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Social behaviour in pigs studied by means of operant conditioning
Anim. Behav., 1979, 27, 947-957
SOCIAL BEHAVIOUR IN PIGS STUDIED BY MEANS OF OPERANT CONDITIONING By B . A . BALDWIN & G . B . MEESE ARC Institute of Animal Physiology, Babraham, Cambridge Abstract. Pigs were trained to obtain food by pressing a panel with their snouts . Pairs or trios of animals were tested together with the panel widely separated from the feeding bowl and response rates were examined in relation to social dominance . Dominant animals of the pairs usually did almost all the panel pressing whether the pair lived together or apart . In pairs tested together with only one member trained, the untrained pigs did not learn to respond . In trios tested for 10 min, the dominant pigs did not press most but, if one animal was satiated before testing, the dominant pigs in the remaining pairs tended to press more . In trios tested for 3 h one pig pressed most and this was unrelated to rank . Social behaviour, which can conveniently be defined as behaviour `whose principal function is to influence the behaviour of a conspecific' (Ewer 1968) has usually been studied by observation of animals in natural or relatively unconfined conditions . However, following the initial observations of Mowrer (1940, 1960), using rats, a limited number of experiments have been made to study social interactions among small groups of animals which could obtain food by performing operant responses . The possible advantages of such an approach, which permits the experimenter to control many aspects of the animal's environment, have been discussed by Dimond (1970) and Box (1973) . The usual type of experiment, using rats, has been to arrange an operant conditioning task in which the food tray and the lever are placed at opposite ends of the apparatus and pairs of rats, or groups of up to five animals, are placed in the apparatus and their behaviour observed . Mowrer (1940, 1960) observed what he called a `division of labour' in groups of three rats in that one of the animals did most of the lever pressing and produced food for all the rats . Littman et al . (1954) mentioned a similar phenomenon and used the terms `worker' and `dependent' . A more detailed investigation of the factors such as age of the animals used, size of groups, training procedures, degree of food deprivation, which could influence the behaviour of rats in experiments similar to those mentioned above, have been carried out by Baron & Littman (1961) and Box (1967, 1969, 1970) . The following types of social organization have been described in domesticated pigs . (1) The teat order, which is established among the litter when the piglets compete for the preferred anterior teats (Donald 1937 ; McBride 1963 ;
Fraser 1975) . (2) The dominance order which is most evident after weaning (McBride et al . 1964 ; Beilharze & Cox 1967 ; Bryant 1970 ; Meese & Ewbank 1973a) . (3) Leadership and follower relations (Meese & Ewbank 1973b .) A general review of behaviour in pigs has recently been published (Signoret et al . 1975) . It has been established that groups of pigs have a well defined social organization characterized by a dominance order which is usually considered to be `bidirectional' in view of the fact that subordinate animals frequently retaliate when attacked by dominant pigs (Beilharze & Cox 1967 ; Bryant 1970) although in a well established social group, communication of threat or submission may be formalized and submission may simply be indicated by a lowering of the direction of the eyes (McBride et al . 1964) . The usual experimental determination of a pig's position in the social hierarchy has depended upon the outcome of agonistic encounters to decide which pig takes precedence in a competitive situation in which food is offered . Competitive tests for rewards other than food do not appear to have been used . In the experiments reported here we have observed the behaviour of pigs in operant situations similar to some of those previously used with rats . Pigs make good experimental animals for operant conditioning and will soon learn to press panels with their snouts in order to obtain food (Baldwin & Stephens 1973) or, when in a cold environment, infra-red heat (Baldwin & Ingram 1967) . They are also bold animals and not easily disturbed by the presence of observers and, when starved, are very highly motivated to obtain food . In the present experiments we have used a competitive operant feeding situation for the 947
948
ANIMAL BEHAVIOUR, 27, 3
following : (1) in order to establish whether the operant method would prove useful in the study of social behaviour in the pig, particularly in relation to the dominance order . (2) To compare the results obtained using pigs with some of those previously found in rats in order to evaluate the degree of species specificity in the observed behaviour . (3) To ascertain the effect of factors such as social rank, degree of food deprivation, amount of pretraining and previous social history on the performance of pigs in the operant situation. General Methods A total of 64 pigs was used . They were all young females or young castrated males of the large white breed and they weighed approximately 20 to 40 kg . Except for any experimental procedure such as 24 h without food the animals were maintained under normal husbandry conditions and fed on a commercial pelleted diet at up to 4 . 5 % body weight per day . All groups were made up of pigs of approximately the same size and weight . When necessary, the pigs were taught in a training cage to push a panel with their snouts in order to obtain a small reward of the pelleted food . The behavioural observations were made in a room measuring approximately 2 . 8 m by 1 . 8 m (Fig 1) and fitted with a double `stable'
door . The lower door was I m high so that the observer could watch the pig's behaviour over the open upper door . The panel was identical to that used in the training cage and a small amount of pelleted food (the animals' normal diet) was delivered into the feeding bowl following a single press of the panel . The observers' notes and tape-recorded commentary were sometimes supplemented by a closed circuit TV system connected to a video-tape recorder . Experiment 1 : Effects of 24 h, 48 h without Food on Pairs of Pigs, Living Together and Both Taught to Press (Figs 1, 2, 3, 4, 5) This experiment was carried out in order to establish the relation between social rank and panel pressing and also to evaluate the effects of increasing food deprivation . w b
a f 100
a w 20
SO-
a N
0 10 TESTS
Fig . 2. Results from experiment 1, pairs of pigs living together . The number of panel presses made during a 15 min period by the dominant and subordinate animals after 24 h without food when tested separately (a) and together (b). Results based on the mean of 10 observations. The symbols for the dominant and subordinate animals apply to all figures .
P
6:
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01
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Fig . 1 . Plan of the room used for behavioural observations. P, switch panel ; F, feeder .
10
TESTS
Fig. 3 . Results from experiment 1, pairs of pigs living together . The time (s) spent feeding and the time spent in aggression during a 15 min period by the dominant and subordinate animals after 24 h without food when tested separately (a) and together (b) . Results based on the mean of 10 observations .
BALDWIN & MEESE : SOCIAL BEHAVIOUR AND OPERANT CONDITIONING IN PIGS
Methods Ten pairs of previously unacquainted pigs were used. The pairs of pigs lived together and were taught separately to press the panel in the training cage . The animals were then introduced singly to the experimental room and were taught to press the panel and receive a reward in the bowl fixed near the far wall (Fig . 1). When both pigs were proficient and rapidly pressing the panel and obtaining food, their individual baseline performance was determined in the following way . The pigs were deprived of food for 24 h and then introduced singly to the room where they were observed for 15 min. At the end of each observation the animals received the remainder of their maintenance rations. During this period a record was kept of the number of presses made at the panel and the time spent feeding at the bowl . These baseline observations were made, in
949
this and subsequent experiments, in order to establish that all the pigs could press the panel and feed at similar rates . A total of 10 observations were made on each animal . At the end of these baseline determinations the two pigs were simultaneously placed in the test room and a further 10 x 15 min observations were made. During these periods the number of presses made by each animal and the time spent feeding was recorded. A record was also kept of the duration of all the aggression observed with a minimum of 1 s assumed for all aggressive incidents . At the end of the 10 observations baseline determinations were repeated but after 48 h without food . The tests in which the two pigs were together were repeated with both pigs deprived of food for 48 h . At the completion of all the observations the relative dominance was determined for the two animals in the following way . Both pigs were deprived of food for 24 h and a bowl containing approximately 500 g of food was placed in a neutral pen to which the two animals were introduced . They were observed for up to 3 min and the time spent feeding by each animal, or both together, was recorded . A total of 10 such observations was made . Dominance was calculated by the proportion (between 0 and 1) of the total time that each animal spent feeding singly at the bowl . The above procedure was followed in all cases of dominance determinations .
TESTS
Fig. 4 . Results from experiment 1, pairs of pigs living together . The number of panel presses made during a 15 min period by the dominant and subordinate animals after 48 h without food when tested separately (a) and together (b) . Results based on the means of 10 observations . K' I,
6-0 ~
0 2O~
1
s
o TEST'
Fig. 5 . Results from experiment 1, pairs of pigs living together. The time (s) spent feeding and the time spent in aggression during a 15 min period by the dominant and subordinate animals after 48 h without food when tested separately (a) and together (b). Results based on the mean of 10 observations .
