Methodology for measuring hunger and food needs using operant conditioning in the pig

Applied Animal Behaviour Science, 24 (1989) 273-285 273 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Methodology for Measuring Hunger and Food Needs Using Operant Conditioning in the Pig

A.B. LAWRENCE and A.W. ILLIUS

Edinburgh School of Agriculture, West Mains Road, Edinburgh EH9 3JG (Gt. Britain)

(Accepted for publication 23 June 1989)

ABSTRACT

Lawrence, A.B. and Illius, A.W., 1989. Methodology for measuring hunger and food needs using operant conditioning in the pig. Appl. Anita. Behav. Sci., 24: 273-285.

Two reward schedules were compared for their sensitivity in measuring motivational changes due to food restriction in the pig (Sus scro[a). Six boars were each restricted (proportionally) to 0.8, 0.6 and 0.4 of their predicted ad libitum food intake (PFI) in a Latin square design. Subse- quently, they were offered their PFI (Treatment 1.0). Feeding motivation was measured using a fixed ratio of 10 panel presses for each 6-g reward of food (FR schedule) and operant response rates were measured over 20-min sessions at three times post-feeding on each food level. Another 6 boars were subjected to identical conditions, differing only in their being rewarded on a progressive ratio (PR) where the response contingency was incremented by one on each successive reward. Reward rate per session was strongly affected by food restriction on both FR and PR schedules. On FR, reward rate per session increased with food restriction, but only up to a maximum on Treatment 0.6. On PR, in contrast, it was possible to distinguish between reward rates on Treatments 0.6 and 0.4 up to 5 h post-feeding. Progressive ratio appears to be a more sensitive means that FR of measuring changes in feeding motivation, however the cost was found to be an increase in the variability of the response data. The results suggest that the subjects on Treatment 0.6 were maximally food motivated for at least 19 h of the day. This also applies to sows and boars maintained on similar levels of food restriction under commercial conditions, showing the extreme divergence between food-restricted pigs' motivational need for food and their economically determined food allowances.

INTRODUCTION

The measurement of motivation has recently become important in applied animal behaviour science, as a result of public concern over animal suffering and welfare. The study of the existence and nature of motivational states un- derlying behaviour is often seen as critical to understanding the implications of restrictive husbandry conditions for the welfare of the animal (Dawkins, 1980). Definitions of motivation emphasise the goal-directed nature of motivated behaviour (Teitelbaum, 1966; Epstein, 1982). The blocking or thwarting

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274 A.B. LAWRENCE AND A.W. ILLIUS

of such goal-directed behaviour is said to result in the negative affective state of frustration (e.g. MacKintosh, 1983 ) and the performance of apparently ir- relevant stereotypic activities (Duncan and Wood-Gush, 1972) now widely regarded as behavioural indicators of chronic stress (Dantzer, 1986). Thus, it is argued that the demonstration of motivational states can be used as evidence in favour of environments that allow for the performance of motivated behaviour (Wood-Gush, 1983).

There are a large number of potential methods for measuring motivation. One technique increasingly used is that of operant conditioning where an animal is trained to perform an operant response to obtain a reward. Operant conditioning has previously been used, for example, to measure motivation for space and lighting in hens (Savory and Duncan, 1982; Faure, 1986) and for access to social contact in pigs (Matthews and Ladewig, 1986). It has been suggested that if animals cannot be trained to work for a reward, then their motivation for that reward cannot be great (Beilharz and Zeeb, 1981 ). How- ever, Dawkins and Beardsley (1986) suggest that operant conditioning may be a more problematical method for measuring motivation than is generally ac- knowledged. There is a need, therefore, to improve our understanding of operant conditioning as a technique for measuring motivation.

The schedule on which the animal is reinforced by rewards is generally rec- ognised to have large effects on operant response rates (Silverman, 1978). However, there are relatively few reports of experiments that directly compare operant performance in relation to reward schedule. In many studies, including those cited above, animals are rewarded for performing a fixed number of responses. In a previous experiment (Lawrence et al., 1988), the operant response rates of pigs on a fixed ratio of 10 panel presses for one reward (FR10) were compared across different levels of food restriction. Operant response rates were found to increase with food restriction, but only to a point, as further restriction below 0.6 of ad libitum intake produced no further increase in responding.

