Individual differences in behavioural responses of pigs exposed to non-social and social challenges

Applied Animal Behaviour Science, 30 ( 1991 ) 73-86 73 Elsevier Science Publ ishers B.V., Ams te rdam

Individual differences in behavioural responses of pigs exposed to non-social and social challenges

A.B. Lawrence 1, E.M.C. T e r l o u w I a n d A . W . I l l ius 2

1 The Scottish Agricultural College, West Mains Road, Edinburgh, EH9 3JG (Gt. Britain) ZThe University of Edinburgh, West Mains Road, Edinburgh, EH9 3JG (Gt. Britain)

(Accepted for publication 11 September 1990)

ABSTRACT

Lawrence, A.B., Terlouw, E.M.C. and Illius, A.W., 1991. Individual differences in behavioural responses of pigs exposed to non-social and social challenges. Appl. Anim. Behav. Sci., 30: 73-86.

The existence of individual differences in behaviour is a rarely considered but almost universal phenomenon. Individual differences in behaviour may reflect the existence of stable temperamental characteristics which could be used to improve understanding of behavioural mechanisms and to predict individuals' responses at different ages and in different contexts. In farm animals, the study of temperament has been largely restricted to dairy animals and to a limited range of contexts. This study examined the consistency of individual differences in responsiveness to a number of non-social and social challenges in female pigs. Two separate groups of gilts were exposed to a series of handling tests including willingness to leave the group residential pen, ease of movement through a corridor, response to a sudden human approach, resistance to restraint and vocalisation response to restraint. With the exception of vocalisation during restraint, individual scores for these tests tended to correlate and the scores for the correlated tests were therefore summed and 8 low scoring and 8 high scoring gilts selected from each group. The responses of these low and high responders to a novel object and to a group feeding test were then measured. In the novel object test, high responding gilts expressed overall a greater attention to the novel object. In the group feeding test there was no difference between low and high responders in the number of attacks won or lost but high responders won a significantly greater proportion of encounters. It was possible therefore to identify in female pigs, individuals that responded consistently to a variety of non-social challenges. In a social challenge test, however, not all elements of response were consistent with previously measured responses to non-social challenge. The variation between responses to social and non-social challenge in the present study suggests more than one underlying behavioural dimension controlling responses to challenge in female pigs.

I N T R O D U C T I O N

Studies of animal behaviour have revealed that there are often large individual differences in the performance of behaviour. Intraspecific variability has been shown in the manner in which animals feed (Slater, 1974 ) and also in maternal (Lawrence, 1980), play (French, 1981 ) and sexual behaviour (Dunbar, 1983 ). Often these individual differences are treated as a statistical

0168-1591 /91 /$03 .50 © 1991 - - Elsevier Science Publ ishers B.V.

74 A.B. LAWRENCE ET AL.

inconvenience and in pure and applied behavioural science, studies are conducted to produce a normative view ofbehaviour at the expense of measuring stable temperamental differences between individuals.

Temperament has been defined as the individual's basic stance towards environmental change and challenge (Mason, 1984) and can be assumed to be the product of as yet undefined characteristics of the individual's neural sys- tems that interact with the environment to influence behaviour. Implicit in the concept of temperament is the assumption that individuals are relatively consistent in their response to environmental challenge both across their development and in different situations and contexts. It is exactly the extent to which individuals express consistency of response that has been the subject of considerable debate (McCall, 1986 ). Whilst some studies have found that individuals' responses at one time are correlated to those at a later age (e.g. Lyons et al., 1988), others have found reorganisation and change in behaviour across development (Stevenson-Hinde et al., 1980). The study of temperament has considerable relevance to applied behaviour science. If individual differences in behaviour are a reflection of stable temperamental characteristics then behaviour in one context can be used to predict behaviour in other situations (e.g. Kerr and Wood-Gush, 1985), or used as a tool to investigate the mechanisms and processes that underlie behaviour such as the origins and functions of individual responses to stress (Dantzer, 1989).

