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Division of labor between rats: Influence of differential social rearing conditions
BEHAVIORAL BIOLOGY, 12, 233-241 (1974), Abstract No. 3304R
Division of Labor Between Rats: Influence of Differential Social Rearing Conditions
JANDIRA MASUR and GINA STRUFFALDI 1
Escola Paulista de Medicina, R. Botvcatu, 862, Departamento de Psieobiologia, 04023 S~o Pauio, Brasil When pairs of rats are placed in a modified Skinner box, with the water dipper and the bar attached on opposite walls, one of the rats usually becomes the worker, performing most of the bar presses, while the partner receives the reward without working (parasite rat). The influence of differential social rearing condition on this peculiar division of labor was studied. The development of the worker-parasite relationship was compared between pairs made up of two isolated or two nonisolated raised rats. Isolated rats reacted to the presence of another isolated rat through searching physical contact which led to a different pattern of development of the division of labor when compared to pairs made up by two socially raised rats. In a second experiment, in which each pair was made up of one isolated and one nonisolated rat, the isolated became parasite. The stability of the worker-parasite relationship and the behavior of two parasites or two workers when tested together in the "social situation" was also analyzed.
When rats are paired in a situation where the reward is contingent on a bar-pressing response, it is frequent that one of them will perform most of the bar presses (worker rat) while its partner acquires a parasitic pattern of behavior receiving the reward without working (Mowrer, 1940; Oldfield-Box, 1967; Masur, Mgrtz, and Carlini, 1972; Masur, 1973). Developmental factors involved in the occurrence of this division of labor are still unknown. Since differences in several behavioral situations have been reported between rodents housed alone or in groups (Stimbert, 1969; Johnson, Desisto, and Koenig, 1972; Valzelli, 1973), the purpose of the present study was to determine the influence of differential social rearing conditions on a division of labor between rats. The first experiment was designed to compare the development of the relationship when the pairs were made up of two animals reared in social isolation or of two animals reared in a group situation. The lwith fellowship from Funda¢~o de Amparo ~ Pesquisa do Estado de S~o Paulo (FAPESP). 233 Copyright (g)1974 by Academic Press, Inc. All rights of reproduction in any form reserved.
234
MASUR AND STRUFFALDI
second experiment was designed to look for a correlation between social rearing condition and either the worker pattern or parasite pattern. In order to do this, pairs made up of one animal reared in isolation and one reared in a group situation were tested. Furthermore (1) the stability of worker or parasite behavior, and (2) the behavior of paired worker or paired parasite rats when put together in the "social situation" was tested.
MATERIAL AND METHODS
SubJects. Male Wistar rats from our Department of Psychobiology colony were used. They were all raised to weaning in litters which had been reduced to six pups. After weaning, at 25 days of age, litters were randomly assigned to one of two conditions: (1)Reared in individual wooden cages measuring 26 X 26 X 20 cm in which rats could not establish contact with other rats but could receive olfactory and auditory stimuli, or (2)reared in groups of seven to eight in wooden cages measuring 49 X 35 × 20 cm. The animals remained either isolated (/) or grouped (G) throughout the experiment. All cages had wire mesh tops and fronts and were cleaned daily. Food and water were provided ad lib. Rats were maintained on a 12-hr day-night cycle at a room temperature of 23°+-1°C. Apparatus. The apparatus used for the division-of-labor task was a modified Skinner box, identical to that utilized by Masur et al. (1972) and Masur (1973). The box was constructed of Plexiglas, measuring 60 X 30 X 30 cm; the water dipper and the bar could be attached to either the same wall or to opposite walls. Experiment 1. When 65 days old, 24 I rats and 24 G rats were individually trained in the modified Skinner box, at 20-22 hr of water deprivation. First they had a 30-rain shaping session and second, they were trained on a continuous reinforcement schedule (CRF) every 48 hr for a 15-rain period. In the shaping training, as in the first two sessions, the bar and water dipper were on the same wall in order to facilitate learning; after that they were located on opposite walls. In this situation the animal had to press the bar and run across the box to drink the water. After 10 training sessions (the last three with a variability in bar-pressing rate of at most 10%) the animals were placed in the box in pairs, each consisting of two I or two G which had similar individual baselines (the last training session was considered); the G pairs were made up of animals that were not cagemates. Eleven I and 10 G pairs were formed and the remaining animals were excluded from the experiment. Sessions occurred every 48 hr and lasted for 15 min. Measures were (1)the frequency and sequence of bar presses and rewards obtained by each rat of the pair and (2) the occurrence of fighting
SOCIAL ISOLATION AND LABOR DIVISION
235
episodes. After 13 paired sessions, the rats o f each pair were categorized as workers, parasite, coactive or atypical according to a criterion established as shown in the first four columns of Table 1. A pair was placed in category A when both rats maintained themselves within the required criterion for three consecutive sessions. After being categorized the type A pairs were left undisturbed for a l-too period whereupon they were retested in order to verify the stability of the division of labor. The animals which maintained their previous category classification were rearranged into new pairs, which were either two isolated or grouped Worker rats or two isolated or grouped parasite rats. Nine sessions were run, after which the rats were recategorized according to their new division of labor.
