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Further studies concerning an experimentally induced labor division between rats
BEHAVIORAL BIOLOGY 16, 527-533 (1976), Abstract No. 5187
BRIEF REPORT Further Studies Concerning an Experimentally Induced Labor Division between Rats I
GINA STRUFFALDI 2 and JANDIRA MASUR
Departamento de Psicobiologia, Escola Paulista de Medicina, Rua Botucatu, 862, 04023 S~o Paulo, Brasil When rats are paired in a situation where the reward is contingent on a bar-pressing response, one of the rats performs most of the bar pressing (worker rat), while its partner receives the reward without working (parasite rat). The possible influence of the persistence of a learned response and of the schedule of reinforcement, was studied. No correlation was found between persistence of response and the categorization of the rats as workers or parasites. On the other hand, when rats individually trained in a variable interval (VI) schedule of reinforcement were paired with rats trained under continuous reinforcement (CRF), most, although not all, VI trained animals became workers. This result shows that rats can be induced to behave as workers through the manipulation of the schedule of reinforcement at the individual training.
Mowrer (1940) described that when rats individually trained to press a bar are put in groups, one of them becomes the worker rat performing most of the bar pressing, while the partners receive the reward without working. I n our laboratory, the characteristics of the development of this labor division are being investigated, through pairing rats previously trained in a modified Skinner box, with the lever and the water dipper attached to opposite walls. As a general pattern, after several paired sessions, one of the rats stops almost completely the bar pressing, remaining near the water deliverer and, thus, obtaining most of the rewards (parasite rat), while its partner continues pressing the bar and running across the box toward the water deliverer (worker rat). Sometimes the worker rat is able to dislocate the partner from the water dipper, reaching part of the reward. A rat is categorized as worker when pressing more than 80% of the total responses made by the pair, receives less than 20% of the reward throughout consecutive
1This research was submitted by G. S. in partial fulfillment of the requirement for Master of Sciences degree at Escola Paulista de Medicina. 2With a fellowship from Funda¢go de Amparo ~t Pesquisa do Estado de S~io Paulo (FAPESP). 527 Copyright © 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
528
STRUFFALDI AND MASUR
sessions (as a criterion, we consider the rat which first reaches the water dipper as the one that receives the reward). On the contrary, the partner which presses less than 20% and receives more than 80% of the reward is categorized as parasite. The worker-parasite relationship, which is characterized mainly by the decrease of bar presses by one of the rats, occurs in about 80% of the pairs after 10-15 paired sessions (Masur et alo, 1972; Masur, 1973; Masur and Struffaldi, 1974). Once developed, it remains stable throughout the sessions and it is maintained even after a period of 1 month without testing (Masur and Struffaldi, 1974). One important point is that when water is used as reinforcer (Masur et al., 1972; Masur, 1973; Masur and Struffaldi, 1974) the worker-parasite relationship is not exactly the same as when food is contingent to the bar-press response (Mowrer, 1940; Littman et al., 1954; Baron and Littman, 1961; Oldfield-Box, 1967; 1970). When food is used, the worker rat is able, through a high rate of response, to release many food pellets. This allows it to reach the food tray before the partner has consumed all the reward. Instead, when water is used, there is no possibility of accumulation. In this case, the worker is able to drink only when the parasite is not near the water deliverer, which occurs infrequently, or when the worker is able, through pushing, to dislocate its partner from the water deliverer. This last possibility is not very efficient, as the water deliverer contains only 0.08 ml of water that is easily licked before the worker rat is able to push the parasite. It has been our purpose: ( 1 ) t o test whether a differential previous experience between the members of the pair would affect the development of such relationship, and (2) to detect which are the factors contributing to the development of the worker-parasite relationship when the same previous experience is provided for both members of the pair. Within our first purpose, it was studied whether different schedules of reinforcement given to each member of a pair, at the individual bar pressing training sessions, would interfere with the later categorization of the animals (Expt. 1). It was investigated whether a rat individually trained under a variable interval (VI) schedule of reinforcement would behave as worker when paired with a rat trained under continuous reinforcement (CRF). A VI schedule was chosen as it resembles the conditions of a worker rat when water is used as reinforcer. Concerning the second purpose, one possibility would be that the worker rat could be the one which is less able to inhibit a previously acquired response, in this case, the bar press response. This hypothesis was tested through the analysis of the rate of bar pressing extinction showed by workers and parasites (Expt. 2).