As with other species, farm animals have been shown to express considerable individual variability in response to challenge. For example, Syme and Elphick (1982) classified sheep as being unresponsive, vocal or uncooperative to handling. Uncooperative individuals which had higher hearts rates during handling, tended to avoid human contact and be found at the back of the order. However, relatively few studies in farm animals have considered the extent to which such individual variability in response to a challenge such as handling is consistent over t ime and contexts. Those studies that have looked for stable differences between individuals have been largely restricted to species involved in dairy production, where individual responses to hus- bandry procedures are seen as potentially contributing to loss of productivity (e.g. Kilgour, 1975; Kerr and Wood-Gush, 1987; Lyons et al., 1988). For example, Lyons et al. ( 1988 ) found that individual dairy goats showed a consistent level of response to human presence and approach by a human observer over a 55-week period. Factor analysis suggested that the goats' response to humans could be largely described by a single latent variable that Lyons et al. ( 1988 ) labelled "timidity". There have been no comparable studies of temperament in pigs.

Previous studies in farm animals have also tended only to consider individuals' responses in a limited number of contexts and none have compared individual differences in non-social and social challenges. Recent work in rodents has found that individuals which respond with aggressive behaviour in

BEHAVIOURAL RESPONSES OF PIGS TO CHALLENGES 7 5

social contexts also perform better in non-social tests such as active avoidance of electric shock (Benus et al., 1990) and maze running (Benus et al., 1987). This work has important implications for applied animal behaviour as it suggests that response to a non-social challenge, for example handling, might predict subsequent behaviour in a social group such as individuals' pre- disposition to aggressive behaviour. Furthermore, there are indications in rodents that aggressive individuals may also be more likely to develop repetitive behaviour patterns such as schedule-induced polydipsia (Dantzer et al., 1988 ). This observation is of relevance to the study of the environmentally induced stereotypies that develop in animals such as closely confined sows. Consid- erable individual differences in the development of stereotypic behaviours have been reported (e.g. Appleby and Lawrence, 1987), yet the origins and consequences of these individual differences remain obscure. It might be that responses to intensive housing could be better understood by taking account of the temperamental characteristics of individuals prior to them being exposed to the housing stress.

The objective of this study was to extend the study of temperament by in- vestigating individual differences in behavioural responsiveness in pigs. Spe- cifically the study aimed to explore the consistency of behavioural responses between social and non-social contexts. The objective was also to compare responses in non-social tests that did or did not involve human presence and handling. We report that individual female pigs show consistent patterns of response to a series of handling tests and to a novel object test, but that their responses to a social food competi t ion test is not consistent with these previous non-social challenges. This work is part of a larger project to determine the extent to which pigs' responses to intensive housing such as stereotypic behaviour, are determined by predisposing temperamental characteristics that influence the capacity of the individual to respond to stressors.

ANIMALS, MATERIALS AND METHODS

Animals and housing

The subjects were two separate groups (Groups A and B; n = 2 6 and 36, respectively) of modern hybrid (Landrace and Large White) non-pregnant nulliparous females (gilts) reared by the Cotswold Breeding Company, Lin- coln, U.K. Initially, the gilts were subjected to a number of handling tests at the rearing unit to select gilts for subsequent experimentation. Following these initial tests, the selected gilts were moved to an experimental unit where all subsequent experimentation was conducted. The gilts were maintained on the rearing unit in groups in large pens bedded with straw (Fig. 1 ). They had ad libitum access to food and water. At the experimental unit the gilts were also group-housed in strawed pens (Fig. 2 ). Except during experimentation, the

76 A.B. LAWRENCE ETAL.

Collection pen 2

Gilts pen

12

# a 5

Collection pen 1

~Corridor

411 7ro

Fig. 1. Spatial arrangement of handling tests. Distances in metres. ( 1, Order 1; 2, Order 2; 3, Transit; 4, Threat; 5, Restraint and Vocalisation. )

PENS

1 2 3

OBS __. ~ i '~" (~) Gates Feeders

2"5 D, q 3-5 4 3 I~

Fig. 2. Spatial arrangement of the novel object test and social competition test. Distances in metres. ( 1, novel object test; Obs. l, observation point for novel object test; 2, social competition test; Obs. 2, observation points for social competition test. )

a n i m a l s h a d f r e e d o m to m o v e b e t w e e n t h e p e n s a n d e a c h w e r e o f f e r e d 4 kg d a y - l o f a s t a n d a r d c o m m e r c i a l c o n c e n t r a t e f o o d o n c e dai ly . W a t e r w a s a v a i l a b l e ad l i b i t u m .