The present paper reports on an experiment to test if this response ceiling was a function of the feeding motivational system, whereby at 0.6 of ad libitum intake the pigs were maximally food motivated, or alternatively a result of FR being relatively insensitive as a measure of feeding motivation. As a comparison with FR, a progressive ratio (PR) was used, where the animal has to work cumulatively harder for successive rewards (Hodos, 1961). Although infre- quently used, results suggest that PR is a sensitive measure of motivation (Kennedy and Baldwin, 1972; Dantzer, 1976). There is additionally an important applied aspect to this work. Under commercial conditions, producers are recommended for production purposes [e.g. Agricultural Research Council (A.R.C.), 1981 ] to restrict their pregnant sows and boars to food levels equivalent to 0.6 of ad libitum intake, the level found previously (Lawrence et al., 1988) to result in maximal rates of operant responding. This may indicate that

MEASURING HUNGER AND FOOD NEEDS IN THE PIG 275

commercial breeding populations of pigs are maintained in states of maximal food motivation. Other evidence suggests a relationship between food restriction and the development of stereotypic behaviour in intensively housed sows (Appleby and Lawrence, 1987). It is therefore a matter of considerable impor- tance to devise a methodology which accurately represents motivational changes resulting from food restriction in the pig and which defines the motivational need for food as opposed to the production requirement.

METHODS

The subjects were 12 Large White × Landrace (Cotswold Pig Development Co. Ltd., U.K. ), entire males weighing on average 48 kg at the start of experimentation. They were housed in individual pens with part-slatted concrete floors, in environmentally controlled rooms {mean temperature 18 ° C). Light- ing was controlled automatically and was on between 07.00 and 21.00 h. The diet was a standard pelleted food containing on average 164 g protein, 22 g oil, 45 g fibre, 57 g ash k g - 1 and 13.2 MJ digestible energy (DE) kg -1. The boars were offered this food once daily at 10.00 h in their pens; cleaning of pens and weighing of animals took place between 10.30 and 11.00 h.

Following a 2-week period when the ad libitum intake of the boars was measured, the boars were food restricted (see below) and conditioned on an FR10 schedule for 6-g rewards of the same pelleted food. Conditioning took place in two separate rooms containing feeding devices operated in each case by a panel beside the feeding devices. A BBC Model B microcomputer both controlled delivery of rewards and recorded the time of each panel press (see Lawrence et al., 1988 for further details of equipment).

Throughout this period and in all subsequent experimentation, it was nec- essary, as the boars were immature and growing rapidly, to assign levels of feeding that took account of the pat tern of ad libitum food intake that followed the boars' growth. To achieve this, feeding level was determined with a regres- sion model using data collected in the present and two previous experiments (Lawrence et al., 1988, 1989) where boars' ad libitum intake was measured over a range of body weights, from 62 to 216 kg, in the same house. Forty-two measurements, of which 12 were repeats, were used. The equation that best fitted these data was

PFI = - 1.28 + 6.89 × 10- 2 W - 1.92 × 10- 4 W 2 [ S (y X x) -- 0.45; r 2 -- 70.0% ]

where PFI is the predicted ad libitum food intake and W is the liveweight in kg. During the conditioning period, boars were restricted to 0.6 of their PFI.

Characterisation period

Information related to individual variation in responding was gained by con- t inuing to restrict boars to 0.6 of their PFI following completion of condition-


ing and characterising individual response rates on FR10 over a 1-week period. During this characterisation, each boar was tested at two times of the day (08.00-10.00 and 13.00-15.00 h) with two duplicates at each time, each session lasting for 20 min. Individual boar sessions were balanced across test rooms. As far as possible, pigs were tested only once per day. Following characterisation, the boars were ranked in two groups of six for level of operant responding and then allocated within these groups to the following schedules.