SOCIAL PARASITISM BETWEEN RATS
529
EXPERIMENT 1 Methods
Thirty-two male Wistar rats from our own colony, 90 days old at the beginning of the experiment, were utilized; their weight ranged between 250-310 g. After weaning, at 25 days of age, they were housed by pairs in wire cages and maintained at a room temperature of 23 +-- 1° C. The apparatus used was a modified Skinner box made of Plexiglas, measuring 60 X 30 X 30 cm; the water dipper (0.08 ml) and the bar could be attached to either the same or to opposite walls. After one shaping session, one of the rats of each cage was trained under CRF schedule, while the other animal was trained on a VI schedule which ranged from 2 to 60 sec (average 32 sec). The rats were water deprived for 20-22 hr. In the shaping session, as in the first two following training sessions, the bar and the water dipper were on the same wall; after that they were located on opposite walls. Fourteen individual training sessions of 15 rain each, at every 48 hr, were performed. Afterwards they were placed in the box in pairs, each one made up by one VI and one CRF trained rat (cagemates). At the paired sessions, water was delivered at each bar press. For nine pairs, 11 paired sessions of 15 min were run, for the remaining seven pairs the number of sessions was 17. Results and Discussion
As expected, at the individual training sessions, VI trained rats had a higher frequency of bar pressing than CRF trained rats (Figs. 1 and 2). After pairing one VI and one CRF trained rat, the worker-parasite relationship had a rapid onset in nine pairs (Fig. 1), the VI rat behaving as worker and the CRF rat as parasite from the first sessions on. A rat was categorized as worker when pressing more than 80% of the total responses made by the pair, receives less than 20% of the reward throughout at least three consecutive sessions (Masur and Struffaldi, 1974). However, although bar pressing criterion for categorizing a rat as worker or parasite was easily achieved and maintained throughout the 11 sessions performed, the same did not happen concerning the reward criterion. Thus, for pairs 2, 3, and 10, in a few sessions the worker rat was able to reach more than 20% of the reward delivered. Figure 4 shows the results obtained with the remaining seven pairs, in which the influence of the schedule of reinforcement was not so clear. In pairs 5, 9, and 16 the rats did not establish a worker-parasite relationship throughout the 17 sessions performed. In pairs 7, 11, 12, and 15, the VI rats,
530
STRUFFALDI AND MASUR • VI DCRF
- - BAR PRESSINGS . . . . REWARDS
80 j
1
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2
4
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3
6 •
i
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SESSIONS Fig. t. Division of labor in nine pairs of rats in which the worker-parasite relationship developed rapidly. The rats trained individually in a VI schedule behaved as workers. The isolated symbols indicate the individual performance of the rats at the last individual session. The pair number is given above each curve. which showed a higher performance at the first paired sessions, began to decrease it in the following sessions, while the CRF trained partners had also a very low performance (near zero). Consequently, both rats remained near the water deliverer showing sporadic fighting episodes. This pattern was maintained for a few sessions. After that, in pairs 7, 12, and 15, the VI trained rat increased bar pressing; whereas in pair 11, it was the CRF rat which increased it. Thus, after the 11-13th session the worker-parasite relationship developed in pairs 7, 11, 12, and 15. In three of them the VI trained rat was categorized as worker, and in pair 11 the CRF trained rat behaved as such. Thus, in the majority of the pairs tested, the worker rat had been reinforced in a VI schedule, whereas the parasite rat was trained under CRF (P ~< 0.04; binomial test). These results indicate that rats can be induced to behave as workers or
SOCIAL PARASITISM BETWEEN RATS • vI aCRF
531
--BAR I~IES,~NGS . . . . . . REWARDS 9
5 i
Q
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.