RESULTS There was no difference between [ and G rats during the individual acquisition of the bar-press response. In the shaping session these animals had, respectively, 14+7 (mean+SD) and 18+8 rain ( P > 0 . 0 5 ; Student's t test) as shaping time; that is, the time for them to give the first three bar presses followed by the consumption of the respective rewards. Their bar-pressing performances were also similar during the 10 training sessions that followed. It was also observed, when I-I and G-G rats were paired, that rearing condition did not affect the final categorization into types A, B, or C (last two columns of Table 1). However, more I pairs displayed aggressive behavior
TABLE 1 Division of Labor of Isolated and Grouped-reared Pairs of Rats According to a Criterion of Categorization Beahvioral category
Bar presses
% of pairs in each categorya isolated grouped
Reward
Pair types A
55
70
Worker
>/80
~<20
Parasite
~<20
/> 80
Coactive
~ 50
~ 50
B
18
10
Atypic
~>80 ~<20 >/80
/> 80 < 20 ~ 50
C
27
20
aNo significant difference was found between I and G concerning the number of pairs in each category (Chi-square test).
236
MASUR AND STRUFFALDI
characterized by boxer posture, biting; and vocalization in several sessions (Fig. 1), irrespective of their pair-type categorization. Bar pressing was suppressed for a few seconds after fighting episodes. There were differences in the development of the relationship of type A according to whether the pairs were made up of two I rats or 2 G rats. Grouped animals classified as workers (Table 2; column 2) maintained similar performances throughout the 13 paired sessions which were comparable to their previous individual baselines (last individual training session). Their parasite partners (Table 2; column 3) gradually decreased bar presses reaching very low frequencies at the last sessions. On the other hand, worker and parasite isolated rats (Table 2; columns 6 and 7) showed an accentuated decrease of bar pressing at the first sessions when compared to their individual baselines. The animals later classified as workers returned to their initial rate while their partners further decreased their performances. This different pattern of development of the type A relationship can be better observed in Fig. 2 (panels I and II) in which the performance of the rats is given as a percentage of their individual baselines. The decrease of bar presses shown by the isolated rats at the first paired sessions was apparently due to the fact that they remained in physical contact, expressed mainly by rolling over and touching each other, almost all the time. One month elapsed between session 13 and sessions 14-15 (Table 2). During this period the animals were left undisturbed either in their isolated or grouped conditions. The type A relationship was to be very stable. In sessions 14 and 15 the isolated animals again exhibited the same behavioral pattern of physical contact as described for the first paired sessions. The fighting episodes in the I-I pairs were also as intense as in the first sessions. No fighting was observed between grouped rats. I~ r-'t
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_¢ 8 0
o ,"r
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I¢1
1
2
3
4
5
6
7
8
9
10
11 12 13 SESSIONS
Fig. 1. Percentage o f fighting pairs made up b y two rats reared in social isolation or two rats raised in groups. Asterisks indicate significant differences between the frequencies at a level of at least 5% (Fisher test for proportions).
SOCIAL ISOLATION AND LABOR DIVISION
237
TABLE 2 Bar Pressing and Reward Obtained (Mean -+ SD) by Pairs Made Up by Two Grouped Housed (G) or Two Isolated Reared (/) Rats. The 14th and 15th Sessions were Performed 1 mo after the 13th Grouped Paired Bar pressing sessions number Worker Parasite a
Isolated Reward
Bar pressing
Reward
Worker
Parasite
WorkX.r Parasite
Worker
Parasite
1 2 3 4 5 6 7 8 9 10 11 12 13
30-+ 3 28-+7 23 -+ 4 22-+7 24-+7 25-+8 25-+9 27-+12 25-+10 33-+9 33-+8 29-+9 29-+8 38-+10
30±9 23-+11 14 -+ 7 8-+5 9-+8 6-+5 7-+6 4-+3 4-+4 2-+3 3-+4 2-+2 1-+1 1-+1
27-+3 20-+7 17 -+ 6 8-+5 8-+8 7-+5 8-+7 5-+2 6-+5 4-+3 6-+4 4-+4 4-+4 3-+3
24-+5 22-+4 20 ± 4 22-+6 22-+6 24-+8 26-+10 26-+12 22-+9 32-+9 30-+7 26-+9 26-+7 36-+9
33-+5 16-+8 19 -+ 9 17-+7 15-+5 18-+7 21-+7 24-+9 30-+12 28-+11 33-+3 32-+8 37-+5 30-+6
35-+ 11 11-+11 12 -+ 8 12-+8 5-+4 9-+7 5-+6 3-+3 6-+6 2-+3 2-+2 2-+3 3-+2 2-+3
27-+4 14-+8 15 -+ 5 13±5 8-+3 11-+7 8-+5 5-+4 14-+6 5-+2 4-+3 4-+2 5-+1 4-+3
32-+8 13-+7 15 -+ 5 16-+4 12-+4 16-+7 20-+6 22-+9 21-+8 28-+14 30-+4 30-+7 36-+4 28-+4
14 15
29-+16 30-+17
1-+2 2-+2
2-+2 3-+2
29-+9 34-+10
18-+7 20-+8
1-+3 2-+4
3-+3 4-+2
16-+6 16-+9
aLast session of individual training. • GROUPED ISOLATED 120I-
GROUPED X GROUPED
__WORKER .... PARASITE ISOLATED X ISOLATED
GROUPED X ISOL AT E D
II
~ ...1 z~ ~e~
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20
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Fig. 2. Mean bar pressing of worker and parasite rats raised either socially or in isolation as a percentage of their individual performance.
