taste preferences in pigs for nutritive and non-nutritive sweet solutions

Anirn. Behav., 1972, 20, 706-718

T A S T E P R E F E R E N C E S I N P I G S F O R N U T R I T I V E A N D N O N - N U T R I T I V E

S W E E T S O L U T I O N S

BY J. M. KENNEDY & B. A. BALDWIN* A.R.C. Institute of Animal Physiology, Babraham, Cambridge

Abstract. Taste preferences in young pigs have been examined using both long term and short term two choice preference tests. The substances used were examined over a wide range of concentrations; they included sucrose, glucose, sodium saccharin and sodium cyclamate. Relative to water the pigs had a very strong preference for solutions of sucrose and glucose and a strong preference for saccharin. They displayed no preference for solutions of sodium cyclamate. Using operant conditioning methods it was possible to demonstrate that the pigs would learn to perform operant responses in order to obtain sucrose solutions and their motivation to respond on progressive ratio schedules of reinforcement was related to the concentration of sucrose which they obtained.

Many species of animals display a marked preference for sweet tasting solutions. The preferences for nutritive substances such as sugars and non-nutritive substances such as saccharin have been extensively studied in rats (Richter & Campbell 1940). Less detailed information on sweet taste preferences is available for several other species including hamsters, rabbits (Carpenter 1956), squirrel monkeys (Clark & Harriman 1969), calves (Bell & Williams 1959), sheep (Goatcher & Church 1970) and goats (Bell 1959). Some animals, such as cats (Carpenter 1956), pigeons, fowl (Kare 1961) and armadillos (MaUer & Kare 1967) appear to lack a sweet taste sense. In contrast to goats, sheep do not exhibit strong preferences for sugar solutions (Goatcher & Church 1970).

Relatively little quantitative information is available on the sense of taste in the pig. The taste preferences of groups of young pigs have been examined by Kare, Pond & Campbell (1965) and the effects of adding sweetness to the diet of pigs has been studied by Diaz et al. (1956) and Aldinger et al. (1959). As a pre- liminary to further studies on the behavioural and physiological factors influencing sweet taste preferences in pigs we have examined their preferences for two nutritive sugars and two non-nutritive sweeteners. The sugars used were sucrose and glucose and the non-nutritive substances were saccharin and cyclamate.

In some of the preference experiments we have used long term, two choice, preference tests of the type originated by Richter (1939).

*Requests for reprints to be sent to Dr B. A. Baldwin, A.R.C. Institute of Animal Physiology, Babraham, Cambridge.

However, one disadvantage of long term tests is that the relatively large volumes of fluid consumed gives rise to post ingestive effects and therefore, in other experiments, we have at- tempted to minimize post ingestive factors by using relatively short exposures to the test substances (Christensen 1961).

The work of Guttman (1953), using rats, has emphasized the usefulness of operant conditioning methods in providing information on the motivation of animals to work for sweet solutions and we have also used operant techniques in the present study. It has recently been demonstrated that pigs make good experi- mental animals for operant conditioning (Bald- win & Ingram 1967; Baldwin & Stephens 1970) and we have been able to use the progressive ratio methods previously used by Hodos (1961) in his studies on taste in rats. It has been necessary to devise appropriate apparatus and training techniques for work with pigs and, where necessary full details are given in the methods section.

Methods Animals

A total of sixty-six Large White or Landrace pigs were used in this study. Both females and castrated males were used and many of the pigs, used for particular tests, were littermates. The animals used in the preference tests ranged in age from 2 to 4 months and in weight from 15 to 30 kg. The pigs used in the operant conditioning experiments were selected at about 2 months- old but were kept for longer periods, in some cases reaching weights of over 100 kg.

For the duration of the preference tests the pigs were fed once each day at 17.00 hours with

706

KENNEDY & BALDWIN: TASTE PREFERENCES IN PIGS 707

640 g of commercial pelleted food containing 17 per cent protein, 3 per cent oil, 4 per cent fibre and 180 ppm of copper.