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Fig. 2. Division of labor in seven pairs in which the influence of the schedule of reinforcement at the individual training sessions was not so drastic as in those pairs shown in Fig. 1. To be read as in Fig. 1. parasites through the manipulation of the schedule of reinforcement at the individual training. Littman et al. (1954) pairing rats trained under fixed-ratio versus rats trained under CRF, and using food as reinforcer (see Introduction), reported no correlation between the differential training and the worker or parasite behavior. Oldfield-Box (1970) also manipulated the schedule of reinforcement at the individual training. However, no conclusion can be reached from his observations as an insufficient number of animals were used. The present experiment provided further data concerning the influence of a differential previous experience between the members o f the pair. It has been already described the influence of postweaning social isolation in the development of the worker-parasite relationship (Masur and Struffaldi, 1974). However, there is still a lack o f data concerning which are the factors contributing to the development of the worker-parasite relationship when the same previous experience is provided for both members of a pair. The hypothesis that rats less able to inhibit an acquired response will more likely behave as workers is tested in Expt. 2.
532
STRUFFALDI AND MASUR EXPERIMENT 2
Methods
Male Wistar rats were used. The conditions were the same as in Expt. 1, with the exception that after weaning they remained in groups of six in wooden cages. The rats had been used to test a possible correlation between "learning ability" and the worker-parasite relationship. When 80 days old they were submitted to a T-maze, being classified into slow or fast learners. Afterwards they were individually trained in the modified Skinner box, as described in Expt. 1, with the exception that all rats were trained under CRF schedule. Pairs were made up by one slow and one fast learner (not cagemates). After 10 paired sessions, they were categorized as workers or parasites. As negative results were found concerning the correlation between learning ability and the worker-parasite relationship, they were used to test a possible difference in the rate of bar pressing extinction between workers and parasites. Forty-eight hours after the last paired session, 19 workers and 19 parasites were again individually placed in the Skinner box. Eight such individual sessions were performed, at every 48 hr. After that, seven extinction sessions were given individually. Results and Discussion
The results can be seen in Fig. 3. Panel A shows the performance of the rats at the individual bar pressing training. No difference was noted in the individual performance of the rats which were afterwards classified as workers or parasites. The development of the worker-parasite relationship can be seen in panel B. Panel C shows the performance of the animals when again submitted to individual sessions; parasite rats when again alone in the box started bar pressing with a performance similar to that of the workers. Panel D A
~
B
C
60,
uJ ~40'
~2o.
LZ
• " ] i ~ 4 ~ SESSIONS
Fig. 3. Bar pressing frequency of rats categorized as workers ( m m) or parasites ( D t3) at the individual training (A), when paired (B), when individually retrained (C), and during extinction (D). Bars indicate standard errors.
SOCIAL PARASITISM BETWEEN RATS
533
indicate no difference in the extinction rate of workers and parasites. Thus, the hypothesis that the worker rat is the one less able to inhibit an acquired response found no experimental evidence. The present experiment, although providing negative data to the problem of which are the factors determining the worker-parasite relationship, answers two other relevant questions to the understanding of the social interaction studied. First, workers and parasites have a similar performance at the individual training (panel A). Second, the decrease o f bar pressing to near zero showed b y parasite rats occurs only in the presence o f the worker partner, since in its absence the frequency of responding of the parasite rats returned to the baseline level observed before pairing (panel C).
REFERENCES Baron, A., and Littman, R. A. (1961). Studies of individual and paired interactional problem-solving behavior of rats: III. Solitary and social controls. Genet. Psychol. Monogr. 64, 129-209. Littman, R. A., Lanski, L. M., and Thine, R. J. (1954). Studies of individual and paired interaetional problem solving behavior of rats. Behaviour 7, 189-206. Masur, J., Mgrtz, R. M. W., and Carlini, E. A. (1972). The behavior of worker and nonworker rats under the influence of ( - ) A9 -trans-tetrahydroeannabinol, chlorpromazine and amylobarbitone. Psychopharmacologia (Berlin) 25, 57-68. Masur, J. (1973). Labor division of rats under the influence of prolonged administration of ( - ) £x9 -trans-tetrahydrocannabinol. Pharmacology 9, 35-40. Masur, J., and Struffaldi, G. (1974). Division of labor between rats: Influence of differential social rearing conditions. Behav. Biol. 12, 233-241. Mowrer, O. H. (1940). Animal studies in the genesis of personality. Trans. N.Y. Acad. Sei. 3, 8-11. Oldfield-Box, H. (1967). Social organization of rats in a "social problem" situation. Nature [London) 213, 533-534. Oldfield-Box, H. (1970). Experimental manipulation of individual performance within groups of rats engaged in a social problem. Psychol. Rep. 26, 219-225.