238
MASUR AND STRUFFALDI
When two worker rats or two parasite rats which had similar bar-pressing performances were paired a new division of labor occurred (as can be seen in Table 3). When two worker rats are paired one o f them begins to behave as a parasite and when two parasite rats are paired one of them begins to behave as a worker. This result occurred irrespective o f the I or G rearing condition. An analysis was Performed in order to observe whether there is some relationship between the behavior of the rats in the first 10 bar presses and rewards of the first paired session and their later classification as workers or parasites (Fig. 3). F r o m the 130 first bar presses performed by the rats of 13 pairs, 69 were performed by rats later classified as workers while 61 were performed b y parasites. On the other hand, for the first 130 rewards, 73 were received by workers while parasites received 57. No significant difference was found ( C h i - square test; P > 0.5). In Fig. 3 the sequence o f the 10 first bar presses and rewards given b y workers and parasites can be seen. However, when two workers or two parasites were paired it could be predicted from the first 10 bar pressings which one would become the worker in the new division of labor. In all pairs the rat later classified as worker performed most o f the first 10 bar pressings. The first rewards obtained were casually distributed between b o t h rats of each pair. Experiment 2. Fourteen I rats and 14 G rats were individually trained as in Expt. 1. After eight training sessions they were placed in the testing box in pairs, each pair consisting o f one I and one G rat which had similar
TABLE 3 Performance (Mean + SD) of Previous Worker and Parasite Rats When Paired with Another Worker or Parasite, Respectively Previous worker rats Paired session number a 1 2 3 4 5 6 7 8 9
Bar pressing
Previous parasite rats
Reward
Bar pressing
Reward
Worker
Parasite
Worker
Parasite
Worker
Parasite
Worker
Parasite
37±9 25±9 21±8 23~10 20±3 26±3 28~12 36±7 30~6 30±4
40±9 18~11 16±7 10±6 10±7 8~4 6±4 1±1 2±1 2±1
2±1 21±6 20±5 12±5 9±4 10±3 6±5 1±1 1±1 2±1
4±3 21±10 17~10 21±11 20±2 25±2 27±3 36±7 31±5 30~4
1±1 17~7 18±12 13±6 20~9 27±8 24±6 24±5 26±8 29±3
1±1 2~3 3±4 3±3 1±2 1±1 2~2 1±1 1±1 0
32±6 3±3 7±7 3±4 2±2 1±1 2±2 1±1 1~2 0
30±7 15±5 14±9 13±6 16±13 27±8 24±6 24±5 21±14 29±3
aLast paired session with the original partner.
SOCIAL ISOLATION AND LABOR DIVISION
FIRST
SESSION
LAST SESSION
FIRST 10 BAR-PRESSINGS
LAST 10 BAR-PRESSING5
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239
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[] PARASITE
Fig. 3. Frequency of the first 10 bar pressing and reward obtained in the first paired session by rats later classified into workers or parasites. The last 10 bar pressings and reward obtained by the same animals in the last paired session are also indicated. Bar pressing and reward are equally distributed through the rats at the first session. No relationship could be estabfished with their later classification. Pairs 1-7 and 8-13 are made up by group-reared and isolation reared rats, respectively.
bar-press performance during individual training. In each session, as in Expt. 1, the following measures were taken: ( 1 ) f r e q u e n c y and sequence of bar-press and reward obtained b y each rat of the pair, ( 2 ) w h i c h animal attacked in each fighting episode, and (3)fighting time. After 12 sessions each pair was categorized according to the criteria shown in Table 1.