Results 24 h of food deprivation . The results obtained when the 10 different pairs of pigs were starved for 24 h are shown in Fig . 2 . It can be seen that, when tested alone, the dominant pigs tended to press the panel more often during the 15 min test period (Fig . 2a) although the difference was not statistically significant when examined by `t' test following analysis of variance . When tested together after 24 h without food, as can be seen from Fig 2b, the dominant pigs did most of the panel pressing and, after five trials, the subordinate pigs seldom pressed the panel. The time spent feeding and the time spent in aggression by the pairs of pigs after 24 h without food are shown in Fig . 3 where it can be seen that the time spent feeding, by both dominant and subordinate members of each pair, was the same when they were tested alone (Fig . 3a) . When the pigs were tested together, the dominant animals spent much more time feeding (Fig . 3b) but the amount of aggression was low and almost constant during the 15 min trials .
950
ANIMAL BEHAVIOUR,
The aggression occurred entirely at the feeding bowl and no instances were seen in which competition and aggression took place in order to obtain access to the response panel . After the first test together the aggression became ritualized, i .e. there was not much fighting, just displacement from the feeder by pushing and threats . The displaced subordinate pigs often briefly retaliated or refused to move . Several instances of `play fighting' were seen and occasionally the pigs would scratch themselves by rubbing along the walls . 48 h of food deprivation . The results obtained when the same ten pairs of pigs were tested alone, or together, after 48 h starvation are shown in Figs. 4 and 5 . As may be seen in Fig . 4a, the dominant pigs pressed the panel more often than subordinate pigs when tested alone although the difference was not statistically significant . The rate of panel pressing by both dominant and subordinate pigs after 48 h of food deprivation was higher than after 24 h without food as can be seen by comparison of Fig . 4a and Fig . 2a and it is probable that this difference is due to the longer deprivation period although it must be noted that, at the time of the 48 h tests, the pigs were more experienced in panel pressing. The results obtained when the pairs of pigs were tested together after 48 h deprivation of food are shown in Fig . 4b from which it can be seen that the dominant pigs did almost all the panel pressing and responded at a slightly higher rate than when they were tested alone . In all groups the dominant pig pressed the panel most on the first and subsequent tests . In marked contrast, the subordinate pigs hardly ever pressed the panel . The time spent feeding when the pigs were alone or together, after 48 h starvation, is shown in Fig. 5 which also displays the amount of aggression that occurred . It can be seen that, when tested alone, the time spent feeding was about the same as after 24 h without food (Figs. 3a and 5a), but as the response rate was considerably higher (Figs . 4a and 2a) and hence more food was delivered, the rate of ingestion was higher in the pigs after 48 h starvation . When tested together after 48 h without food the dominant pigs spent much more time feeding than the subordinates (Fig. 5b) . The amount of aggression was low and constant throughout the 10 tests and was about the same level as seen after 24 h deprivation of food (Fig . 5b) . All the aggression occurred at the feeding bowl and
27, 3
there was no fighting to gain access to the response panel . Experiment 2 ; Effects of 24 h of Food Deprivation on Pairs of Pigs, Living Together with one only of Each Pair Taught to Press (Table I) This experiment was done in order to establish whether the untrained members of the pairs would learn to respond by observing the behaviour of the trained pigs . Method Six pairs of previously unacquainted pigs were used. The animals lived together in pairs and only one of each pair was taught to press the panel . Following 24 h without food a baseline was determined for the animals which could press the panel ; a total of 10 x 5 min observations were made on separate occasions . At the end of these observations a trained and an untrained pig were simultaneously introduced to the experimental situation and 10 x 5 min observations were made. After the paired tests, a repeat was made of the baseline determinations using both the trained and untrained pigs . Finally, the relative dominance values were determined using the method previously described . Of the six animals trained to press the panel, four were the dominant of their pair, one was the subordinate and one was of approximately equal rank . Results The results are presented in Table I from which it can be seen that the untrained pigs did not learn to press the panel . No vigorous aggression was seen at any stage, and the total amount of aggression was low as even the most aggressive pairs did not exceed an average of 8 % of the available time in aggressive activity . Almost all the aggression occurred near the food bowl . In one group (B) panel pressing almost ceased after four runs ; they began to respond again before the end of the series of trials but both pigs spent a considerable time nibbling and scratching on the walls . In group C the trained pig took six trials before it started to press the panel and during this time both pigs moved around the pen rooting in the sawdust and emitting agitated grunts . The trained pig, which was the dominant animal (Table I) often attempted to escape by trying to leap over the door . As can be seen in Table I the trained pigs were dominant in groups C, D, E and F . The untrained pig was clearly dominant in group B
BALDWIN & MEESE : SOCIAL BEHAVIOUR AND OPERANT CONDITIONING IN PIGS V
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ANIMAL BEHAVIOUR, 27, 3
952
and of almost equal rank in group A . In group B the dominant untrained pig spent more time feeding. When the trials were over the pigs were retested alone and it can be seen from Table I that, despite the tests in which they had an opportunity to observe the trained pigs pressing the panels, none of the untrained animals had learned to press the panels. Experiment 3: Effects of 24 h of Food Deprivation on Pairs of Pigs, Living Separately and both Taught to Press (Figs . 6 and 7) This experiment was carried out in order to investigate the effect of social rank on panel pressing in previously unacquainted pigs . Method Four pairs of previously unacquainted pigs were used . Both animals of each pair were taught to press but the pigs were kept in separate pens and never met except in the experimental situation . The previously described observations were made (in all cases after 24 h starvation) i .e . 10 x 5 min baseline determinations on both pigs separately, 10 x 5 min observations with the two pigs together, followed by repetition of the baseline observations . The relative dominance was determined between the two pigs after the tests using the competitive feeding procedure . Results The results obtained are shown in Figs . 6 and 7. Figure 6 displays the number of responses made by the dominant and subordinate pigs
when tested separately (Fig . 6a) or together (Fig. 6b) and also when they were tested separately after completing the tests in which they were together (Fig . 6c) . It can be seen that the dominant pigs tended to press the panel at a more rapid rate than did subordinates even when they were tested alone . However, this difference was mainly due to one of the pairs in which the dominant animal pressed much faster than the subordinate pig . When they were tested together (Fig . 6b) it can be seen that, after the first two tests, the dominant pigs did almost all the panel pressing . In three groups of the four, the a pig did most pressing in the first test and usually was the sole presser in subsequent tests . In the remaining group the a and (3 pressed about an equal amount although the a was clearly dominant in the subsequent food competition tests . It can be seen from Fig . 7, which displays the time spent feeding by the dominant and subordinate pigs when tested alone (Fig . 7a), together (Fig . 7b) or separately again after the paired tests (Fig. 7c) that when the two pigs were tested together the dominant pigs spent much more time feeding than the subordinates . The amount of aggression seen was low and relatively constant after the initial encounter . It was obvious that one pig usually became the main panel presser in this situation . In the subsequent competitive feeding (dominance) tests it was clear that the animals which did most panel pressing were the dominant pigs .
40b
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Fig. 6 . Results from experiment 3, pairs of pigs previously unacquainted before meeting in the experimental situation . The number of responses (panel presses) made during a 5 min period by the dominant and subordinate animals after 24 h without food when tested seperately (a) or together (b) and also when they were tested separately again (c) . Each point is based upon the mean of four observations .
Fig. 7 . Results from experiment 3, pairs of pigs, previously unacquainted before meeting in the experimental situation . The time (s) spent feeding and the time spent in aggression during a 5 min period by the dominant and subordinate animals after 24 h without food when tested separately (a) together (b) and also tested separately again (c) . Results based upon the means of four observation . Dominance relationships were determined at the end of the observations .