BRIEF REPORT Further Studies Concerning an Experimentally Induced Labor Division between Rats I
GINA STRUFFALDI 2 and JANDIRA MASUR
Departamento de Psicobiologia, Escola Paulista de Medicina, Rua Botucatu, 862, 04023 S~o Paulo, Brasil When rats are paired in a situation where the reward is contingent on a bar-pressing response, one of the rats performs most of the bar pressing (worker rat), while its partner receives the reward without working (parasite rat). The possible influence of the persistence of a learned response and of the schedule of reinforcement, was studied. No correlation was found between persistence of response and the categorization of the rats as workers or parasites. On the other hand, when rats individually trained in a variable interval (VI) schedule of reinforcement were paired with rats trained under continuous reinforcement (CRF), most, although not all, VI trained animals became workers. This result shows that rats can be induced to behave as workers through the manipulation of the schedule of reinforcement at the individual training.
Mowrer (1940) described that when rats individually trained to press a bar are put in groups, one of them becomes the worker rat performing most of the bar pressing, while the partners receive the reward without working. I n our laboratory, the characteristics of the development of this labor division are being investigated, through pairing rats previously trained in a modified Skinner box, with the lever and the water dipper attached to opposite walls. As a general pattern, after several paired sessions, one of the rats stops almost completely the bar pressing, remaining near the water deliverer and, thus, obtaining most of the rewards (parasite rat), while its partner continues pressing the bar and running across the box toward the water deliverer (worker rat). Sometimes the worker rat is able to dislocate the partner from the water dipper, reaching part of the reward. A rat is categorized as worker when pressing more than 80% of the total responses made by the pair, receives less than 20% of the reward throughout consecutive
1This research was submitted by G. S. in partial fulfillment of the requirement for Master of Sciences degree at Escola Paulista de Medicina. 2With a fellowship from Funda¢go de Amparo ~t Pesquisa do Estado de S~io Paulo (FAPESP). 527 Copyright © 1976 by Academic Press, Inc. All rights of reproduction in any form reserved.
528
STRUFFALDI AND MASUR
sessions (as a criterion, we consider the rat which first reaches the water dipper as the one that receives the reward). On the contrary, the partner which presses less than 20% and receives more than 80% of the reward is categorized as parasite. The worker-parasite relationship, which is characterized mainly by the decrease of bar presses by one of the rats, occurs in about 80% of the pairs after 10-15 paired sessions (Masur et alo, 1972; Masur, 1973; Masur and Struffaldi, 1974). Once developed, it remains stable throughout the sessions and it is maintained even after a period of 1 month without testing (Masur and Struffaldi, 1974). One important point is that when water is used as reinforcer (Masur et al., 1972; Masur, 1973; Masur and Struffaldi, 1974) the worker-parasite relationship is not exactly the same as when food is contingent to the bar-press response (Mowrer, 1940; Littman et al., 1954; Baron and Littman, 1961; Oldfield-Box, 1967; 1970). When food is used, the worker rat is able, through a high rate of response, to release many food pellets. This allows it to reach the food tray before the partner has consumed all the reward. Instead, when water is used, there is no possibility of accumulation. In this case, the worker is able to drink only when the parasite is not near the water deliverer, which occurs infrequently, or when the worker is able, through pushing, to dislocate its partner from the water deliverer. This last possibility is not very efficient, as the water deliverer contains only 0.08 ml of water that is easily licked before the worker rat is able to push the parasite. It has been our purpose: ( 1 ) t o test whether a differential previous experience between the members of the pair would affect the development of such relationship, and (2) to detect which are the factors contributing to the development of the worker-parasite relationship when the same previous experience is provided for both members of the pair. Within our first purpose, it was studied whether different schedules of reinforcement given to each member of a pair, at the individual bar pressing training sessions, would interfere with the later categorization of the animals (Expt. 1). It was investigated whether a rat individually trained under a variable interval (VI) schedule of reinforcement would behave as worker when paired with a rat trained under continuous reinforcement (CRF). A VI schedule was chosen as it resembles the conditions of a worker rat when water is used as reinforcer. Concerning the second purpose, one possibility would be that the worker rat could be the one which is less able to inhibit a previously acquired response, in this case, the bar press response. This hypothesis was tested through the analysis of the rate of bar pressing extinction showed by workers and parasites (Expt. 2).