RESULTS As in Expt. 1 no difference was found between the bar press rates o f / and G rats when tested individually. When paired, G rats immediately started to press the bar and obtain reward and appeared undisturbed by the presence o f the other animal. I partners showed a peculiar behavior. They chased after the G rats and sporadically pressed the bar and rarely obtained reward. This pattern of
240
MASUR AND STRUFFALDI
behavior continued for the first three to four sessions after which the I rats stopped running after their partners and remained near the water dipper drinking the water provided by the partner. In the majority of the sessions, fighting episodes occurred, although they were not as evident as in the pairs made up of I rats (Expt. 1). These episodes were more frequent in the middle of the sessions and were started mostly by I rats. Fighting occurred until the last session, although not as frequently as in the earlier sessions. From the 14 pairs tested 12 were categorized as type A. In 10 of them the isolated rats behaved as parasites (Binomial test; P < 0.02). The remaining two pairs were atypical. Figure 2 (panel III) shows the development of the relationship type A in the I-G pairs. Isolated rats showed, from the first paired session, an accentuated decrease of performance while their grouped partners remained near baseline rates.
DISCUSSION The present experiments have provided some information concerning questions about the division of labor studied. In Expt. 1 there was no relation between the first bar presses or rewards obtained in the beginning of the first session and the later behavior of the animals. This result suggests that factors other than which rat is reinforced first, produces the worker or parasite behavior. It is also interesting to note that the development of the worker-parasite relationship was not observed in all pairs tested. However, this could be due to an insufficient number of paired testing sessions. The relationship of type A is highly stable. No inversion of the worker-parasite relationship occurred throughout the experiment and it was maintained even after a period of 1 mo without testing. In order to classify a rat as a worker or parasite both criteria, bar presses and rewards, must be met (Table 1). As a rule the bar-pressing criterion is the first to be achieved. Usually in sessions two to six one of the animals of the pair shows a decline in bar pressing to about 20% of the total bar presses performed by the pair. At this phase this animal develops a pattern of walking back and forth across the box and not staying by the water dipper. After a few more sessions, however, the same rat obtains more than 80% of the rewards as it now stays near the water dipper most of the time. Although it is known that social deprivation in rodents and primates induces behavioral changes (Turner, Davenport, and Rogers, 1969; Stimbert, 1969; Johnson, Desisto, and Koenig, 1972; ValzeUi and Garattini, 1972; Suomi, Harlow, and McKinney, 1972; Valzelli, 1973) no difference occurred between isolated and socially reared rats when trained to bar press individually (Expts. 1 and 2). The influence of the differential social rearing condition appeared only when the rats were placed in the worker-parasite situation. This
SOCIAL ISOLATION AND LABOR DIVISION
241
could indicate either that initial training was not sensitive enough to detect differences between rearing conditions or that the differences emerge only in "social situations." One behavioral difference between pairs o f two I or G rats became evident in the first testing sessions when the rats were paired; the I pairs sought "physical contact (rolling over and touching) which was predominant over the bar-press response. Although this pattern vanished after a few sessions, it reemerged after a period of 1 mo without testing. Also in the pairs made up of one I and one G rat the continuous chasing o f the I animals after their partners could be interpreted as a search of physical contact which was made difficult as the G partner remained pressing the bar and running across the box in order to receive the reward. Our results suggest that when a socially raised rat is paired with an individually housed rat the worker-parasite behavior is determined b y the different housing conditions. There is a predominance o f parasite behavior among the I rats (Expt. 2). This could be due to a direct effect of isolation producing an unknown behavioral characteristic which would lead to parasitic behavior. A more likely hypothesis would be that o f an indirect effect of isolation. In animals housed alone the search for physical contact would compete with the bar-pressing performance, decreasing it. Their later parasitic behavior when paired with socially raised rats would be a consequence of being reinforced by the receipt o f reward without bar pressing.
REFERENCES Johnson, R. N., Desisto, M. J. Jr., and Koenig, A. B. (1972). Social and development experience and interspecific aggression in rats. J. Comp. Physiol. Psychol. 79, 237-242. Masur, J., M~rtz, R. M. W., and Carlini, E. A. (1972). The behavior of worker and nonworker rats under the influence of (-) ~9 -trans-tetrahydrocannabinol, chlorpromazine and amylobarbitone. Psychopharrnacologia 25, 57-68. Masur, J. (1973). Labor division of rats under the influence of prolonged administration of (-) A9 -trans-tetrahydrocannabinol. Pharmacology 9, 35-40. Mowrer, O. H. (1940). Animal studies in the genesis of personality. Trans. N. K Acad. Sci. 3, 8-11. Oldfield-Box, H. (1967). Social organization of rats in a "social problem" situation. Nature (London), 213, 533-534. Stimbert, V. E. (1969). Effects of early experience on social learning in rats. J. Comp. Physiol. Psychol. 69, 640-643. Suomi, S. J., Harlow, H. F., and McKinney, W. T., Jr. (1972). Monkey psychiatrists. Amer. J. Psychiat. 128, 927-932. Turner, C. H., Davenport, R. K., Jr., Rogers, C. M. (1969). The effect of early deprivation on the social behavior of adolescent chimpanzees. Amer. J. Psychiat. 125, 1531-1536. Valzelli, L., Garattini, S. (1972). Biochemical and behavioral changes induced by isolation in rats. Neuropharmacology 11, 17-22. Valzelli, L. (1973). The "isolation syndrome" in mice. Psychopharmacologia 31, 305-320.