BALDWIN & MEESE : SOCIAL BEHAVIOUR AND OPERANT CONDITIONING IN PIGS
Experiment 4 : (a) Effects of 24 h Food Deprivation on three Pigs, Living Together and all Taught to Press (Table II) This experiment was carried out in order to see whether altering the group size to three pigs would yield different results from those obtained with pairs of pigs . Method Four groups of three previously unacquainted pigs were used . The groups of three pigs lived together and were all taught to panel press . In order to check that all the pigs had learned to use the panel, baseline values were obtained from 5 x 5 min observations on each pig tested alone . After 24 h deprivation of food the three pigs were placed together in the experimental pen and 5 x 10 min observations were made on separate occasions . Dominance tests were carried out before the experimental runs as follows . After a period of 24 h without food each pig in turn competed in a 3 min food competition test with one of the other pigs . These combinations were repeated five times and a dominance value determined based on the amount of time spent by each animal eating alone during the competition . Results The results are shown in Table II which summarizes a statistical analysis of the data (ANOVA) in terms of which pig did more pressing or spent longer feeding. As can be seen from the table, when none of the pigs was satiated, there was no tendency for the dominant a pigs to press the panel significantly more often than the other pigs .
In terms of time spent feeding, no clear result emerged except that the y pigs usually spent less time feeding than the other pigs . The amount of aggression was low with an overall mean for all groups of 36 ± 9 s out of a possible 600 s . During the test occasional short periods occurred in which the pigs would rub themselves against the walls of the pen . Experiment 4 : (b) Effects within Groups of three Pigs of 24 h Food Deprivation of two Animals and Satiation of the Third, Pigs Living Together and all Taught to Press (Table II) This experiment was carried out to establish whether satiation of one member of the group would produce results similar to those seen when pairs of pigs were used . Method The same pigs were used as in the previous experiment (4a) and the procedure with the three pigs together was repeated on five occasions after one of the pigs had been fed to satiation in the experimental pen . The remaining two pigs had undergone the usual 24 h food deprivation . The satiated pig (not eating in the presence of food) received food from the experimenter and not by pressing the panel. This was repeated with the a, (3 and y animals until each of the pigs had been used . At the end of the experiments the dominance values were again determined in order to see whether any marked changes in relative dominance had occurred during the experiments . In none of the groups had the position of the a pigs changed but in one group the (3 and 7 pigs exchanged places . All the
Table II Group and activity Panel pressing
Feeding
Which pigs were satiated None A B C D
y > (3 R>a .y .
A B C D
a.D>y a. R > y R > a .y a.y>13
NS NS
a /3 .y > a 0 > a .y . 13 > a .y. f3>a .y . (3.y>a NS p.y > a a .y>(3
S NS
Q>0>7 a > 13.y. a>R>y a.y > R a .y>0 a.y>R a .y>R
y (3 > a .-I . a > 13 .y . S > a .y. a .R>y a.0 > y a.s>y a.0>y a>y
No significant difference between a, (3 and y pigs at 5 % level or less . a pig differs significantly from 0 and the y lies between a and (3 but not significantly different from either . a > 0 > y : All three pigs differ significantly. NS :
a > R:
953
954
ANIMAL BEHAVIOUR, 27,
analysis in Table II is based on the first set of dominance values . Results These are presented in Table II from which it can be seen that, when the a pig was satiated, the R pigs did most of the panel pressing . When the (3 pigs were satiated the a pigs pressed significantly more often in three of the groups . When the y pigs were satiated the results were more variable with no tendency for the a pigs to do most of the panel pressing . In terms of time spent feeding the two hungry pigs usually did not significantly differ from each other . The overall level of aggression was low with a mean value of 40 ± 4 s out of 600 s . During these experiments `play fighting' and chasing around the pen sometimes occurred and occasionally the pigs would rub themselves against the walls or nibble at the walls . Experiment 5 : Effects of 24 h without Food on Groups of Three pigs, Living Together all Taught to Press and Allowed to Satiate in the Test Situation (Table III) This experiment was done in order to establish whether stable `worker-dependent' relations developed during prolonged tests using three pigs . Method Four groups of three previously unacquainted pigs were used . The groups of three pigs lived together and were all taught to press the panel
3
as described previously . Baseline values were obtained from 5 x 5 min observations made on each pig alone in the test room. In the main series of tests, after 24 h fasting, the three pigs were simultaneously introduced to the experimental pen and the group observed continuously for 3 h in order to record the number of panel presses made by each pig during the session . The procedure was followed on five occasions with each group . Dominance relationships between the three animals were determined before and after the experimental tests using the competitive feeding test, and it was found that in none of the groups had the position of the a pigs changed but in one group the y and 3 exchanged places (Tables III, group B) . Results The results for all groups are summarized in Table III from which it can be seen that initially, when tested alone, the a, R and y pigs all pressed the panel and ate the food at approximately similar rates . When the three pigs were tested together it clearly emerged that one animal tended to do almost all the panel pressing but this tendency was unrelated to social rank . During the tests all the pigs were able to obtain sufficient food because often, while two pigs were fighting or playing near the feeder the third animal would feed . Sometimes all three pigs would feed together . Aggressive behaviour was more frequent during the first parts of the session when
Table III Pre-test baseline Pigs
(responses in 5 min) a (i
Pre-test baseline Y
a
(Time feeding in s) P
0 .34 0 . 17 20±1 .0 33±1 .0 201 ± 12 0 . 27 0 .27
Three together for 3 h Y
a
(responses in 3 h) S
7 ± 3
10 ± 7
Group A (3 pigs)
0 . 491 18±2 . 0 0 . 462
Group B (3 pigs)
0 . 40 0 . 31 24 ± 3 . 0 20 ± 1 . 0 0 . 18 0 . 59
0 .28 18 ± 1 .0 022
218 ± 5
198 ± 10 194 ± 6
GroupC (3 pigs)
0 . 42 16±1 0 .45
0 . 41 16±2 0 . 35
0 . 21 21±1 0 . 19
216±6
229±8
GroupD (3 pigs)
0 .43 31±2 0 .46
0 . 38 37±3 0 . 31
0 . 18 25±1 0 . 31
211±11 225±12 220±6 _536±112
Dominance relations before tests . 2 Dominance relations after tests . 1
206 ± 8
222 ± 10
212±2
37 ± 34 719 ± 128
23±17
y
728 ± 75
9 ± 6
4±3
569±70
13±11
92±66
BALDWIN & MEESE : SOCIAL BEHAVIOUR AND OPERANT CONDITIONING IN PIGS
all the pigs were hungry and always took place near the feeder . As in previous experiments the pigs sometimes displayed `irrelevant' behaviour such as rubbing against the walls . Towards the end of the 3 h session the pigs often rested and sometimes slept. Discussion The results of experiments 1 and 3, in which pairs of trained pigs were tested, clearly showed that dominance in the food competition tests was strongly associated with a tendency to do most of the panel pressing (Figs . 2, 4, 6) . Dominant (a) pigs also tended to obtain more food (Figs . 3, 5, 7) . The amount of aggression remained low even in the experiments (experiment 3) in which pairs of pigs only met in the test room (Fig. 7) . The tendency for the dominant member of each pair to do most of the panel pressing was a sensible strategy for these pigs to adopt since they were able to displace the subordinate animals from the feeder. If the subordinate pigs had done most of the panel pressing they would have been unable to displace the dominant pigs from the feeder and their panel pressing would not have been reinforced . By staying near the feeder the subordinate pigs were able to eat some of the food delivered by the dominant pigs before they were displaced and thus their behaviour was also reinforced . The experiment (experiment 2) in which only one member of the pair was taught to press the panel demonstrated that the untrained pigs did not learn to press the panel by watching the trained pigs . Learning by observation has been reported in monkeys (Darby & Riopelle 1959), cats (John et al . 1968) and rats (GrinbergZylberbaum et al . 1974), but it is possible that, as the pigs were hungry and in a competitive situation, they were in too high a state of arousal for learning to readily occur (Hebb 1955). Experiments in which pairs of rats, with both members previously trained to bar press in order to obtain food, were tested together, revealed that one member of the pair sometimes did all the bar pressing (Littman et al . 1954 ; Baron & Littman 1961) . However, this stable behaviour in which one rat was the `worker' and the other the `dependent' was only observed in one pair out of nine and in the remaining eight pairs extinction of the lever pressing response occurred . In other experiments, Baron & Littman (1961) found that increasing the degree of food