BEHAVIOURAL RESPONSES OF PIGS TO CHALLENGES

Responses to handling

7 7

The tests of responsiveness to handling were conducted when the gilts were 7 months old, weighing ~ 100 kg. The tests were conducted sequentially over a 3-h period in the order given below. The spatial arrangement of the tests is indicated in Fig. 1. Groups A and B were tested on 2 separate days, 3 months apart.

Order from pen (Order 1) The stockworker and two assistants walked one-third of the way up the res-

idential pen where they stood for 3 min to attract the attention of the gilts. The stockworker then opened the pen gate and the gilts were allowed to move out of the pen. The gilts were divided into 4 batches dependent on their latency to leave the pen: 1: voluntarily left pen; 2: voluntarily left pen but with more hesitation; 3: animals were required to be approached from behind but without touching to leave; 4: animals were required to be driven out.

Order of presentation for tests (Order 2) Following their removal from the pen as above, animals were collected in a

pen from which they were removed individually into a corridor for further testing (Fig. 1 ). From the order of presentation for testing five equal batches were derived post hoc where Batch 1 presented themselves first for testing and Batch 5 were those gilts tested last, often having to be driven into the corridor.

Ease of transit (Transit) The gilts were moved individually down the corridor with an observer

walking behind. They were subjectively scored on their ease of progress down the corridor: 1: walked or ran down the corridor voluntarily; 2: walked easily, only hesitating to show interest; 3: hesitated but recommenced movement without human contact; 4: stopped, but showed no physical resistance to movement on human contact; 5: stopped, offered physical resistance to movement on human contact.

Response to sudden human approach (Threat) At the end of the corridor another observer, attempting to keep the atten-

tion of the gilt approached suddenly to a distance of 0.5 m stamping his foot. Gilts were subjectively scored by the observer for their response to this challenge: 1: no response; 2: gilt showed mild flinching response; 3: gilt retreated from observer; 4: gilt retreated, vocalising; 5: gilt attempted to escape.


Resistance to restraint (Restraint) A restraining noose used commercial ly to restrain conscious pigs for veter-

inary inspection, was applied to the gilt's upper jaw for a period of 60 s. The gilt's response was scored subjectively in terms of the degree and duration of its physical resistance: 1: no resistance after initial short response ( < 10 s ); 2: some resistance for a minori ty of test; 3: some resistance for a majori ty of test; 4: considerable resistance for a majori ty but not all of test; 5: considerable resistance throughout test.

Vocalisation response to restraining noose (Vocalisation) Simultaneously with the previous test, the noise level of vocalisation was

measured at a distance of 1 m using a CEL-193 Impulse and Integrating Sound Level Meter. Two measures were made simultaneously: Leq , the equivalent continuous sound level and Lmax, the maximum level of sound produced. Both measures were recorded in decibels. Animals were given a score for each measure between 1 (low vocalisation) and 5 (high vocalisation) to give a roughly normal distribution of gilts across these values. The mean of the Leq and Lmax was then taken as the individual 's vocalisation score.

Different observers were used to score blindly each test to ensure independ- ence of scoring. On the basis of the above tests, a handling response score for each individual was derived (see Statistical analysis and Results sections) and used to select 8 high and 8 low responders from both Groups A and B. These 32 gilts were subsequently moved to the experimental unit.

Novel object test

Four weeks following the arrival of the gilts at the experimental unit, the gilts' responses to a novel object were measured. The animals were held in Pen 1 and separately tested in Pen 3 (Fig. 2). Pen 2 isolated the test animal from other members of the group. The test gilt was allowed 1 min to habituate to the pen and the novel object (a plastic bin, 1 m deep by 0.5 m diameter) was lowered from the ceiling to a height of 60 cm above the floor. The bucket was chosen to provide a novel object that could be repeatedly investigated without the possibility of it being chewed upon. The test lasted for 10 min after the gilt had first observed the novel object. Instantaneous sampling (Air- mann, 1974) every 10 s was used to record occurrences of: ( 1 ) contact with the novel object; (2) attention directed towards the novel object without physical contact; (3) other activities such as exploring the pen and resting behaviour. Interest in the novel object was defined as the sum of contact and attention. Individual scores were totalled over 30-s periods to give the proportion of samples within 30 s that attention was directed to the novel object.