Division of Labor Between Rats: Influence of Differential Social Rearing Conditions
JANDIRA MASUR and GINA STRUFFALDI 1
Escola Paulista de Medicina, R. Botvcatu, 862, Departamento de Psieobiologia, 04023 S~o Pauio, Brasil When pairs of rats are placed in a modified Skinner box, with the water dipper and the bar attached on opposite walls, one of the rats usually becomes the worker, performing most of the bar presses, while the partner receives the reward without working (parasite rat). The influence of differential social rearing condition on this peculiar division of labor was studied. The development of the worker-parasite relationship was compared between pairs made up of two isolated or two nonisolated raised rats. Isolated rats reacted to the presence of another isolated rat through searching physical contact which led to a different pattern of development of the division of labor when compared to pairs made up by two socially raised rats. In a second experiment, in which each pair was made up of one isolated and one nonisolated rat, the isolated became parasite. The stability of the worker-parasite relationship and the behavior of two parasites or two workers when tested together in the "social situation" was also analyzed.
When rats are paired in a situation where the reward is contingent on a bar-pressing response, it is frequent that one of them will perform most of the bar presses (worker rat) while its partner acquires a parasitic pattern of behavior receiving the reward without working (Mowrer, 1940; Oldfield-Box, 1967; Masur, Mgrtz, and Carlini, 1972; Masur, 1973). Developmental factors involved in the occurrence of this division of labor are still unknown. Since differences in several behavioral situations have been reported between rodents housed alone or in groups (Stimbert, 1969; Johnson, Desisto, and Koenig, 1972; Valzelli, 1973), the purpose of the present study was to determine the influence of differential social rearing conditions on a division of labor between rats. The first experiment was designed to compare the development of the relationship when the pairs were made up of two animals reared in social isolation or of two animals reared in a group situation. The lwith fellowship from Funda¢~o de Amparo ~ Pesquisa do Estado de S~o Paulo (FAPESP). 233 Copyright (g)1974 by Academic Press, Inc. All rights of reproduction in any form reserved.
234
MASUR AND STRUFFALDI
second experiment was designed to look for a correlation between social rearing condition and either the worker pattern or parasite pattern. In order to do this, pairs made up of one animal reared in isolation and one reared in a group situation were tested. Furthermore (1) the stability of worker or parasite behavior, and (2) the behavior of paired worker or paired parasite rats when put together in the "social situation" was tested.
MATERIAL AND METHODS
SubJects. Male Wistar rats from our Department of Psychobiology colony were used. They were all raised to weaning in litters which had been reduced to six pups. After weaning, at 25 days of age, litters were randomly assigned to one of two conditions: (1)Reared in individual wooden cages measuring 26 X 26 X 20 cm in which rats could not establish contact with other rats but could receive olfactory and auditory stimuli, or (2)reared in groups of seven to eight in wooden cages measuring 49 X 35 × 20 cm. The animals remained either isolated (/) or grouped (G) throughout the experiment. All cages had wire mesh tops and fronts and were cleaned daily. Food and water were provided ad lib. Rats were maintained on a 12-hr day-night cycle at a room temperature of 23°+-1°C. Apparatus. The apparatus used for the division-of-labor task was a modified Skinner box, identical to that utilized by Masur et al. (1972) and Masur (1973). The box was constructed of Plexiglas, measuring 60 X 30 X 30 cm; the water dipper and the bar could be attached to either the same wall or to opposite walls. Experiment 1. When 65 days old, 24 I rats and 24 G rats were individually trained in the modified Skinner box, at 20-22 hr of water deprivation. First they had a 30-rain shaping session and second, they were trained on a continuous reinforcement schedule (CRF) every 48 hr for a 15-rain period. In the shaping training, as in the first two sessions, the bar and water dipper were on the same wall in order to facilitate learning; after that they were located on opposite walls. In this situation the animal had to press the bar and run across the box to drink the water. After 10 training sessions (the last three with a variability in bar-pressing rate of at most 10%) the animals were placed in the box in pairs, each consisting of two I or two G which had similar individual baselines (the last training session was considered); the G pairs were made up of animals that were not cagemates. Eleven I and 10 G pairs were formed and the remaining animals were excluded from the experiment. Sessions occurred every 48 hr and lasted for 15 min. Measures were (1)the frequency and sequence of bar presses and rewards obtained by each rat of the pair and (2) the occurrence of fighting
SOCIAL ISOLATION AND LABOR DIVISION
235
episodes. After 13 paired sessions, the rats o f each pair were categorized as workers, parasite, coactive or atypical according to a criterion established as shown in the first four columns of Table 1. A pair was placed in category A when both rats maintained themselves within the required criterion for three consecutive sessions. After being categorized the type A pairs were left undisturbed for a l-too period whereupon they were retested in order to verify the stability of the division of labor. The animals which maintained their previous category classification were rearranged into new pairs, which were either two isolated or grouped Worker rats or two isolated or grouped parasite rats. Nine sessions were run, after which the rats were recategorized according to their new division of labor.