Apparatus The preference tests were carried out with the

pigs housed in metal cages 137 cm long by 61 cm wide and 76 cm high. The taste solutions were presented to the pigs in two 5-1itre buckets which were mounted in brackets on the end wall of the cage. The end of the cage had an aperture in it for the pigs to put their heads through. In order to prevent the pigs chewing the plastic buckets it was necessary to cover them with a sheet of aluminium in which holes were cut to permit access to the solutions. As the pigs drank from the buckets the fluid in them was maintained at a constant level by means of two glass reservoirs mounted on top of the cage and con- nected to the buckets by means of vinyl tubing. The glass aspirator bottles were graduated in 50-ml calibrations so that a record could be kept of the volumes consumed by the pigs.

The cages had metal grid floors and were cleaned each day. The cages used in the experiments on short term preference tests were similar to those described above except that the large glass reservoirs were replaced by 1-1itre graduated measuring cylinders to facilitate measurement of the smaller volumes consumed. The pigs used in these tests normally lived in a group housed in a large pen. In the long term tests the temperature of the room in which the test cages were housed was maintained at approx- imately I~~ (Range 15 ~ to 25~ In the winter months extra heating was provided by suspend- ing infrared lamps above the cages. The pen in which the short term pigs were housed was maintained at about 18~

Long Term Preference Tests For 12 hr (20.00 to 08.00 hours) in each 24 hr

the pigs were provided with water in both buckets, but for the remaining 12 hr the solution under test was substituted for water in one of the buckets. The volumes of fluid in each aspirator were recorded at the beginning and end of each 12-hr period. In order to get the pigs used to living in the cages a period of 1 week elapsed, after entering the cage, before any experiments started.

Many animals, when faced with a choice of two identitical sources of fluid, will show a consistent preference for one particular con- tainer and such 'position preference' has been

studied in rats (Myers & Holman 1966). In the present study the problem was avoided by alternating the position of the test solution and water so that each concentration of each test substance was presented twice to the animal, once to right of the water and once to the left. Each concentration was presented for two successive 12-hr test sessions.

The different concentrations of the test substances were presented in the following order. Initially a very low concentration of the test solution was substituted for water in one of the buckets and its position alternated. In the next exposure the concentration of the test substance was raised and this process continued until the maximum concentration was reached. This series of tests constituted an 'ascending concentration run'. After the highest concentration was reached it was then lowered in the test sessions until the lowest concentration was reached (the 'descending concentration run').

Short Term Preference Tests In these tests, which were designed to mini-

mize post ingestive factors, the pigs were only placed in the test cage for 1 hr each day. For the remainder of the time all the animals being tested lived together in a large pen which was situated near to the test cage. At the time of testing one pig would be removed from the pen and coaxed into the test cage and left for a 1-hr period. In these tests only an 'ascending concentration run' was used. For this type of test it was essential that the pigs were tame and used to being handled. At the beginning of the tests some difficulty was experienced in getting the pigs to sample the fluids and therefore, at the lowest concentrations, they were deprived of water for 24 hr prior to testing. Once the pigs had learned to sample the solutions this practice was discontinued and the results discarded.

Operant Conditioning The pigs used for operant conditioning ex-

periments were kept in separate pens and tested individually, usually for �89 hr each day. Before training commenced the pigs had to be tame and used to being handled.

The cage in which the pigs were trained was placed in the laboratory as no sound shielded chamber was available. The pigs were trained to push a panel switch, mounted at the end of the cage with their snouts in order to obtain small volumes (12 ml) of a sweet tasting solution, 1.0 M sucrose. The solutions were delivered

708 A N I M A L B E H A V I O U R , 20, 4

into a stainless steel bowl by means of a magnetic valve system. The system was controlled by means of relay apparatus and the behaviour was recorded on a print-out counter or on a Ger- brand's cumulative recorder.

Training Procedures In an attempt to ensure that the pigs were

attracted by the taste of the solutions and not their calorific value all pigs were on an adequate diet and with water available ad libitum. The animals were fed each day before the operant tests were carried out. Training was carried out using conventional operant conditioning techniques as described by Ferster & Skinner (1957). When fully trained the pigs were run on variable interval (mean interval 30 s) and progressive ratio schedules of reinforcement.