Fixed ratio (FR)

Six boars (average weight now 90 kg) were restricted to proportionally 0.8, 0.6 and 0.4 of their PFI in a 3 X 3 Latin square design (see Statistical Analysis), each food level treatment lasting for 1 week. Each week, the PFI was recalcu- lated and the levels of feeding altered accordingly to take account of any liveweight change. Measurement of operant responding began 48 h later. Previous observations (A.B. Lawrence, unpublished data, 1988) suggested that there was no measurable carry-over effect if this period of time separated successive treatments. Furthermore, there was no effect of previous treatment in the present work when it was included as a covariate in the analysis of variance (see below). Treatment 0.6 was broadly the equivalent to that offered to pregnant sows and boars under commercial conditions (e.g. Cronin et al., 1986). Each week, operant responding was measured over 20-min sessions on an FR10 schedule at ~ 1 h post-feeding (11.00-12.30 h), 5 h post-feeding (14.00-15.30 h) and 22 h post-feeding (08.30-10.00) with two duplicates at each time. Ob- servations were also balanced across test rooms by two duplicates at each time. In the fourth week, following completion of the Latin square, the subjects were offered their PFI for 1 week (Treatment 1.0). Treatment 1.0 was not included within the Latin square to prevent low levels of responding on the high food allowance causing a loss of conditioning with resultant carry-over effects. Test- ing was repeated as above. The choice of FR10 as the fixed ratio and 20 min as the length of the sessions gave numbers of rewards earned per session in the same range as those obtained on PR (see below).

Progressive ratio (PR)

Six boars (average weight 88 kg ) were subjected to an identical experimental procedure with the same operant equipment used for FR, differing only in that the reward schedule used during tests was a PR, where the response contingency is incremented in a predetermined fashion. In the present case, the boars were required to press once for the first reward, twice for the second, three times for the third, etc. The end of the session came automatically at any time following a lapse in responding > 2 rain. A short time-out period was chosen in accordance with previous work using PR reward schedules in pigs (Kennedy


and Baldwin, 1972; Dantzer, 1976). Previous work (Lawrence et al., 1988) also found that inter-response bout intervals on a fixed ratio schedule were never > 2 min, even on high food levels. To leave the animal in the test room for long periods after cessation of responding was in addition thought to have possible negative consequences on conditioning.

Statistical analysis

The variate analysed was the number of rewards (R) gained per session. For the PR schedule, this was taken as the reward preceding the termination of the session. The corresponding number of responses per session on FR equals

R R × 10, whilst on PR it is given by ~ R. Thus, it required PR subjects to have

R = I

gained 19 rewards, for which they had responded 190 times, before their response rate per session equalled that of the FR treatment.

Rewards earned per session were analysed by an analysis of variance with a Latin square structure. Boars were regarded as columns, week of testing as rows and food allowances as treatments; each main plot (which was defined as the intersection of boars and weeks) was divided into sub-plots by time of day. As the effects of Treatment 1.0 and Week 4 were confounded, Treatment 1.0 was not analysed as part of the Latin square and comparison of Treatment 1.0 to the other food levels is made on the basis of their respective means and standard errors. Rewards per session in the pre-experimental period was tested as a covariate to obtain better estimates of individual boar differences. Pre- vious treatment was also tested as a covariate to test for carry-over effects from preceding treatments. Neither factor proved significant as covariates and are not therefore referred to in the Results. An analysis was carried out on log- transformed data; this gave qualitatively the same results and thus has also not been reported. Post-hoc analysis of the difference between pairs of means was carried out using the Q test (Snedecor and Cochran, 1980).

RESULTS

On FR, food level had a strong influence on rewards earned per session with responding increasing with food deprivation (means of rewards per session: 15.1, 25.9, 33.3 and 33.1 for Treatments 1.0-0.4, respectively). However, the major influence on motivation to earn rewards on FR was seen in the inter- action between time since feeding and food level (Table 1; Fig. 1). One hour post-feeding rewards earned on FR were depressed on Treatments 1.0 and 0.8 ( P < 0.05 in the case of the latter) relative to Treatment 0.6. Five hours post- feeding, rewards per session on 0.8 had recovered and did not differ from that on 0.6. However, rewards per session on Treatment 1.0 remained depressed


40.