RESULTS There was no difference between [ and G rats during the individual acquisition of the bar-press response. In the shaping session these animals had, respectively, 14+7 (mean+SD) and 18+8 rain ( P > 0 . 0 5 ; Student's t test) as shaping time; that is, the time for them to give the first three bar presses followed by the consumption of the respective rewards. Their bar-pressing performances were also similar during the 10 training sessions that followed. It was also observed, when I-I and G-G rats were paired, that rearing condition did not affect the final categorization into types A, B, or C (last two columns of Table 1). However, more I pairs displayed aggressive behavior
TABLE 1 Division of Labor of Isolated and Grouped-reared Pairs of Rats According to a Criterion of Categorization Beahvioral category
Bar presses
% of pairs in each categorya isolated grouped
Reward
Pair types A
55
70
Worker
>/80
~<20
Parasite
~<20
/> 80
Coactive
~ 50
~ 50
B
18
10
Atypic
~>80 ~<20 >/80
/> 80 < 20 ~ 50
C
27
20
aNo significant difference was found between I and G concerning the number of pairs in each category (Chi-square test).
236
MASUR AND STRUFFALDI
characterized by boxer posture, biting; and vocalization in several sessions (Fig. 1), irrespective of their pair-type categorization. Bar pressing was suppressed for a few seconds after fighting episodes. There were differences in the development of the relationship of type A according to whether the pairs were made up of two I rats or 2 G rats. Grouped animals classified as workers (Table 2; column 2) maintained similar performances throughout the 13 paired sessions which were comparable to their previous individual baselines (last individual training session). Their parasite partners (Table 2; column 3) gradually decreased bar presses reaching very low frequencies at the last sessions. On the other hand, worker and parasite isolated rats (Table 2; columns 6 and 7) showed an accentuated decrease of bar pressing at the first sessions when compared to their individual baselines. The animals later classified as workers returned to their initial rate while their partners further decreased their performances. This different pattern of development of the type A relationship can be better observed in Fig. 2 (panels I and II) in which the performance of the rats is given as a percentage of their individual baselines. The decrease of bar presses shown by the isolated rats at the first paired sessions was apparently due to the fact that they remained in physical contact, expressed mainly by rolling over and touching each other, almost all the time. One month elapsed between session 13 and sessions 14-15 (Table 2). During this period the animals were left undisturbed either in their isolated or grouped conditions. The type A relationship was to be very stable. In sessions 14 and 15 the isolated animals again exhibited the same behavioral pattern of physical contact as described for the first paired sessions. The fighting episodes in the I-I pairs were also as intense as in the first sessions. No fighting was observed between grouped rats. I~ r-'t
IO0
ISOLATED GROUPED
_¢ 8 0
o ,"r
40-
I¢1
1
2
3
4
5
6
7
8
9
10
11 12 13 SESSIONS
Fig. 1. Percentage o f fighting pairs made up b y two rats reared in social isolation or two rats raised in groups. Asterisks indicate significant differences between the frequencies at a level of at least 5% (Fisher test for proportions).
SOCIAL ISOLATION AND LABOR DIVISION
237
TABLE 2 Bar Pressing and Reward Obtained (Mean -+ SD) by Pairs Made Up by Two Grouped Housed (G) or Two Isolated Reared (/) Rats. The 14th and 15th Sessions were Performed 1 mo after the 13th Grouped Paired Bar pressing sessions number Worker Parasite a
Isolated Reward
Bar pressing
Reward
Worker
Parasite
WorkX.r Parasite
Worker
Parasite
1 2 3 4 5 6 7 8 9 10 11 12 13
30-+ 3 28-+7 23 -+ 4 22-+7 24-+7 25-+8 25-+9 27-+12 25-+10 33-+9 33-+8 29-+9 29-+8 38-+10
30±9 23-+11 14 -+ 7 8-+5 9-+8 6-+5 7-+6 4-+3 4-+4 2-+3 3-+4 2-+2 1-+1 1-+1
27-+3 20-+7 17 -+ 6 8-+5 8-+8 7-+5 8-+7 5-+2 6-+5 4-+3 6-+4 4-+4 4-+4 3-+3
24-+5 22-+4 20 ± 4 22-+6 22-+6 24-+8 26-+10 26-+12 22-+9 32-+9 30-+7 26-+9 26-+7 36-+9
33-+5 16-+8 19 -+ 9 17-+7 15-+5 18-+7 21-+7 24-+9 30-+12 28-+11 33-+3 32-+8 37-+5 30-+6
35-+ 11 11-+11 12 -+ 8 12-+8 5-+4 9-+7 5-+6 3-+3 6-+6 2-+3 2-+2 2-+3 3-+2 2-+3
27-+4 14-+8 15 -+ 5 13±5 8-+3 11-+7 8-+5 5-+4 14-+6 5-+2 4-+3 4-+2 5-+1 4-+3
32-+8 13-+7 15 -+ 5 16-+4 12-+4 16-+7 20-+6 22-+9 21-+8 28-+14 30-+4 30-+7 36-+4 28-+4
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29-+9 34-+10
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1-+3 2-+4
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16-+6 16-+9
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Fig. 2. Mean bar pressing of worker and parasite rats raised either socially or in isolation as a percentage of their individual performance.