Group-feeding competition test

One week after the novel object test, daily food intake was reduced to 3 kg day- 1 offered at 14:00 h. On 9 successive days, starting 1 day after the altered feeding regime, a group-feeding test was conducted between 09:30 and 10:30 h. During the test the animals were held in Pens 2 and 3. A test consisted of six sessions, each lasting 5 min and starting with the placing of 1 kg of food onto the floor at a fixed point in Pen 2 (Fig. 2). This point was chosen to give feeding access to only 6 pigs simultaneously. Between Days 3 and 9, all occurrences of agonistic interactions were recorded. Gilts were scored for whether they attacked or were attacked, and the outcome. A second observer also scored the following behavioural categories every 30 s: feeding: feeding from the feeding site; trying: trying to gain access to the feeding site by pushing and/or biting other gilts; waiting: standing or rooting near the feeding site or having access to the feeding site without using it; no interest: no apparent interest in food. These scores were cumulated over tests and over days.

Statistical analysis

Spearman rank order correlation coefficients were used to determine whether individual differences in responsiveness to handling tests were consistent across the response tests. As responses across tests tended to correlate (see Results) individual scores for the tests were summed to produce a handling response score. Eight high and 8 low responders were selected from each group on the basis of this score. As high and low responders were previously selected in this way, thereby affecting the distribution of responsiveness and possibly biasing correlations, analysis of variance was used to test for differences between low and high responders in the novel object test and the group feeding test.

RESULTS

Handling response tests

There was considerable individual variability in all of the handling response tests (e.g. Group A: range in Threat and Restraint 1-4 vs. 1-5, respectively). In both Groups A and B, there was a strong tendency for individual scores across the tests to correlate (Table 1 ). The strongest correlations were with Threat which correlated with all other tests, and Restraint which correlated with all tests with the exception of Transit. Vocalisation showed little correlation with tests other than Restraint and Threat. The major difference


TABLE 1

Correlation matrix of handling tests (combined data from Groups A and B: n = 62 )

Test Test

1 2 3 4 5

Order 1 1 Order 2 2 0.22* Transit 3 0.21 0.25* Threat 4 0.22* 0.33** 0.44*** Restraint 5 0.33** 0.41"** 0.04 Vocalisation 6 0.02 0.00 0.14

0.42*** 0.31"* 0.37***

*P< 0.05; **P< 0.01; ***P< 0.001.

Frequency 10

0 10 15 20 Behavioural Response Score

Fig. 3. Frequency distribution o f handling response scores.

between Groups A and B was in Order 1 which correlated with all tests in Group A with the exception of Vocalisation but only correlated with Re- straint in Group B. Order 2 showed a similar pattern in both groups.

As individual scores for the tests tended to correlate, the correlated scores for Order 1, Order 2, Transit, Threat and Restraint were summed to give a handling response score. The distribution of handling response scores is shown in Fig. 3. On the basis of the handling response score, the 8 highest and 8 lowest scoring animals were selected. Group A low and high responders had mean handling response scores of 5.4 ( + 0.3 ) and 12.5 ( + 0.7 ), respectively. Group B low and high responders mean scores were 6.0 ( + 0 . 4 ) and 13.4 ( _+ 0.6 ), respectively.

Novel object test

The general response of animals to the novel object was to express interest initially by looking at it and approaching, followed by nosing and attempted

BEHAVIOURAL RESPONSES OF PIGS TO CHALLENGES 81

1.0-

0

E

"8 0.5-

0 e~ 0

0 .

0 0 1 2 3 4 5 6 7 8 9 10

M i n u t e s of t e s t

Fig. 4. The proportion of time interested in the novel object for low (---) (n = 16) and high responders ( ) (n = 16). Means and standard errors are shown.

biting. This interest declined rapidly in the first period of the test (Fig. 4). Total interest in the novel object was greater for high than low responding gilts in both groups ( F = 17.47; d.f .= 1,28; P<0 .001) . This difference between low and high responders existed only during the early part of the test when interest in the novel object was high (Fig. 3 ). Analysis of the separate components of interest in the novel object found that high responders paid attention to the novel object more ( F = 11.36; P < 0.05 ) but did not have more contact ( F = 3.12: NS ). Although there was no difference in the total interest in the novel object or attention paid to the novel object between Groups A and B, Group A had considerably more contact with the novel object than Group B ( F = 16.34:P<0.001 ).