30

== ==

@ I¢

. . . . ~ 0 5 10 15 2 2

Time since feeding ( h )

Fig. 1. FR schedule: the effect of time since feeding on rewards earned per session for different food levels. Means and standard errors are shown.

below that of other t reatments. Twenty- two hours post-feeding, there were only small differences in rewards earned per session between treatments, with rewards earned on 1.0 having recovered. Reward per session on 0.8 were slightly lower than on 0.6 and 0.4 (P < 0.05). Rewards earned per session on 0.6 and 0.4 did not vary significantly from one another or with t ime of day.

In contrast to FR, on P R the major influence on motivation was food level. Rewards earned per session increased between Trea tments 1.0 and 0.8, and between 0.8 and 0.6 (P < 0.05). However, in contrast to FR, where a plateau in responding was found between 0.6 and 0.4, rewards earned per session on P R increased significantly between t rea tments 0.6 and 0.4 ( P < 0.05). There was also a considerably greater range of rewards earned across the different treatments on P R (means: 2.9, 15.1, 25.0 and 28.8 for Treatments 1.0-0.4, respectively). The corresponding rates of responding illustrate how work rate per session on P R increases steeply at the high ratios achieved on the low food levels in comparison to FR (FR: mean responses per sessions: 150, 260, 330

MEASURING HUNGER AND FOOD NEEDS IN THE PIG

T A B L E 1

Results of analysis of variance of rewards earned on FR

279

Term DF Sums of squares F P

Boars. s t ra tum 5 9625

Week. s t ra tum 2 295

Boars. week. s t ra tum Rat ion 2 1281 Residual 8 353

Boars. week. t ime- s t ra tum Time 2 109 Time- Rat ion 4 513 Residual 30 307

Total 13268

Grand mean of rewards earned 31

14.52

5.30 12.53

<0.001

< 0.05 < 0.001

40-

30

==

Food level

• 0 '4 • 0.6 • 0-8 • 1.o

/ /

A I I I I I I

0 5 10 15 20 25

Time since feeding ( h )

Fig. 2. PR schedule: the effect of t ime since feeding on rewards earned per session for different food levels. Means and s tandard errors are shown.

280

TABLE 2

Results of analysis of variance of rewards earned on PR

A.B. LAWRENCE AND A.W. ILLIUS

Term DF Sums of squares F P

Boars. s t ratum 5 9916

Week. s t ratum 2 2835

Boars- week. s tratum Ration 2 3582 Residual 8 321

Boars- week. t ime. s t ratum Time 2 337 Time" Ration 4 307 Residual 30 966

Total 20144

Grand mean of rewards earned 21

44.67 < 0.001

5.24 < 0.05 2.39 < 0.10

and 330; PR: 3, 120, 325 and 435 for Trea tments 1.0-0.4, respectively, in both cases). The mean session time on P R increased from 3 min on Trea tment 1.0 to 23 min on Trea tment 0.4 and was strongly correlated to rewards earned (Pearson correlation coefficient: r-- 0.995, d f = 2, P < 0.05 ).

The pat tern of rewards earned per session on P R with time since feeding is shown in Fig. 2. Rewards per session were depressed 1 h post-feeding on all t reatments other than 0.4 (P < 0.05). In contrast to FR, reward earned on 0.6 were also found to be lower 1 h post-feeding than on 0.4 (P < 0.05). Five hours post-feeding, rewards per session on 1.0 and 0.8 still remained depressed relative to 0.6 ( P < 0 . 0 5 in the case of the latter). However, on 0.6, rewards per session had recovered and now did not differ from that on 0.4. Twenty- two hours post-feeding, rewards earned on Trea tments 1.0 and 0.8 still remained depressed (P < 0.05 in the case of the latter) and again there was no difference between Trea tments 0.6 and 0.4. Rewards earned on Trea tment 0.4 showed no tendency to vary with time since feeding.

An additional difference between FR and P R was in the overall variability generated by the two schedules. The overall mean number of rewards earned on FR was 48% greater than that on PR, yet the total sums of scores from the analysis of the FR data were 66% less (Tables 1 and 2). The variation in responding on P R was thus proportionally considerably larger than that on FR. The source of the extra variability in P R was not boars which had similar sums of squares in both t rea tments (sums of squares for boars: 9624 versus 9916 for FR versus PR) , but in the week × boar space (Tables 1 and 2).