955
deprivation in the pairs of rats increased the occurrence of stable `worker-dependent' relations . Baron & Littman (1961) carried out an extensive series of experiments to determine the factors influencing the outcome when pairs of trained rats were tested together . In well established `worker-dependent' pairs, they observed, as we have seen in pigs, that the `dependent' animal tends to stay near the feeder, and eats some of the delivered food but enough is left for the `worker' to obtain reinforcement for bar pressing . Thus both animals are having their behaviour reinforced by obtaining food . From the description of the rat experiments it would appear that the formation of stable `worker-dependent' relations occurs much more readily in pigs than in rats but it must be noted that the details of the training procedures and the layout of the testing chambers could influence the results obtained. In our experiments, using young pigs, we have found the 'workerdependent' relation to be a readily reproducible effect and one which is clearly influenced by the dominance relations between the animals involved . In marked contrast to the relatively clear and consistent results obtained when pairs of pigs were tested together (experiments 1, 2 and 3), the results of experiments 4 and 5 in which groups of 3 pigs were tested together were more variable . The most obvious difference was that, in the experiments involving three pigs, there was no evidence that dominant animals did most of the panel pressing (Tables II and III) . In experiment 4a, in which three hungry pigs were tested together for 10 min periods, there was no consistent tendency for any of the pigs to do most of the panel pressing (Table II) . However, when the a or R pigs were satiated and presumably had little motivation to feed or press the panel the animal of highest rank in the remain-ing pair tended to press more often (Table II) . Thus, with the a or R satiated in a trio the outcome was similar to that seen when pairs of pigs were used . This result was not seen when the y pig was satiated as either the a or R pigs then pressed more often . It was apparent in these experiments that the possible combinations of social interactions between three pigs are much more complex than those between pairs of animals . In experiment 5, in which three hungry pigs were tested together for 3 h periods, it clearly emerged that one pig did almost all the panel
956
ANIMAL BEHAVIOUR, 27,
pressing and this tendency was unrelated to social rank (Table III) . Complex social interactions can occur when three pigs are together and it was usually possible for a subordinate `worker' pig to eat some of the delivered food while the other pigs were fighting or otherwise distracted . The above results confirm and extend some of the previous work in which groups of three rats were tested together (Mowrer 1940 ; Box 1967, 1969) and one of the rats tended to do all the bar pressing and produce food for the group . The factors influencing the behaviour of rats in what Box (1967) terms a `social problem' have been extensively investigated (Mowrer 1940 ; Littman et al . 1954 ; Baron & Littman 1961 ; Box, 1967, 1969, 1970) and more work on pigs is necessary before detailed comparison of the behaviour of the two species in `social problems' can be made . In many of the present experiments when the pairs or trios of pigs were tested together, seemingly `irrelevant' behaviour was sometimes seen, for example, when the pigs ceased attempting to obtain food and started rubbing themselves on the walls. Pigs normally like to scratch themselves by rubbing against upright objects and this may be considered part of their grooming behaviour (Signoret et al . 1975) . In our experiments it may have been a `displacement activity' as grooming is often considered to be such an activity in animals subjected to conflict situations (see Hinde 1970) The experimental tests contained the elements of an approachavoidance conflict in which the tendency of the hungry pigs to approach the food would conflict with fear of the aggression likely to be encountered at the feeder . Frustration can also increase the probability of `displacement activity' occurring (McFarland 1966) and in the present experiments the hungry pigs may have been frustrated by the periodic delivery of only small amounts of food . Although no experimental work on the factors influencing `displacement activity' in pigs has been carried out, it seems probable that, in our experiments both conflict and frustration could occur and make `displacement activities' more likely to appear . One outstanding feature of the whole series of experiments, in which pairs or trios of pigs were tested together, was the low level of aggression which indicates that the social organization in pigs is adaptive enough to reduce aggression and optimize food intake in competitive situations of this type . The present experiments dem-
3
onstrate that operant methods promise to be useful in the study of the factors influencing dominance relations in pigs . Acknowledgments One of the authors (G.B .M.) wishes to thank the Wellcome Trust/Animal Health Trust for financial support. We also wish to thank Mr D . E. Walters of the ARC Statistics Group, University of Cambridge, for analysing the results and Miss K. Brotby for technical assistance . REFERENCES Baldwin, B . A . & Stephens, D . B. 1973 . The effect of conditioned behaviour and environmental factors on plasma corticosteroid levels in pigs . Physiol. Behav., 10, 267-274. Baldwin, B . A . & Ingram, D . L . 1967 . Behavioural thermoregulation in pigs . Physiol. Behav., 2,15-21 . Baron, A. & Littman, R. A . 1961 . Studies of individual and paired interactional problem solving behaviour of rats : II Solitary and social controls . Genetic Psychology Monographs, 64, 129-209 . Beilharz, R . G . & Cox, D . F . 1967. Social dominance in swine . Anim. Behav .,15,117-122. Box, H . O . 1967 . Social organisation of Rats in a "Social Problem" situation. Nature, Lond., 213, 533-534 . Box, H . O . 1969 . The influence of specific group membership upon individual performance in a "Social Problem" for rats . Psychon. Sci., 14 (1), 39-40. Box, H . O. 1970 . Experimental manipulation of individual performance within groups of rats engaged in a `social problem' . Psychol. Reports, 26, 219-225 . Box, H. O . 1973 . Organisation in Animal Communities . London : Butterworths . Bryant, M . J. 1970. The influence of population density and group size upon the behaviour of the growing pig . Ph .D . Thesis, Faculty of Veterinary Science, University of Liverpool . Darby, C. L. & Riopelle, A . J . 1959 . Observational learning in the Rhesus Monkey . J. comp . physiol. Psychol., 52, 94-97 . Dimond, S . 1970 . The Social Behaviour of Animals . London : Batsford. Donald, H. P. 1937 . Suckling and suckling preference in pigs . Emp. J. exp. Agric., 5, 361-368 . Ewer, R. F . 1968 . Ethology of Mammals . London : Logos Press. Fraser, D . 1975 . The `teat order' of suckling pigs . II Fighting during suckling and the effects of clipping the eye teeth. J. agric. Sci., Camb ., 84, 393-399 . Grinberg-Zylberbaum, J., Carranza, M . B., Cepeda, G . V ., Vale, T. C . & Steinberg, N. N . 1974. Caudate nucleus stimulation impairs the process of perceptual integration . Physiol. Behav., 12, 913-918 . Hebb, D . O. 1955 . Drive and the CNS (conceptual nervous system). Psycholog. Rev ., 62, 243-254 . Hinde, R . A . 1970 . Animal Behaviour. A Synthesis of Ethology and Comparative Psychology. 2nd edn, Chapt. 17. London : McGraw-Hill . John, E. R. Chester, P., Bartlett, F . & Victor, I . 1968 . Observation learning in cats . Science, N. Y., 159, 1489-1491 .
BALDWIN & MEESE : SOCIAL BEHAVIOUR AND OPERANT CONDITIONING IN PIGS Littman, R . A ., Lanski, L. M . & Rhine, R. J . 1954 . Studies of individual and paired interactional problem solving behaviour of rats (Norvegicus Albinos). Behaviour, 7,189-206. McBride, G. 1963 . The `teat order' and communications in young pigs . Anim. Behav., 11, 53-56 . McBride, G ., James, J. & Hodgens, N . 1964. Social behaviour of domestic animals. IV Growing pigs . Anim. Prod., 6,129-140. McFarland, D . J . 1966. The role of attention in the disinhibition of displacement activities . Q . J. exp . Psychol., 18,19-30 . Meese, G. B . & Ewbank, R. 1973a . The extablishment and nature of the dominance hierarchy in the domesticated pig. Anim . Behav ., 21, 326-334 .