Group-feeding competition test

At the start of each session, a large number of gilts at tempted to obtain access to the feeding spot by indiscriminate pushing. During the course of the session, density around the feeding spot declined while agonistic interactions increased. Animals varied greatly in the number of attacks won and lost per session (e.g. Group A: range 0-27 vs. 0-17 for attacks won and attacks lost, respectively). The majority of individuals showed little change in their behaviour over the test days. Number of attacks won was correlated with feeding and trying and negatively correlated with no interest (Table 2 ). Number of attacks lost was correlated with trying and waiting and negatively correlated with no interest. Feeding, trying and waiting were all negatively correlated with no interest and feeding was also correlated with trying. There were significantly more attacks won and lost, feeding and trying in Group A than in Group B (e.g. mean number of attacks lost per test: 8.18 vs. 5.15 for Group A vs. Group B: F = 15.60: d.f. = 1,179: P < 0.001 ).


T A B L E 2

Correlation matrix of behaviour in the group-feeding test (combined data: n = 32 )

Behaviour Behaviour

l 2 3 4 5

Attacks won 1

Attacks lost 2 0.28

Feeding 3 0.55"* 0.28

Trying 4 0.68** 0.67** 0.53**

Waiting 5 0.06 0.51"* - 0.04

No interest 6 - 0.56"* - 0.66"* - 0.74"* 0.22

-0 .76** -0 .58**

**P<0.01.

T A B L E 3

Comparison ofbehaviour between low and high responders in the group-feeding test

Behaviour High Low F

responders responders ( n = 1 6 ) ( n = 1 6 )

P

Attacks won ~ 8.05 5.75 2.92 NS Attacks lost ~ 6.20 7.19 1.67 NS Proportion of

attacks won 0.47 0.34 8.47 ** Feeding 2 0.67 1.07 25.19 ***

Trying 2 0.48 0.43 0.89 NS

Waiting 2 1.60 1.31 11.02 ***

Not interested 2 2.28 2.20 0.49 NS

Mean frequency per test. 2Mean number of observations per test. NS, not significant at P < 0.05; **P< 0.01: ***P< 0.001.

There was no difference between low and high responders in the number of attacks won or lost (Table 3 ) but high responders won a significantly greater proportion of encounters in which they were involved as individuals. High responders also spent significantly more time waiting on the edge of the feeding group. However, it was low responders that had the greatest feeding success (Table 3 ). Amongst the high responders there was a tendency for one or two to win the greatest number of attacks whilst the other high responders tended to win very few attacks. Low responders tended to win either a median or low number of attacks.

DISCUSSION

The study of temperament has been rather neglected in farm animals, despite them showing considerable individual differences in response to non-

BEHAVIOURAL RESPONSES OF PIGS TO CHALLENGES 83

social challenges such as handling (Syme and Elphick, 1982) and to social challenges such as competi t ion for food (Hunter et al., 1988 ). The extent to which these individual differences reflect stable behavioural differences between individuals is an unresolved but important question with a number of implications for applied animal behaviour. This study has examined the consistency of responses of female pigs in both non-social and social contexts. The results show that gilts' responses across a series of handling tests tended to correlate with those animals that left the residential pen most easily or presented themselves first for subsequent testing, showing least response to a sudden human approach or application of a nose sling. The handling tests were deliberately arranged in order of ascending severity to exclude intra- individual carryover effects between tests. Furthermore, the later Threat and Restraint tests were conducted in visual isolation and at some distance from the untested gilts. It seems unlikely, therefore, that visual or auditory stimuli from these later tests would have influenced the motivational state of gilts waiting to be tested. The lack of correlation between Order 1 and other handling tests in Group B may have been influenced by differences in social cohe- siveness between groups rather than differences in individual or group responsiveness (e.g. Lawrence and Wood-Gush, 1988 ). Order of presentation for subsequent testing (Order 2) appeared to represent a more consistent measure of individual differences in responsiveness.