Boars were a major source of variation in both schedules. The rewards earned per session of the least active animals on both schedules were considerably lower than those of the most active animals. For example, on FR the lowest


responding boar only achieved 31% of the rewards of the highest responder on Treatment 0.4. These individual differences were carried across treatments. There was a significant tendency on both schedules for the ranking of boars on rewards earned per session to be similar across treatments (Kendall coefficient of Concordance: FR: W--0.79, N=6, k=4, P<0.01; PR: W=0.69, P<0.01). Rewards earned per session in the pre-session period (when food level was 0.6 of the PFI) also correlated with rewards earned on Treatment 0.6 (Pearson correlation coefficient: FR: r=0.90, n=6, P<0.05; PR: r=0.83, P<0.05).

D I S C U S S I O N

The pattern of rewards per session in relation to food level on FR found here closely matches that reported by Lawrence et al. (1988). In each case, overall rewards earned only increased with food restriction up to a point with no further increment between Treatments 0.6 and 0.4. Both studies also found very similar patterns of rewards per session with time since feeding, with only Treatments 1.0 and 0.8 depressing rewards earned immediately post-feeding and only Treatment 1.0 continuing to do so 5 h post-feeding. In both studies, rewards per session on Treatments 0.6 and 0.4 did not vary from one another or with time of day. However, the present results suggest that the response ceiling at 0.6 found using FR is not motivational in origin, but the result of experimental constraints placed upon the animal by the reward schedule. Us- ing PR as reward schedule, it was possible to distinguish rewards earned on Treatment 0.6 from that on 0.4 at 1 h post-feeding, but not 5 h post-feeding. PR is therefore a more sensitive means that FR of measuring feeding motivation in that it is more likely to distinguish between differing motivational levels. This is supported by the range over which the two schedules measured rewards earned. PR produced rewards per session over a considerably greater range than FR and, in addition to distinguishing between Treatments 0.6 and 0.4 at 1 h post-feeding. PR was particularly efficient in widely separating high food levels (1.0 and 0.8) from low food levels (0.6 and 0.4), even at 22 h post- feeding. However, whilst PR may be a more severe test of motivation, the pro- portional increase in the sums of squares suggests that the cost is an increase in the variability of the response data. This enhanced variability increases the difficulty of discerning how individuals have responded to the PR schedule and implies that the precision with which treatment means are estimated is less than on FR. These two conflicting effects of PR require to be considered in its choice as reward schedule.

There are a number of possible reasons for PR being more sensitive to feeding motivational changes than FR. FR schedules encourage high rates of responding, reinforcing the animal to complete bouts to obtain successive rewards. Lawrence et al. (1988) found that the maximum rate at which a bout


of 10 panel presses was completed (the local response rate) was reached at Treatment 0.8. There was also little variability in the length of bouts, indicat- ing that once a bout was initiated it was completed without a pause. Such stereotypy of responding is typical of FR schedules (Herrnstein, 1961). The local response rate on FR is therefore insensitive to changes in motivation, at least partly as a result of a physical limitation on the speed with which the panel can be operated. However, clearly the major difference between the two schedules is that PR demands increasing work for successive rewards or greater cost for the same benefit. There is now considerable evidence from operant work that animals' behaviour is strongly affected by changes in the ratio of costs to benefits (Collier, 1980). Typically, using a free feeding paradigm {where the animal controls the initiation and termination of meals) increasing the cost of meals, by increasing the work requirement, causes the animal to decrease meal frequency and increase meal size to maintain intake and body weight {Collier et al., 1972). Extremely large response contingencies result in the animal ceasing to be able to maintain intake and body weight {Kanarek and Collier, 1979). Similarly, Dantzer (1976) showed that pigs on PR working for small food rewards, as in the present experiment (i.e. not free feeding), increased their post-reinforcement intervals as the response contingency rose, it is presumed spending longer on consuming rewards. PR therefore brings about changes in behaviour suggesting that as the ratio increases the animal is responding to the increased cost of the reward, and it would seem that PR largely owes its sensitivity to it measuring the animal's at tempts to optimise its allocation of time and energy. If this is so, then PR ought to be, as it has been in the present experiment (see below), a useful tool in the "economic" analysis of a variety of animals' motivations (Dawkins, 1983 ).