238
MASUR AND STRUFFALDI
When two worker rats or two parasite rats which had similar bar-pressing performances were paired a new division of labor occurred (as can be seen in Table 3). When two worker rats are paired one o f them begins to behave as a parasite and when two parasite rats are paired one of them begins to behave as a worker. This result occurred irrespective o f the I or G rearing condition. An analysis was Performed in order to observe whether there is some relationship between the behavior of the rats in the first 10 bar presses and rewards of the first paired session and their later classification as workers or parasites (Fig. 3). F r o m the 130 first bar presses performed by the rats of 13 pairs, 69 were performed by rats later classified as workers while 61 were performed b y parasites. On the other hand, for the first 130 rewards, 73 were received by workers while parasites received 57. No significant difference was found ( C h i - square test; P > 0.5). In Fig. 3 the sequence o f the 10 first bar presses and rewards given b y workers and parasites can be seen. However, when two workers or two parasites were paired it could be predicted from the first 10 bar pressings which one would become the worker in the new division of labor. In all pairs the rat later classified as worker performed most o f the first 10 bar pressings. The first rewards obtained were casually distributed between b o t h rats of each pair. Experiment 2. Fourteen I rats and 14 G rats were individually trained as in Expt. 1. After eight training sessions they were placed in the testing box in pairs, each pair consisting o f one I and one G rat which had similar
TABLE 3 Performance (Mean + SD) of Previous Worker and Parasite Rats When Paired with Another Worker or Parasite, Respectively Previous worker rats Paired session number a 1 2 3 4 5 6 7 8 9
Bar pressing
Previous parasite rats
Reward
Bar pressing
Reward
Worker
Parasite
Worker
Parasite
Worker
Parasite
Worker
Parasite
37±9 25±9 21±8 23~10 20±3 26±3 28~12 36±7 30~6 30±4
40±9 18~11 16±7 10±6 10±7 8~4 6±4 1±1 2±1 2±1
2±1 21±6 20±5 12±5 9±4 10±3 6±5 1±1 1±1 2±1
4±3 21±10 17~10 21±11 20±2 25±2 27±3 36±7 31±5 30~4
1±1 17~7 18±12 13±6 20~9 27±8 24±6 24±5 26±8 29±3
1±1 2~3 3±4 3±3 1±2 1±1 2~2 1±1 1±1 0
32±6 3±3 7±7 3±4 2±2 1±1 2±2 1±1 1~2 0
30±7 15±5 14±9 13±6 16±13 27±8 24±6 24±5 21±14 29±3
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Fig. 3. Frequency of the first 10 bar pressing and reward obtained in the first paired session by rats later classified into workers or parasites. The last 10 bar pressings and reward obtained by the same animals in the last paired session are also indicated. Bar pressing and reward are equally distributed through the rats at the first session. No relationship could be estabfished with their later classification. Pairs 1-7 and 8-13 are made up by group-reared and isolation reared rats, respectively.
bar-press performance during individual training. In each session, as in Expt. 1, the following measures were taken: ( 1 ) f r e q u e n c y and sequence of bar-press and reward obtained b y each rat of the pair, ( 2 ) w h i c h animal attacked in each fighting episode, and (3)fighting time. After 12 sessions each pair was categorized according to the criteria shown in Table 1.
RESULTS As in Expt. 1 no difference was found between the bar press rates o f / and G rats when tested individually. When paired, G rats immediately started to press the bar and obtain reward and appeared undisturbed by the presence o f the other animal. I partners showed a peculiar behavior. They chased after the G rats and sporadically pressed the bar and rarely obtained reward. This pattern of
240
MASUR AND STRUFFALDI
behavior continued for the first three to four sessions after which the I rats stopped running after their partners and remained near the water dipper drinking the water provided by the partner. In the majority of the sessions, fighting episodes occurred, although they were not as evident as in the pairs made up of I rats (Expt. 1). These episodes were more frequent in the middle of the sessions and were started mostly by I rats. Fighting occurred until the last session, although not as frequently as in the earlier sessions. From the 14 pairs tested 12 were categorized as type A. In 10 of them the isolated rats behaved as parasites (Binomial test; P < 0.02). The remaining two pairs were atypical. Figure 2 (panel III) shows the development of the relationship type A in the I-G pairs. Isolated rats showed, from the first paired session, an accentuated decrease of performance while their grouped partners remained near baseline rates.