The gilts that had high scores for the handling tests also tended to show greater amounts of interest but not to spend longer in contact with a novel object. This suggests that the high responders, despite being more responsive to the novel object, may also have found it more aversive than low responding gilts. Approach and investigation of a novel object in familiar surroundings often occurs only after a period of avoidance which can be interpreted as in- dicating a state of fear (Russell, 1979). Lyons et al. (1988) found in goats that the latency to approach a novel object also correlated with the latency to approach a human observer and to pituitary-adrenal responsiveness. One factor, therefore, that may have controlled the response of gilts to the non- social tests was the level of their state of fear.

The present work, similar to other studies on species including primates (Stevenson-Hinde et al., 1980), rodents (e.g. Benus, 1988 ) and goats (Lyons et al., 1988 ), suggests that in female pigs there are stable temperamental characteristics that control responses to non-social challenges involving both humans or inanimate objects. However, this does not necessarily imply that individuals would show a similar response to the same challenge at different ages (e.g. Brim and Kagan, 1980). The present work did not directly test for consistency of behaviour at different developmental stages although the initial handling response tests were separated from subsequent testing by a period of 4 weeks and also by transportation of the subjects to a new environment. Work with other ungulates (Kerr and Wood-Gush, 1987; Lyons et al.,


1988 ) has found that behavioural responsiveness to non-social challenge involving humans is consistent over time. For example, in cattle, reaction to being touched by a human observer showed a consistent individual pattern over a 98-week period (Kerr and Wood-Gush, 1987 ).

Studies of a number of species, including humans (Eysenck, 1967), non- human primates (Suomi, 1989) and other mammals (Kerr and Wood-Gush, 1987; Benus, 1988 ) have suggested a bimodal distribution of behavioural responses to challenge. In the present study the distribution of handling response scores does not suggest the existence of bimodally separate populations of individuals as found for aggressive behaviour in male mice (Benus, 1988 ) and response to human contact in cattle (Kerr and Wood-Gush, 1987 ). Where bimodally separate populations have been identified it has been pro- posed that these represent discrete types of individual. For example, in male mice it has been suggested that there are active and passive types of individual that respond to challenge in distinct ways (Benus, 1988). The present results which show a continuous distribution of response to the handling tests, indicate that in pigs, individuals may differ more in their degree of responsiveness rather than being separable into wholly distinct classes of individual.

This study has also indicated that individuals tend to respond consistently to a social challenge with individuals showing little change in their behaviour over the length of the group feeding tests. Few studies, and none in farm animals, have considered the relationship between responses in social and non- social contexts. This study found no strong evidence that individuals were consistent in their responses to non-social challenges and to a social group- feeding test, suggesting that responses to non-social and social challenges in female pigs may be controlled by separate behavioural processes. Whilst high responders won a greater proportion of encounters, low responders had the highest feeding success. Furthermore, there were no differences between low and high responders in other variables such as number of attacks won or lost, trying to get access to the feeding site and the level of non-involvement in the competition. This is apparently not consistent with work on rats and mice that found aggressive individuals to also show more active responses in non- social challenges (e.g. Bohus et al., 1987 ). However, neither the non-social or social tests used in these separate studies are directly comparable. For example, the non-social tests used in rodents, such as active avoidance, involve learning (e.g. Benus et al., 1990) whereas in the present study the handling and novel object test had little or no learning component. Some species and sex differences are also implicated. In rodents, tests of response to social challenge rely on territorial behaviour, whereas in pigs which do not tend to express territoriality except at parturition (Stolba and Wood-Gush, 1984 ), it is required to examine response to social challenge in terms of competit ion over


resources such as food. Female pigs also often show a lack of a structured hierarchy with a tendency towards intransitive relationships (Meese and Ew- bank, 1972; Hunter et al., 1988).

The present results suggest that whilst it is possible in female pigs, as in other species, to identify stable individual differences in response to non-social challenge, these do not predict responses in social groups. Responses to environmental challenges in female pigs appear, therefore, to be controlled by more than one underlying behavioural dimension and descriptions of temperament require to take account of individuals' responses in both non-social and social contexts. Future work will report on the relationships between social and non-social temperamental characteristics and the development of environmentally induced stereotypic behaviour in pregnant sows.

ACKNOWLEDGEMENTS

We thank the following: Cotswold Pig Development Company Ltd., U.K., for their assistance in this project; G. de Rosa for his help in collecting data and P. Phillips for statistical advice. This research was largely funded by a research grant from the Agriculture Food Research Council.

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Individual differences in behavioural responses of pigs exposed to non-social and social challenges