957
Meese, G . B . & Ewbank, R . 1973b . Exploratory behaviour and leadership in the domesticated pig. Br . vet . J.,129, 251-259. Mowrer, O . H . 1940 . Animal studies in the genesis of personality. Trans . N. Y. Acad. Sci., 3, 8-11 . Mowrer, O . H . 1960. Learning, Theory and the Symbolic Processes . New York : Wiley. Signoret, J . P ., Baldwin, B . A., Fraser, D. & Hafez, E . S . E. 1975 . The behaviour of swine . In : The Behaviour of Domestic Animals (Ed . by E. S . E . Hafez) . London : Bailliere Tindall . (Received 26 January 1978 ; revised 18 September 1978 ; MS. number : 1716)
SOCIAL BEHAVIOUR IN PIGS STUDIED BY MEANS OF OPERANT CONDITIONING By B . A . BALDWIN & G . B . MEESE ARC Institute of Animal Physiology, Babraham, Cambridge Abstract. Pigs were trained to obtain food by pressing a panel with their snouts . Pairs or trios of animals were tested together with the panel widely separated from the feeding bowl and response rates were examined in relation to social dominance . Dominant animals of the pairs usually did almost all the panel pressing whether the pair lived together or apart . In pairs tested together with only one member trained, the untrained pigs did not learn to respond . In trios tested for 10 min, the dominant pigs did not press most but, if one animal was satiated before testing, the dominant pigs in the remaining pairs tended to press more . In trios tested for 3 h one pig pressed most and this was unrelated to rank . Social behaviour, which can conveniently be defined as behaviour `whose principal function is to influence the behaviour of a conspecific' (Ewer 1968) has usually been studied by observation of animals in natural or relatively unconfined conditions . However, following the initial observations of Mowrer (1940, 1960), using rats, a limited number of experiments have been made to study social interactions among small groups of animals which could obtain food by performing operant responses . The possible advantages of such an approach, which permits the experimenter to control many aspects of the animal's environment, have been discussed by Dimond (1970) and Box (1973) . The usual type of experiment, using rats, has been to arrange an operant conditioning task in which the food tray and the lever are placed at opposite ends of the apparatus and pairs of rats, or groups of up to five animals, are placed in the apparatus and their behaviour observed . Mowrer (1940, 1960) observed what he called a `division of labour' in groups of three rats in that one of the animals did most of the lever pressing and produced food for all the rats . Littman et al . (1954) mentioned a similar phenomenon and used the terms `worker' and `dependent' . A more detailed investigation of the factors such as age of the animals used, size of groups, training procedures, degree of food deprivation, which could influence the behaviour of rats in experiments similar to those mentioned above, have been carried out by Baron & Littman (1961) and Box (1967, 1969, 1970) . The following types of social organization have been described in domesticated pigs . (1) The teat order, which is established among the litter when the piglets compete for the preferred anterior teats (Donald 1937 ; McBride 1963 ;
Fraser 1975) . (2) The dominance order which is most evident after weaning (McBride et al . 1964 ; Beilharze & Cox 1967 ; Bryant 1970 ; Meese & Ewbank 1973a) . (3) Leadership and follower relations (Meese & Ewbank 1973b .) A general review of behaviour in pigs has recently been published (Signoret et al . 1975) . It has been established that groups of pigs have a well defined social organization characterized by a dominance order which is usually considered to be `bidirectional' in view of the fact that subordinate animals frequently retaliate when attacked by dominant pigs (Beilharze & Cox 1967 ; Bryant 1970) although in a well established social group, communication of threat or submission may be formalized and submission may simply be indicated by a lowering of the direction of the eyes (McBride et al . 1964) . The usual experimental determination of a pig's position in the social hierarchy has depended upon the outcome of agonistic encounters to decide which pig takes precedence in a competitive situation in which food is offered . Competitive tests for rewards other than food do not appear to have been used . In the experiments reported here we have observed the behaviour of pigs in operant situations similar to some of those previously used with rats . Pigs make good experimental animals for operant conditioning and will soon learn to press panels with their snouts in order to obtain food (Baldwin & Stephens 1973) or, when in a cold environment, infra-red heat (Baldwin & Ingram 1967) . They are also bold animals and not easily disturbed by the presence of observers and, when starved, are very highly motivated to obtain food . In the present experiments we have used a competitive operant feeding situation for the 947
948
ANIMAL BEHAVIOUR, 27, 3
following : (1) in order to establish whether the operant method would prove useful in the study of social behaviour in the pig, particularly in relation to the dominance order . (2) To compare the results obtained using pigs with some of those previously found in rats in order to evaluate the degree of species specificity in the observed behaviour . (3) To ascertain the effect of factors such as social rank, degree of food deprivation, amount of pretraining and previous social history on the performance of pigs in the operant situation. General Methods A total of 64 pigs was used . They were all young females or young castrated males of the large white breed and they weighed approximately 20 to 40 kg . Except for any experimental procedure such as 24 h without food the animals were maintained under normal husbandry conditions and fed on a commercial pelleted diet at up to 4 . 5 % body weight per day . All groups were made up of pigs of approximately the same size and weight . When necessary, the pigs were taught in a training cage to push a panel with their snouts in order to obtain a small reward of the pelleted food . The behavioural observations were made in a room measuring approximately 2 . 8 m by 1 . 8 m (Fig 1) and fitted with a double `stable'
door . The lower door was I m high so that the observer could watch the pig's behaviour over the open upper door . The panel was identical to that used in the training cage and a small amount of pelleted food (the animals' normal diet) was delivered into the feeding bowl following a single press of the panel . The observers' notes and tape-recorded commentary were sometimes supplemented by a closed circuit TV system connected to a video-tape recorder . Experiment 1 : Effects of 24 h, 48 h without Food on Pairs of Pigs, Living Together and Both Taught to Press (Figs 1, 2, 3, 4, 5) This experiment was carried out in order to establish the relation between social rank and panel pressing and also to evaluate the effects of increasing food deprivation . w b
a f 100
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a N
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Fig . 2. Results from experiment 1, pairs of pigs living together . The number of panel presses made during a 15 min period by the dominant and subordinate animals after 24 h without food when tested separately (a) and together (b). Results based on the mean of 10 observations. The symbols for the dominant and subordinate animals apply to all figures .
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Fig. 3 . Results from experiment 1, pairs of pigs living together . The time (s) spent feeding and the time spent in aggression during a 15 min period by the dominant and subordinate animals after 24 h without food when tested separately (a) and together (b) . Results based on the mean of 10 observations .
BALDWIN & MEESE : SOCIAL BEHAVIOUR AND OPERANT CONDITIONING IN PIGS
Methods Ten pairs of previously unacquainted pigs were used. The pairs of pigs lived together and were taught separately to press the panel in the training cage . The animals were then introduced singly to the experimental room and were taught to press the panel and receive a reward in the bowl fixed near the far wall (Fig . 1). When both pigs were proficient and rapidly pressing the panel and obtaining food, their individual baseline performance was determined in the following way . The pigs were deprived of food for 24 h and then introduced singly to the room where they were observed for 15 min. At the end of each observation the animals received the remainder of their maintenance rations. During this period a record was kept of the number of presses made at the panel and the time spent feeding at the bowl . These baseline observations were made, in
949
this and subsequent experiments, in order to establish that all the pigs could press the panel and feed at similar rates . A total of 10 observations were made on each animal . At the end of these baseline determinations the two pigs were simultaneously placed in the test room and a further 10 x 15 min observations were made. During these periods the number of presses made by each animal and the time spent feeding was recorded. A record was also kept of the duration of all the aggression observed with a minimum of 1 s assumed for all aggressive incidents . At the end of the 10 observations baseline determinations were repeated but after 48 h without food . The tests in which the two pigs were together were repeated with both pigs deprived of food for 48 h . At the completion of all the observations the relative dominance was determined for the two animals in the following way . Both pigs were deprived of food for 24 h and a bowl containing approximately 500 g of food was placed in a neutral pen to which the two animals were introduced . They were observed for up to 3 min and the time spent feeding by each animal, or both together, was recorded . A total of 10 such observations was made . Dominance was calculated by the proportion (between 0 and 1) of the total time that each animal spent feeding singly at the bowl . The above procedure was followed in all cases of dominance determinations .
