food and water. The aggregated mice were housed in groups of five in cages identical with those in which the isolated animals were housed. At 22 days of age activity testing was begun. The apparatus consisted of individual boxes (16 • 26 • 12.5 cm) with clear lucite walls and a floor consisting of four 7 • 11 cm aluminium plates, mounted on lucite rods, separated by a space of 5 mm. The plates were wired through an external power supply requiring approximately 35 t~A to activate the input to an external six-digit counter panel. Contact between any two adjacent plates by a mouse completed the circuit and registered a single count on the panel. The activity boxes were located on individual shelves in an insulated chamber, maintained at 26 ~ with a low ambient level of illumination. Isolation- and aggregation-housed mice were individually given daily testing sessions in the activity boxes for 15 rain over twenty-two consecutive days. A total daily cumulative activity record was recorded for the 15 min testing session. All testing was done between 9.30 and 11.30 a.m., and on each day the sequence in which individual animals were tested was randomly varied. After removal from the activity boxes mice were returned to their respective isolated or aggregated housing conditions. Cage bedding was changed daily and, 406
ESSMAN: ACTIVITY DIFFERENCES IN MICE
except for the activity testing, the animals were not otherwise handled during the experiment. Results
The mean activity record for the isolated and aggregated mice is shown for each of the twentytwo daily testing days in Fig. 1. A comparison 24C 22C 20C
18C 16C 140
~20 I00
407
decrements in activity for the twenty-two testing days was significant (p = 0.446, P < 0.05) when linearly transformed difference scores were correlated between groups. The transformed increment or decrement levels between successive testing days differed significantly as a function of the housing condition, when these differences were evaluated with a Mann-Whitney test (U = 20, P < 0.002). Although the incidence of increments (ten) and decrements (eleven) was equal for both the groups, the magnitude of the activity decrements among the aggregated animals was significantly greater. Discussion
go 6O 40 I 2
I
I 4
I
I 6
l
I 8
I
I IO
I
1 12 0Au
I
I 14
I
I 16
~
l 18
I
I 20
I
J 22
Fig. 1. Mean daily activity level for isolated (0) and aggregated (O) mice.
of the differences in activity between groups over the 22 day period yielded a significant difference (F = 4-95, P < 0.10). The major differences which contributed to this significance were accounted for by 15 out of 19 days, after the first 3 days of testing, where the isolated animals were significantly more active than the aggregates (P = 0.046). For the first 3 days of testing the differences between the isolated and aggregated animals were insignificant (t = 1.02, 0.89, and 0.27, respectively, for days I, 2 and 3; P > 0.20). Those days, after day 3, for which acceptable levels of statistical significance did not emerge between the groups were days 9, 10, 11 and 19. These represent days on which the activity of the aggregated mice increased from the level of the previous testing day. For all other days after day 3 the activity differences between isolated and aggregated mice were all significant; the isolated mice, after the third day of testing, had mean activity counts which ranged from 7.85 (P > 0.50) to 84.46 (P < 0.001) more than the mean activity count of the aggregated animals. An analysis of the trend in distribution of activity over the 22 days indicated that the aggregated mice showed a significant reduction in motor activity (F = 6.19, df = 1, P < 0.001), whereas the isolated animals did not show any significant alteration in trend (F = 0.46, df = 1, P > 0"50). The relationship between aggregated and isolated animals for daily increments or de-
The data suggest that differences in activity level develop in weanling Swiss-Webster strain mice as a function of the post-weaning housing conditions. The differences in activity were observed after 3 days of isolation or aggregation; and, whereas isolated animals showed a trend toward maintaining activity level within the range shown just following weaning, the aggregated mice showed a significant trend toward decreased activity. Daily fluctuations in activity level were significantly correlated between groups and the aggregated animals tended toward greater activity decrement. Differences between isolated and aggregated animals which manifest themselves behaviourally may be related to at least one hypothesis-that either condition may be behaviourally stressful and that activity differences may then reflect the effect of stress. Isolation may be viewed as a stress insofar as reduced sensory and social stimulation are concerned,; aggregation has also been assigned a stressful role insofar as crowding, competition, and fighting may constitute stressors. A previous study (Essman, 1966) has indicated that isolated mice are more susceptible to starvation-induced gastric ulcers and also tend to show a somewhat greater degree of depletion in adrenal ascorbic acid than do aggregated animals. These findings support the suggestion that isolation is more stressful for mice than aggregation. The relationship between activity level and stress may be posited on both behavioural as well as physiological levels. Motor activity could, in one sense, be interpreted as a response to heightened motivational factors introduced by isolation. On still another level, biochemical changes have been indicated as a function of the housing conditions. Welch & Welch (1965) have indicated that Swiss mice, 2-3 months of age, became more excitable and
408
ANIMAL
BEHAVIOUR,