As with other operant work (e.g. Will, 1974; Dawkins and Beardsley, 1986), the present study found considerable and consistent individual variability in the absolute levels of responding. The pat tern of individual variation on FR was again very similar to that found by Lawrence et al. { 1988). Currently, there is little understanding of the basis for these individual differences in operant responding. Here, a pre-experimental characterisation period was found to be essential to gain additional information about individuals. This information was used to group individuals prior to t reatment allocation, but was not subsequently a significant covariate in the analysis, given that boar effects were already included in the analysis. It would seem that stringent control over individual variability is an essential feature of operant experiments designed to distinguish between motivational changes.

The present results illustrate the usefulness of applying operant conditioning to motivational questions related to issues of animal welfare. Using PR it was possible, despite the increased variability associated with this schedule, to produce a sensitive representation of the changes in feeding motivation that occur with time since feeding. PR distinguished reductions in responding on


Treatment 0.6 in the immediate post-feeding period. On Treatment 0.4, there was no depression in responding immediately after feeding. It is important that in the present context PR was an open-ended test with no limit placed upon the effort an animal might expend for rewards. This and the lack of response depression post-feeding suggests that responding on PR on Treatment 0.4 may represent a response maximum. Therefore, by the criterion of response to PR, pigs fed at levels equivalent to Treatment 0.4 may be maximally and continu- ously food motivated, whilst pigs on levels equivalent to 0.6, such as sows and boars under commercial conditions, may be maximally motivated for at least 19 h of the day.

This work raises important questions regarding the need of pigs for food. Feeding motivation is influenced by a number of internal and environmental variables (Le Magnen, 1985) and the measurement of this motivation is also dependent on the technique used (Miller, 1955; this study). There are at least three definitions of pigs' food needs, arising from motivational and nutritional studies, which can be viewed hierarchically: commercial, work related and ad libitum. The A.R.C. (1981) defines food needs for sows and boars on the basis of commercial levels of nutrient intake, at ~ 0.6 of ad libitum intake. Although this level of feeding is sufficient for maintenance and some growth, and in the case of sows also for foetal development, it results in maximal food motivation for much of the day (see above) and is related to the development of stereotypic behaviour in tethered gilts (Appleby and Lawrence, 1987). At the other extreme, ad libitum intake of the animal provides another definition of food needs. This in itself is not, however, a demonstration of a strong need and in the case of sows can give rise to levels of fat that interfere with reproductive efficiency (e.g. Whittemore, 1987). Dawkins (1983) has argued that a demonstration of need requires that the animal be willing to pay some cost for the consummatory goal of its motivation. Work-related intake, measured by operant conditioning, therefore provides a third and more useful definition of food needs. The present study has used operant responding on PR to demon- strate a continuing food need on A.R.C. (1981) recommended food levels. Meanwhile, food levels approximating to ad libitum intake (Treatment 1.0) gave rise to response levels not differing from zero, while pigs on 0.8 had inter- mediate levels of responding. The present results thus show that sow and boar food needs, as defined by responding on PR, are currently not being met under commercial conditions. However, commercial considerations would appear to make increases in food allowances to levels that would significantly decrease work-related intake unlikely and indeed recommended food levels for sows and boars may be further reduced (Close, 1987). For this reason, further research should focus on the extent to which operant responding for food may result from residual foraging motivation, i.e. the behavioural need to seek food, and how alternative stimuli such as foraging substrates (e.g. soil ) might substitute for food (c.f. Lea, 1980).


ACKNOWLEDGEMENTS

The authors would like to thank A. McAndrew for his technical assistance, H.A. Macleod for his help with computing, E.A. Hunter and P. Phillips of the Scottish Agricultural Statistics for advice on statistics, and G.C. Emmans for helpful comments on the nutrition and growth of pigs.

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Methodology for measuring hunger and food needs using operant conditioning in the pig