TESTS
Fig. 4 . Results from experiment 1, pairs of pigs living together . The number of panel presses made during a 15 min period by the dominant and subordinate animals after 48 h without food when tested separately (a) and together (b) . Results based on the means of 10 observations . K' I,
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Fig. 5 . Results from experiment 1, pairs of pigs living together. The time (s) spent feeding and the time spent in aggression during a 15 min period by the dominant and subordinate animals after 48 h without food when tested separately (a) and together (b). Results based on the mean of 10 observations .
Results 24 h of food deprivation . The results obtained when the 10 different pairs of pigs were starved for 24 h are shown in Fig . 2 . It can be seen that, when tested alone, the dominant pigs tended to press the panel more often during the 15 min test period (Fig . 2a) although the difference was not statistically significant when examined by `t' test following analysis of variance . When tested together after 24 h without food, as can be seen from Fig 2b, the dominant pigs did most of the panel pressing and, after five trials, the subordinate pigs seldom pressed the panel. The time spent feeding and the time spent in aggression by the pairs of pigs after 24 h without food are shown in Fig . 3 where it can be seen that the time spent feeding, by both dominant and subordinate members of each pair, was the same when they were tested alone (Fig . 3a) . When the pigs were tested together, the dominant animals spent much more time feeding (Fig . 3b) but the amount of aggression was low and almost constant during the 15 min trials .
950
ANIMAL BEHAVIOUR,
The aggression occurred entirely at the feeding bowl and no instances were seen in which competition and aggression took place in order to obtain access to the response panel . After the first test together the aggression became ritualized, i .e. there was not much fighting, just displacement from the feeder by pushing and threats . The displaced subordinate pigs often briefly retaliated or refused to move . Several instances of `play fighting' were seen and occasionally the pigs would scratch themselves by rubbing along the walls . 48 h of food deprivation . The results obtained when the same ten pairs of pigs were tested alone, or together, after 48 h starvation are shown in Figs. 4 and 5 . As may be seen in Fig . 4a, the dominant pigs pressed the panel more often than subordinate pigs when tested alone although the difference was not statistically significant . The rate of panel pressing by both dominant and subordinate pigs after 48 h of food deprivation was higher than after 24 h without food as can be seen by comparison of Fig . 4a and Fig . 2a and it is probable that this difference is due to the longer deprivation period although it must be noted that, at the time of the 48 h tests, the pigs were more experienced in panel pressing. The results obtained when the pairs of pigs were tested together after 48 h deprivation of food are shown in Fig . 4b from which it can be seen that the dominant pigs did almost all the panel pressing and responded at a slightly higher rate than when they were tested alone . In all groups the dominant pig pressed the panel most on the first and subsequent tests . In marked contrast, the subordinate pigs hardly ever pressed the panel . The time spent feeding when the pigs were alone or together, after 48 h starvation, is shown in Fig. 5 which also displays the amount of aggression that occurred . It can be seen that, when tested alone, the time spent feeding was about the same as after 24 h without food (Figs. 3a and 5a), but as the response rate was considerably higher (Figs . 4a and 2a) and hence more food was delivered, the rate of ingestion was higher in the pigs after 48 h starvation . When tested together after 48 h without food the dominant pigs spent much more time feeding than the subordinates (Fig. 5b) . The amount of aggression was low and constant throughout the 10 tests and was about the same level as seen after 24 h deprivation of food (Fig . 5b) . All the aggression occurred at the feeding bowl and
27, 3
there was no fighting to gain access to the response panel . Experiment 2 ; Effects of 24 h of Food Deprivation on Pairs of Pigs, Living Together with one only of Each Pair Taught to Press (Table I) This experiment was done in order to establish whether the untrained members of the pairs would learn to respond by observing the behaviour of the trained pigs . Method Six pairs of previously unacquainted pigs were used. The animals lived together in pairs and only one of each pair was taught to press the panel . Following 24 h without food a baseline was determined for the animals which could press the panel ; a total of 10 x 5 min observations were made on separate occasions . At the end of these observations a trained and an untrained pig were simultaneously introduced to the experimental situation and 10 x 5 min observations were made. After the paired tests, a repeat was made of the baseline determinations using both the trained and untrained pigs . Finally, the relative dominance values were determined using the method previously described . Of the six animals trained to press the panel, four were the dominant of their pair, one was the subordinate and one was of approximately equal rank . Results The results are presented in Table I from which it can be seen that the untrained pigs did not learn to press the panel . No vigorous aggression was seen at any stage, and the total amount of aggression was low as even the most aggressive pairs did not exceed an average of 8 % of the available time in aggressive activity . Almost all the aggression occurred near the food bowl . In one group (B) panel pressing almost ceased after four runs ; they began to respond again before the end of the series of trials but both pigs spent a considerable time nibbling and scratching on the walls . In group C the trained pig took six trials before it started to press the panel and during this time both pigs moved around the pen rooting in the sawdust and emitting agitated grunts . The trained pig, which was the dominant animal (Table I) often attempted to escape by trying to leap over the door . As can be seen in Table I the trained pigs were dominant in groups C, D, E and F . The untrained pig was clearly dominant in group B
BALDWIN & MEESE : SOCIAL BEHAVIOUR AND OPERANT CONDITIONING IN PIGS V
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and of almost equal rank in group A . In group B the dominant untrained pig spent more time feeding. When the trials were over the pigs were retested alone and it can be seen from Table I that, despite the tests in which they had an opportunity to observe the trained pigs pressing the panels, none of the untrained animals had learned to press the panels. Experiment 3: Effects of 24 h of Food Deprivation on Pairs of Pigs, Living Separately and both Taught to Press (Figs . 6 and 7) This experiment was carried out in order to investigate the effect of social rank on panel pressing in previously unacquainted pigs . Method Four pairs of previously unacquainted pigs were used . Both animals of each pair were taught to press but the pigs were kept in separate pens and never met except in the experimental situation . The previously described observations were made (in all cases after 24 h starvation) i .e . 10 x 5 min baseline determinations on both pigs separately, 10 x 5 min observations with the two pigs together, followed by repetition of the baseline observations . The relative dominance was determined between the two pigs after the tests using the competitive feeding procedure . Results The results obtained are shown in Figs . 6 and 7. Figure 6 displays the number of responses made by the dominant and subordinate pigs
when tested separately (Fig . 6a) or together (Fig. 6b) and also when they were tested separately after completing the tests in which they were together (Fig . 6c) . It can be seen that the dominant pigs tended to press the panel at a more rapid rate than did subordinates even when they were tested alone . However, this difference was mainly due to one of the pairs in which the dominant animal pressed much faster than the subordinate pig . When they were tested together (Fig . 6b) it can be seen that, after the first two tests, the dominant pigs did almost all the panel pressing . In three groups of the four, the a pig did most pressing in the first test and usually was the sole presser in subsequent tests . In the remaining group the a and (3 pressed about an equal amount although the a was clearly dominant in the subsequent food competition tests . It can be seen from Fig . 7, which displays the time spent feeding by the dominant and subordinate pigs when tested alone (Fig . 7a), together (Fig . 7b) or separately again after the paired tests (Fig. 7c) that when the two pigs were tested together the dominant pigs spent much more time feeding than the subordinates . The amount of aggression seen was low and relatively constant after the initial encounter . It was obvious that one pig usually became the main panel presser in this situation . In the subsequent competitive feeding (dominance) tests it was clear that the animals which did most panel pressing were the dominant pigs .
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Fig. 7 . Results from experiment 3, pairs of pigs, previously unacquainted before meeting in the experimental situation . The time (s) spent feeding and the time spent in aggression during a 5 min period by the dominant and subordinate animals after 24 h without food when tested separately (a) together (b) and also tested separately again (c) . Results based upon the means of four observation . Dominance relationships were determined at the end of the observations .
BALDWIN & MEESE : SOCIAL BEHAVIOUR AND OPERANT CONDITIONING IN PIGS
Experiment 4 : (a) Effects of 24 h Food Deprivation on three Pigs, Living Together and all Taught to Press (Table II) This experiment was carried out in order to see whether altering the group size to three pigs would yield different results from those obtained with pairs of pigs . Method Four groups of three previously unacquainted pigs were used . The groups of three pigs lived together and were all taught to panel press . In order to check that all the pigs had learned to use the panel, baseline values were obtained from 5 x 5 min observations on each pig tested alone . After 24 h deprivation of food the three pigs were placed together in the experimental pen and 5 x 10 min observations were made on separate occasions . Dominance tests were carried out before the experimental runs as follows . After a period of 24 h without food each pig in turn competed in a 3 min food competition test with one of the other pigs . These combinations were repeated five times and a dominance value determined based on the amount of time spent by each animal eating alone during the competition . Results The results are shown in Table II which summarizes a statistical analysis of the data (ANOVA) in terms of which pig did more pressing or spent longer feeding. As can be seen from the table, when none of the pigs was satiated, there was no tendency for the dominant a pigs to press the panel significantly more often than the other pigs .