aggressive with isolation; a higher level of brain norepinepherine was indicated to account for this finding. Geller, Yuwiler & Zolman (1965), in comparing environmentally stimulated grouphoused rats with individually-housed rats, observed higher levels of brain norepinepherine in the latter. The isolated animals also had somewhat heavier adrenals and increased liver transaminase and pyrrolase activity; these latter differences are characteristic of a stress response. Other measures which have been shown to differ in differentially-housed mice are blood glucose and tissue glycogen; these were shown to be reduced in group-housed adult mice, rendering them more susceptible to toxicity from d-amphetamine (Moore, Sawdy & Shaul, 1965). It appears reasonable to assume that hypoglycaemic mice would also be less active. A related consideration is the question of emotionality and differential housing which, while possibly applicable to the present findings, holds some inconsistency as far as several previous studies are concerned. Activity level has sometimes been indicated as a measure of emotionality. Several investigators have suggested that reduced activity or exploratory behaviour in rodents indicates a greater level of 'emotionality' (Thiessen, 1963; Stern et al., 1960). These same investigators have suggested that 'emotionality' reduction is associated with an increase in activity. Whereas Stern et al. (1960) observed a greater level of activity, measured in a relatively larger field than that employed in the present study, in group-housed rats, and interpreted this finding as 'less "fearful" behaviour in this situation', these animals developed more gastric lesions when immobilized; this finding appears somewhat inconsistent in that it suggests that reduced 'emotionality', as measured by increased activity, is associated with a more severe somatic response to stressor stimulation. Most of the previous studies which have suggested that activity levels are higher for either isolated or aggregated animals have utilized adult populations with considerable variation in the duration of the differential housing and the size of the aggregated group. Strain differences may possibly account for some of these differences, although results consistent with those presented in the present study with SwissWebster strain mice have also been obtained in this laboratory with CF-I strain mice. The present findings indicate that differential post-weaning housing conditions in SwissWebster mice lead to differences in the develop-
14, 4
ment of motor activity levels as measured in a field approximately the size of the home cage. These data suggest that group-housed mice tend to decrease their activity with time, from an initially high level of activity at 22 days of age, whereas isolated mice tend to maintain higher activity levels over time. The initially high and equivalent level of motor activity shown by both isolated and aggregated animals on the first testing day may reflect the change in housing conditions just following weaning. It also seems likely that the first 3 days of motor activity, being equivalent under both housing conditions, represent a response to altered housing conditions, in addition to or rather than a response to differential housing p e r se. With a relatively stable level of motor activity developed in differentiaUy-housed mice by 44 days of age, the isolated animals showed a significantly higher level of activity than the aggregated mice. Summary Male Swiss-Webster strain mice, housed under isolated or aggregated conditions from weaning (21 days), were tested daily for activity levels. By the fourth post-weaning day significant differences in motor activity between groups emerged and the activity of the isolated mice was generally greater thhn that of the aggregates; the latter showed a significant trend toward activity decrement over days, whereas the former tended to maintain higher stable activity levels. Daily fluctuations in activity were correlated between groups. The activity differences which developed as a function of post-weaning differential housing are considered in view of the possible contribution of 'stress', biochemical differences, and motivational factors. Acknowledgment This study was supported in part by U.S.P.H.S Grant MH 08698-02. The author wishes to thank D. Glaubinger for his technical assistance. REFERENCES Chance, M. R. A. (1946). Aggregation as a factor influencing the toxicityof sympathomimeticamines in mice. J. PharmacoL, 87, 214-219. Chance, M. R. A. (1947). Factors influencingthe toxicity of sympathomimetic amines to solitary mice. J. Pharmacol., 89, 289-296. Essman, W. B. (1966). Gastric ulceration as a function of food deprivation in isolated vs. aggregated mice. Psychon. Sci., 4, 251-252.
ESSMAN: ACTIVITY DIFFERENCES IN MICE Essman, W. B. & Frisone, J. (1966). Isolation-induced facilitation of gastric ulcero-genesis in mice. J. psychosom. Res., (in press). Geller, E., Yuwiler, A. & Zolman, J. F. (1965). Effects of environmental complexity on constituents of brain and liver. J. Neurochem., 12, 949-955. Gunn, J. A. & Gurd, J. (1940). The action of some amines related to adrenaline: cyclohexylalkylamines. J. Physiol., Lond., 7, 463-470. Moore, K. E., Sawdy, L. C. & Shaul, S. R. (1965). Effects of d-amphetamine on blood glucose and tissue glycogen levels of isolated and aggregated mice. Biochem. Pharmac., 14, 197-204. Myers, R. D. & Fox, J. (1963). Differences in maze performance of group- vs. isolation-reared rats. Psychol. Rep., 12, 199-202. Stern, J. A., Winokur, G., Eisenstein, A., Taylor, R. & Sly, M. (1960). The effect of group vs. individual housing on behavior and physiological responses to stress in the albino rat. J. psychosom. Res., 4, 187-190.
409
Thiessen, D. D. (1963). Varying sensitivity of C57B1/Crgl mice to grouping. Science, N. Y., 141,825-828. Thiessen, D. D. (1964). Population density mouse genotype, and endocrine function in behavior. J. comp. physiol. PsychoL, 57, 412-416. Welch, B. L. & Welch, A. S. (1965). Effect of grouping on the level of brain norepinepherine in White Swiss Mice. Life Sciences, 4, 1011-1018. Woods, P. J. (1959). The effects of free and restricted environmental experience on problem-solving behavior in the rat. J. comp. physiol. PsychoL, 52, 399-402. Zimbardo, P. & Montgomery, K. C. (1957). Effects of 'free environment' rearing upon exploratory behavior. PsychoL Rep., 3, 589-594.
(Received 4 February 1966, revised 9 April 1966; Ms. number: A417)