SOCIAL PARASITISM BETWEEN RATS
529
EXPERIMENT 1 Methods
Thirty-two male Wistar rats from our own colony, 90 days old at the beginning of the experiment, were utilized; their weight ranged between 250-310 g. After weaning, at 25 days of age, they were housed by pairs in wire cages and maintained at a room temperature of 23 +-- 1° C. The apparatus used was a modified Skinner box made of Plexiglas, measuring 60 X 30 X 30 cm; the water dipper (0.08 ml) and the bar could be attached to either the same or to opposite walls. After one shaping session, one of the rats of each cage was trained under CRF schedule, while the other animal was trained on a VI schedule which ranged from 2 to 60 sec (average 32 sec). The rats were water deprived for 20-22 hr. In the shaping session, as in the first two following training sessions, the bar and the water dipper were on the same wall; after that they were located on opposite walls. Fourteen individual training sessions of 15 rain each, at every 48 hr, were performed. Afterwards they were placed in the box in pairs, each one made up by one VI and one CRF trained rat (cagemates). At the paired sessions, water was delivered at each bar press. For nine pairs, 11 paired sessions of 15 min were run, for the remaining seven pairs the number of sessions was 17. Results and Discussion
As expected, at the individual training sessions, VI trained rats had a higher frequency of bar pressing than CRF trained rats (Figs. 1 and 2). After pairing one VI and one CRF trained rat, the worker-parasite relationship had a rapid onset in nine pairs (Fig. 1), the VI rat behaving as worker and the CRF rat as parasite from the first sessions on. A rat was categorized as worker when pressing more than 80% of the total responses made by the pair, receives less than 20% of the reward throughout at least three consecutive sessions (Masur and Struffaldi, 1974). However, although bar pressing criterion for categorizing a rat as worker or parasite was easily achieved and maintained throughout the 11 sessions performed, the same did not happen concerning the reward criterion. Thus, for pairs 2, 3, and 10, in a few sessions the worker rat was able to reach more than 20% of the reward delivered. Figure 4 shows the results obtained with the remaining seven pairs, in which the influence of the schedule of reinforcement was not so clear. In pairs 5, 9, and 16 the rats did not establish a worker-parasite relationship throughout the 17 sessions performed. In pairs 7, 11, 12, and 15, the VI rats,
530
STRUFFALDI AND MASUR • VI DCRF
- - BAR PRESSINGS . . . . REWARDS
80 j
1
IZ80
•
2
4
80
3
6 •
i
rt
m
10
r
13
1 3 5 7 ~
8
'
'~
14
I
SESSIONS Fig. t. Division of labor in nine pairs of rats in which the worker-parasite relationship developed rapidly. The rats trained individually in a VI schedule behaved as workers. The isolated symbols indicate the individual performance of the rats at the last individual session. The pair number is given above each curve. which showed a higher performance at the first paired sessions, began to decrease it in the following sessions, while the CRF trained partners had also a very low performance (near zero). Consequently, both rats remained near the water deliverer showing sporadic fighting episodes. This pattern was maintained for a few sessions. After that, in pairs 7, 12, and 15, the VI trained rat increased bar pressing; whereas in pair 11, it was the CRF rat which increased it. Thus, after the 11-13th session the worker-parasite relationship developed in pairs 7, 11, 12, and 15. In three of them the VI trained rat was categorized as worker, and in pair 11 the CRF trained rat behaved as such. Thus, in the majority of the pairs tested, the worker rat had been reinforced in a VI schedule, whereas the parasite rat was trained under CRF (P ~< 0.04; binomial test). These results indicate that rats can be induced to behave as workers or
SOCIAL PARASITISM BETWEEN RATS • vI aCRF
531
--BAR I~IES,~NGS . . . . . . REWARDS 9
5 i
Q
W •
0
Z~
15
12
.