DISCUSSION The present experiments have provided some information concerning questions about the division of labor studied. In Expt. 1 there was no relation between the first bar presses or rewards obtained in the beginning of the first session and the later behavior of the animals. This result suggests that factors other than which rat is reinforced first, produces the worker or parasite behavior. It is also interesting to note that the development of the worker-parasite relationship was not observed in all pairs tested. However, this could be due to an insufficient number of paired testing sessions. The relationship of type A is highly stable. No inversion of the worker-parasite relationship occurred throughout the experiment and it was maintained even after a period of 1 mo without testing. In order to classify a rat as a worker or parasite both criteria, bar presses and rewards, must be met (Table 1). As a rule the bar-pressing criterion is the first to be achieved. Usually in sessions two to six one of the animals of the pair shows a decline in bar pressing to about 20% of the total bar presses performed by the pair. At this phase this animal develops a pattern of walking back and forth across the box and not staying by the water dipper. After a few more sessions, however, the same rat obtains more than 80% of the rewards as it now stays near the water dipper most of the time. Although it is known that social deprivation in rodents and primates induces behavioral changes (Turner, Davenport, and Rogers, 1969; Stimbert, 1969; Johnson, Desisto, and Koenig, 1972; ValzeUi and Garattini, 1972; Suomi, Harlow, and McKinney, 1972; Valzelli, 1973) no difference occurred between isolated and socially reared rats when trained to bar press individually (Expts. 1 and 2). The influence of the differential social rearing condition appeared only when the rats were placed in the worker-parasite situation. This
SOCIAL ISOLATION AND LABOR DIVISION
241
could indicate either that initial training was not sensitive enough to detect differences between rearing conditions or that the differences emerge only in "social situations." One behavioral difference between pairs o f two I or G rats became evident in the first testing sessions when the rats were paired; the I pairs sought "physical contact (rolling over and touching) which was predominant over the bar-press response. Although this pattern vanished after a few sessions, it reemerged after a period of 1 mo without testing. Also in the pairs made up of one I and one G rat the continuous chasing o f the I animals after their partners could be interpreted as a search of physical contact which was made difficult as the G partner remained pressing the bar and running across the box in order to receive the reward. Our results suggest that when a socially raised rat is paired with an individually housed rat the worker-parasite behavior is determined b y the different housing conditions. There is a predominance o f parasite behavior among the I rats (Expt. 2). This could be due to a direct effect of isolation producing an unknown behavioral characteristic which would lead to parasitic behavior. A more likely hypothesis would be that o f an indirect effect of isolation. In animals housed alone the search for physical contact would compete with the bar-pressing performance, decreasing it. Their later parasitic behavior when paired with socially raised rats would be a consequence of being reinforced by the receipt o f reward without bar pressing.
REFERENCES Johnson, R. N., Desisto, M. J. Jr., and Koenig, A. B. (1972). Social and development experience and interspecific aggression in rats. J. Comp. Physiol. Psychol. 79, 237-242. Masur, J., M~rtz, R. M. W., and Carlini, E. A. (1972). The behavior of worker and nonworker rats under the influence of (-) ~9 -trans-tetrahydrocannabinol, chlorpromazine and amylobarbitone. Psychopharrnacologia 25, 57-68. Masur, J. (1973). Labor division of rats under the influence of prolonged administration of (-) A9 -trans-tetrahydrocannabinol. Pharmacology 9, 35-40. Mowrer, O. H. (1940). Animal studies in the genesis of personality. Trans. N. K Acad. Sci. 3, 8-11. Oldfield-Box, H. (1967). Social organization of rats in a "social problem" situation. Nature (London), 213, 533-534. Stimbert, V. E. (1969). Effects of early experience on social learning in rats. J. Comp. Physiol. Psychol. 69, 640-643. Suomi, S. J., Harlow, H. F., and McKinney, W. T., Jr. (1972). Monkey psychiatrists. Amer. J. Psychiat. 128, 927-932. Turner, C. H., Davenport, R. K., Jr., Rogers, C. M. (1969). The effect of early deprivation on the social behavior of adolescent chimpanzees. Amer. J. Psychiat. 125, 1531-1536. Valzelli, L., Garattini, S. (1972). Biochemical and behavioral changes induced by isolation in rats. Neuropharmacology 11, 17-22. Valzelli, L. (1973). The "isolation syndrome" in mice. Psychopharmacologia 31, 305-320.