In terms of time spent feeding, no clear result emerged except that the y pigs usually spent less time feeding than the other pigs . The amount of aggression was low with an overall mean for all groups of 36 ± 9 s out of a possible 600 s . During the test occasional short periods occurred in which the pigs would rub themselves against the walls of the pen . Experiment 4 : (b) Effects within Groups of three Pigs of 24 h Food Deprivation of two Animals and Satiation of the Third, Pigs Living Together and all Taught to Press (Table II) This experiment was carried out to establish whether satiation of one member of the group would produce results similar to those seen when pairs of pigs were used . Method The same pigs were used as in the previous experiment (4a) and the procedure with the three pigs together was repeated on five occasions after one of the pigs had been fed to satiation in the experimental pen . The remaining two pigs had undergone the usual 24 h food deprivation . The satiated pig (not eating in the presence of food) received food from the experimenter and not by pressing the panel. This was repeated with the a, (3 and y animals until each of the pigs had been used . At the end of the experiments the dominance values were again determined in order to see whether any marked changes in relative dominance had occurred during the experiments . In none of the groups had the position of the a pigs changed but in one group the (3 and 7 pigs exchanged places . All the
Table II Group and activity Panel pressing
Feeding
Which pigs were satiated None A B C D
y > (3 R>a .y .
A B C D
a.D>y a. R > y R > a .y a.y>13
NS NS
a /3 .y > a 0 > a .y . 13 > a .y. f3>a .y . (3.y>a NS p.y > a a .y>(3
S NS
Q>0>7 a > 13.y. a>R>y a.y > R a .y>0 a.y>R a .y>R
y (3 > a .-I . a > 13 .y . S > a .y. a .R>y a.0 > y a.s>y a.0>y a>y
No significant difference between a, (3 and y pigs at 5 % level or less . a pig differs significantly from 0 and the y lies between a and (3 but not significantly different from either . a > 0 > y : All three pigs differ significantly. NS :
a > R:
953
954
ANIMAL BEHAVIOUR, 27,
analysis in Table II is based on the first set of dominance values . Results These are presented in Table II from which it can be seen that, when the a pig was satiated, the R pigs did most of the panel pressing . When the (3 pigs were satiated the a pigs pressed significantly more often in three of the groups . When the y pigs were satiated the results were more variable with no tendency for the a pigs to do most of the panel pressing . In terms of time spent feeding the two hungry pigs usually did not significantly differ from each other . The overall level of aggression was low with a mean value of 40 ± 4 s out of 600 s . During these experiments `play fighting' and chasing around the pen sometimes occurred and occasionally the pigs would rub themselves against the walls or nibble at the walls . Experiment 5 : Effects of 24 h without Food on Groups of Three pigs, Living Together all Taught to Press and Allowed to Satiate in the Test Situation (Table III) This experiment was done in order to establish whether stable `worker-dependent' relations developed during prolonged tests using three pigs . Method Four groups of three previously unacquainted pigs were used . The groups of three pigs lived together and were all taught to press the panel
3
as described previously . Baseline values were obtained from 5 x 5 min observations made on each pig alone in the test room. In the main series of tests, after 24 h fasting, the three pigs were simultaneously introduced to the experimental pen and the group observed continuously for 3 h in order to record the number of panel presses made by each pig during the session . The procedure was followed on five occasions with each group . Dominance relationships between the three animals were determined before and after the experimental tests using the competitive feeding test, and it was found that in none of the groups had the position of the a pigs changed but in one group the y and 3 exchanged places (Tables III, group B) . Results The results for all groups are summarized in Table III from which it can be seen that initially, when tested alone, the a, R and y pigs all pressed the panel and ate the food at approximately similar rates . When the three pigs were tested together it clearly emerged that one animal tended to do almost all the panel pressing but this tendency was unrelated to social rank . During the tests all the pigs were able to obtain sufficient food because often, while two pigs were fighting or playing near the feeder the third animal would feed . Sometimes all three pigs would feed together . Aggressive behaviour was more frequent during the first parts of the session when
Table III Pre-test baseline Pigs
(responses in 5 min) a (i
Pre-test baseline Y
a
(Time feeding in s) P
0 .34 0 . 17 20±1 .0 33±1 .0 201 ± 12 0 . 27 0 .27
Three together for 3 h Y
a
(responses in 3 h) S
7 ± 3
10 ± 7
Group A (3 pigs)
0 . 491 18±2 . 0 0 . 462
Group B (3 pigs)
0 . 40 0 . 31 24 ± 3 . 0 20 ± 1 . 0 0 . 18 0 . 59
0 .28 18 ± 1 .0 022
218 ± 5
198 ± 10 194 ± 6
GroupC (3 pigs)
0 . 42 16±1 0 .45
0 . 41 16±2 0 . 35
0 . 21 21±1 0 . 19
216±6
229±8
GroupD (3 pigs)
0 .43 31±2 0 .46
0 . 38 37±3 0 . 31
0 . 18 25±1 0 . 31
211±11 225±12 220±6 _536±112
Dominance relations before tests . 2 Dominance relations after tests . 1
206 ± 8
222 ± 10
212±2
37 ± 34 719 ± 128
23±17
y
728 ± 75
9 ± 6
4±3
569±70
13±11
92±66
BALDWIN & MEESE : SOCIAL BEHAVIOUR AND OPERANT CONDITIONING IN PIGS
all the pigs were hungry and always took place near the feeder . As in previous experiments the pigs sometimes displayed `irrelevant' behaviour such as rubbing against the walls . Towards the end of the 3 h session the pigs often rested and sometimes slept. Discussion The results of experiments 1 and 3, in which pairs of trained pigs were tested, clearly showed that dominance in the food competition tests was strongly associated with a tendency to do most of the panel pressing (Figs . 2, 4, 6) . Dominant (a) pigs also tended to obtain more food (Figs . 3, 5, 7) . The amount of aggression remained low even in the experiments (experiment 3) in which pairs of pigs only met in the test room (Fig. 7) . The tendency for the dominant member of each pair to do most of the panel pressing was a sensible strategy for these pigs to adopt since they were able to displace the subordinate animals from the feeder. If the subordinate pigs had done most of the panel pressing they would have been unable to displace the dominant pigs from the feeder and their panel pressing would not have been reinforced . By staying near the feeder the subordinate pigs were able to eat some of the food delivered by the dominant pigs before they were displaced and thus their behaviour was also reinforced . The experiment (experiment 2) in which only one member of the pair was taught to press the panel demonstrated that the untrained pigs did not learn to press the panel by watching the trained pigs . Learning by observation has been reported in monkeys (Darby & Riopelle 1959), cats (John et al . 1968) and rats (GrinbergZylberbaum et al . 1974), but it is possible that, as the pigs were hungry and in a competitive situation, they were in too high a state of arousal for learning to readily occur (Hebb 1955). Experiments in which pairs of rats, with both members previously trained to bar press in order to obtain food, were tested together, revealed that one member of the pair sometimes did all the bar pressing (Littman et al . 1954 ; Baron & Littman 1961) . However, this stable behaviour in which one rat was the `worker' and the other the `dependent' was only observed in one pair out of nine and in the remaining eight pairs extinction of the lever pressing response occurred . In other experiments, Baron & Littman (1961) found that increasing the degree of food
955