40
Z N~
a',
b..A"
W i
i
~
f
l
~
~
r
31517
SESSIONS
Fig. 2. Division of labor in seven pairs in which the influence of the schedule of reinforcement at the individual training sessions was not so drastic as in those pairs shown in Fig. 1. To be read as in Fig. 1. parasites through the manipulation of the schedule of reinforcement at the individual training. Littman et al. (1954) pairing rats trained under fixed-ratio versus rats trained under CRF, and using food as reinforcer (see Introduction), reported no correlation between the differential training and the worker or parasite behavior. Oldfield-Box (1970) also manipulated the schedule of reinforcement at the individual training. However, no conclusion can be reached from his observations as an insufficient number of animals were used. The present experiment provided further data concerning the influence of a differential previous experience between the members o f the pair. It has been already described the influence of postweaning social isolation in the development of the worker-parasite relationship (Masur and Struffaldi, 1974). However, there is still a lack o f data concerning which are the factors contributing to the development of the worker-parasite relationship when the same previous experience is provided for both members of a pair. The hypothesis that rats less able to inhibit an acquired response will more likely behave as workers is tested in Expt. 2.
532
STRUFFALDI AND MASUR EXPERIMENT 2
Methods
Male Wistar rats were used. The conditions were the same as in Expt. 1, with the exception that after weaning they remained in groups of six in wooden cages. The rats had been used to test a possible correlation between "learning ability" and the worker-parasite relationship. When 80 days old they were submitted to a T-maze, being classified into slow or fast learners. Afterwards they were individually trained in the modified Skinner box, as described in Expt. 1, with the exception that all rats were trained under CRF schedule. Pairs were made up by one slow and one fast learner (not cagemates). After 10 paired sessions, they were categorized as workers or parasites. As negative results were found concerning the correlation between learning ability and the worker-parasite relationship, they were used to test a possible difference in the rate of bar pressing extinction between workers and parasites. Forty-eight hours after the last paired session, 19 workers and 19 parasites were again individually placed in the Skinner box. Eight such individual sessions were performed, at every 48 hr. After that, seven extinction sessions were given individually. Results and Discussion
The results can be seen in Fig. 3. Panel A shows the performance of the rats at the individual bar pressing training. No difference was noted in the individual performance of the rats which were afterwards classified as workers or parasites. The development of the worker-parasite relationship can be seen in panel B. Panel C shows the performance of the animals when again submitted to individual sessions; parasite rats when again alone in the box started bar pressing with a performance similar to that of the workers. Panel D A
~
B
C
60,
uJ ~40'
~2o.
LZ
• " ] i ~ 4 ~ SESSIONS
Fig. 3. Bar pressing frequency of rats categorized as workers ( m m) or parasites ( D t3) at the individual training (A), when paired (B), when individually retrained (C), and during extinction (D). Bars indicate standard errors.
SOCIAL PARASITISM BETWEEN RATS
533
indicate no difference in the extinction rate of workers and parasites. Thus, the hypothesis that the worker rat is the one less able to inhibit an acquired response found no experimental evidence. The present experiment, although providing negative data to the problem of which are the factors determining the worker-parasite relationship, answers two other relevant questions to the understanding of the social interaction studied. First, workers and parasites have a similar performance at the individual training (panel A). Second, the decrease o f bar pressing to near zero showed b y parasite rats occurs only in the presence o f the worker partner, since in its absence the frequency of responding of the parasite rats returned to the baseline level observed before pairing (panel C).
REFERENCES Baron, A., and Littman, R. A. (1961). Studies of individual and paired interactional problem-solving behavior of rats: III. Solitary and social controls. Genet. Psychol. Monogr. 64, 129-209. Littman, R. A., Lanski, L. M., and Thine, R. J. (1954). Studies of individual and paired interaetional problem solving behavior of rats. Behaviour 7, 189-206. Masur, J., Mgrtz, R. M. W., and Carlini, E. A. (1972). The behavior of worker and nonworker rats under the influence of ( - ) A9 -trans-tetrahydroeannabinol, chlorpromazine and amylobarbitone. Psychopharmacologia (Berlin) 25, 57-68. Masur, J. (1973). Labor division of rats under the influence of prolonged administration of ( - ) £x9 -trans-tetrahydrocannabinol. Pharmacology 9, 35-40. Masur, J., and Struffaldi, G. (1974). Division of labor between rats: Influence of differential social rearing conditions. Behav. Biol. 12, 233-241. Mowrer, O. H. (1940). Animal studies in the genesis of personality. Trans. N.Y. Acad. Sei. 3, 8-11. Oldfield-Box, H. (1967). Social organization of rats in a "social problem" situation. Nature [London) 213, 533-534. Oldfield-Box, H. (1970). Experimental manipulation of individual performance within groups of rats engaged in a social problem. Psychol. Rep. 26, 219-225.