deprivation in the pairs of rats increased the occurrence of stable `worker-dependent' relations . Baron & Littman (1961) carried out an extensive series of experiments to determine the factors influencing the outcome when pairs of trained rats were tested together . In well established `worker-dependent' pairs, they observed, as we have seen in pigs, that the `dependent' animal tends to stay near the feeder, and eats some of the delivered food but enough is left for the `worker' to obtain reinforcement for bar pressing . Thus both animals are having their behaviour reinforced by obtaining food . From the description of the rat experiments it would appear that the formation of stable `worker-dependent' relations occurs much more readily in pigs than in rats but it must be noted that the details of the training procedures and the layout of the testing chambers could influence the results obtained. In our experiments, using young pigs, we have found the 'workerdependent' relation to be a readily reproducible effect and one which is clearly influenced by the dominance relations between the animals involved . In marked contrast to the relatively clear and consistent results obtained when pairs of pigs were tested together (experiments 1, 2 and 3), the results of experiments 4 and 5 in which groups of 3 pigs were tested together were more variable . The most obvious difference was that, in the experiments involving three pigs, there was no evidence that dominant animals did most of the panel pressing (Tables II and III) . In experiment 4a, in which three hungry pigs were tested together for 10 min periods, there was no consistent tendency for any of the pigs to do most of the panel pressing (Table II) . However, when the a or R pigs were satiated and presumably had little motivation to feed or press the panel the animal of highest rank in the remain-ing pair tended to press more often (Table II) . Thus, with the a or R satiated in a trio the outcome was similar to that seen when pairs of pigs were used . This result was not seen when the y pig was satiated as either the a or R pigs then pressed more often . It was apparent in these experiments that the possible combinations of social interactions between three pigs are much more complex than those between pairs of animals . In experiment 5, in which three hungry pigs were tested together for 3 h periods, it clearly emerged that one pig did almost all the panel
956
ANIMAL BEHAVIOUR, 27,
pressing and this tendency was unrelated to social rank (Table III) . Complex social interactions can occur when three pigs are together and it was usually possible for a subordinate `worker' pig to eat some of the delivered food while the other pigs were fighting or otherwise distracted . The above results confirm and extend some of the previous work in which groups of three rats were tested together (Mowrer 1940 ; Box 1967, 1969) and one of the rats tended to do all the bar pressing and produce food for the group . The factors influencing the behaviour of rats in what Box (1967) terms a `social problem' have been extensively investigated (Mowrer 1940 ; Littman et al . 1954 ; Baron & Littman 1961 ; Box, 1967, 1969, 1970) and more work on pigs is necessary before detailed comparison of the behaviour of the two species in `social problems' can be made . In many of the present experiments when the pairs or trios of pigs were tested together, seemingly `irrelevant' behaviour was sometimes seen, for example, when the pigs ceased attempting to obtain food and started rubbing themselves on the walls. Pigs normally like to scratch themselves by rubbing against upright objects and this may be considered part of their grooming behaviour (Signoret et al . 1975) . In our experiments it may have been a `displacement activity' as grooming is often considered to be such an activity in animals subjected to conflict situations (see Hinde 1970) The experimental tests contained the elements of an approachavoidance conflict in which the tendency of the hungry pigs to approach the food would conflict with fear of the aggression likely to be encountered at the feeder . Frustration can also increase the probability of `displacement activity' occurring (McFarland 1966) and in the present experiments the hungry pigs may have been frustrated by the periodic delivery of only small amounts of food . Although no experimental work on the factors influencing `displacement activity' in pigs has been carried out, it seems probable that, in our experiments both conflict and frustration could occur and make `displacement activities' more likely to appear . One outstanding feature of the whole series of experiments, in which pairs or trios of pigs were tested together, was the low level of aggression which indicates that the social organization in pigs is adaptive enough to reduce aggression and optimize food intake in competitive situations of this type . The present experiments dem-
3
onstrate that operant methods promise to be useful in the study of the factors influencing dominance relations in pigs . Acknowledgments One of the authors (G.B .M.) wishes to thank the Wellcome Trust/Animal Health Trust for financial support. We also wish to thank Mr D . E. Walters of the ARC Statistics Group, University of Cambridge, for analysing the results and Miss K. Brotby for technical assistance . REFERENCES Baldwin, B . A . & Stephens, D . B. 1973 . The effect of conditioned behaviour and environmental factors on plasma corticosteroid levels in pigs . Physiol. Behav., 10, 267-274. Baldwin, B . A . & Ingram, D . L . 1967 . Behavioural thermoregulation in pigs . Physiol. Behav., 2,15-21 . Baron, A. & Littman, R. A . 1961 . Studies of individual and paired interactional problem solving behaviour of rats : II Solitary and social controls . Genetic Psychology Monographs, 64, 129-209 . Beilharz, R . G . & Cox, D . F . 1967. Social dominance in swine . Anim. Behav .,15,117-122. Box, H . O . 1967 . Social organisation of Rats in a "Social Problem" situation. Nature, Lond., 213, 533-534 . Box, H . O . 1969 . The influence of specific group membership upon individual performance in a "Social Problem" for rats . Psychon. Sci., 14 (1), 39-40. Box, H . O. 1970 . Experimental manipulation of individual performance within groups of rats engaged in a `social problem' . Psychol. Reports, 26, 219-225 . Box, H. O . 1973 . Organisation in Animal Communities . London : Butterworths . Bryant, M . J. 1970. The influence of population density and group size upon the behaviour of the growing pig . Ph .D . Thesis, Faculty of Veterinary Science, University of Liverpool . Darby, C. L. & Riopelle, A . J . 1959 . Observational learning in the Rhesus Monkey . J. comp . physiol. Psychol., 52, 94-97 . Dimond, S . 1970 . The Social Behaviour of Animals . London : Batsford. Donald, H. P. 1937 . Suckling and suckling preference in pigs . Emp. J. exp. Agric., 5, 361-368 . Ewer, R. F . 1968 . Ethology of Mammals . London : Logos Press. Fraser, D . 1975 . The `teat order' of suckling pigs . II Fighting during suckling and the effects of clipping the eye teeth. J. agric. Sci., Camb ., 84, 393-399 . Grinberg-Zylberbaum, J., Carranza, M . B., Cepeda, G . V ., Vale, T. C . & Steinberg, N. N . 1974. Caudate nucleus stimulation impairs the process of perceptual integration . Physiol. Behav., 12, 913-918 . Hebb, D . O. 1955 . Drive and the CNS (conceptual nervous system). Psycholog. Rev ., 62, 243-254 . Hinde, R . A . 1970 . Animal Behaviour. A Synthesis of Ethology and Comparative Psychology. 2nd edn, Chapt. 17. London : McGraw-Hill . John, E. R. Chester, P., Bartlett, F . & Victor, I . 1968 . Observation learning in cats . Science, N. Y., 159, 1489-1491 .
BALDWIN & MEESE : SOCIAL BEHAVIOUR AND OPERANT CONDITIONING IN PIGS Littman, R . A ., Lanski, L. M . & Rhine, R. J . 1954 . Studies of individual and paired interactional problem solving behaviour of rats (Norvegicus Albinos). Behaviour, 7,189-206. McBride, G. 1963 . The `teat order' and communications in young pigs . Anim. Behav., 11, 53-56 . McBride, G ., James, J. & Hodgens, N . 1964. Social behaviour of domestic animals. IV Growing pigs . Anim. Prod., 6,129-140. McFarland, D . J . 1966. The role of attention in the disinhibition of displacement activities . Q . J. exp . Psychol., 18,19-30 . Meese, G. B . & Ewbank, R. 1973a . The extablishment and nature of the dominance hierarchy in the domesticated pig. Anim . Behav ., 21, 326-334 .
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Meese, G . B . & Ewbank, R . 1973b . Exploratory behaviour and leadership in the domesticated pig. Br . vet . J.,129, 251-259. Mowrer, O . H . 1940 . Animal studies in the genesis of personality. Trans . N. Y. Acad. Sci., 3, 8-11 . Mowrer, O . H . 1960. Learning, Theory and the Symbolic Processes . New York : Wiley. Signoret, J . P ., Baldwin, B . A., Fraser, D. & Hafez, E . S . E. 1975 . The behaviour of swine . In : The Behaviour of Domestic Animals (Ed . by E. S . E . Hafez) . London : Bailliere Tindall . (Received 26 January 1978 ; revised 18 September 1978 ; MS. number : 1716)