In a progressive ratio schedule of reinforcement the number of responses which the animal must make before the behaviour is reinforced increases by a fixed increment with each reinforcement. Eventually, when the ratio gets very high the animal stops responding. When it does so for a predetermined period of time the last completed ratio is recorded and used as a measure of performance. The criterion adopted here to determine the cessation of working was when the pig went for a period of 30 s without responding. It was found essential to have quiet conditions during trials using progressive ratios as any disturbances could result in a pre- mature cessation of responding, especially on high ratios. When using a variable interval schedule of reinforcement it was found that mean intervals longer than 30 s tended to result in irregular responding for the sugar solutions.

Solutions Used All the test solutions used in the preference

test experiments were made up in water obtained directly from an artesian well situated at the Institute. It was not practical to use distilled water because of the large volume needed. The well water had not undergone any chemical treatment.

Two methods of representing the results of the preference tests have been used. The first method was to plot the volumes consumed of the various substance against the concentrations of the substance being tested. Also, in accordance with the usual practice in the study of taste preference by the two choice methods, the following system has been used.

Volume of test solution consumed. + 100 =

Total volume consumed.

Percentage preference.

The percentage preference is plotted against the concentration of the substance being tested. The percentage preference may be interpreted as follows. A percentage preference of 50 per cent indicates that the animal consumed equal volumes of test solution and water. A value higher than 50 per cent indicates a preference and a 100 per cent preference implies that the animal drank no water but only consumed the test solution.

In order to facilitate comparisons with other published work the following terms will be used in accordance with the work of Goatcher & Church (1970): 60 to 80 per cent indicates a moderate or weak preference; 80 to 100 per cent indicates a strong preference.

Results Sucrose

Long term preference tests. As may be seen from Fig. 1 pigs display a strong preference for sucrose. Their preference threshold appears to be between 0.005 and 0.01 M and a strong preference is exhibited at any concentration above 0.03 M. At concentrations greater than 0,03 r~ all animals exhibited consistent preference with very little variation. Over the lower range of concentrations (0.005 to 0.03 M) there was a wide variation between different pigs but there was a definite general tendency towards preference and the pigs may be said to display a weak or moderate preference. The volumes of sucrose solution consumed by the pigs are shown in Fig. 2 and this measure also gives a clear picture of the preference behaviour. As the concentration of sucrose rises the intake increases throughout the range 0.005 r~ up to 0.225 M while at the same time the intake of water diminishes. It can be seen that although on the preference graph (Fig. 1) the percentage preference is relatively constant above 0.03 ~ this is in fact, due to the low water intake and the volume graph (Fig. 2) indicates that the amount of sucrose drunk continues to increase. One of the most striking findings was the very large increase in fluid intake caused by the addition of sucrose. The intake of the pigs over a 12-hr exposure period rose from a baseline level of about 3 litres to over 17 litres.


|

100.

9 0 .

so2

70 .

50

40_

30 .

20_

t0_ Molari iy(Log. scale)

1.001 0.01 0.1 0 ~ a I J I ,

if001 0'0025 0,005 0.01 0"03 0.06 0.12

Sucrose test solution (Molari ty)

Fig. 1. Sucrose.--long term preference test. Mean, • sD of results from ten pigs. Based on results obtained using an ascending and descending series of concentrations.

! 0.22S

E

18.

16.

15.

14.

13.

1i.

11.

10.

9"

0_

7_

2.

I_ ~001

0 0-001

0.0t ~ 'l l I I , 0 2 , o.oos o., ~os ~.oe o.12' o.~s' 6.s

Test solution (Molarl ty)

Fig. 2. Mean volumes consumed of sucrose, glucose and water in the 12-hr test periods during the long term preference tests.

,Glucose

O.0

,/ater(G) 1.0 I

7 1 0 A N I M A L B E H A V I O U R , 20, 4

Both an 'ascending concentration run' and a 'descending concentration run' were carried out. Statistical analysis o f variance revealed that there was no significant difference between the results obtained on the 'ascending concentration run' and the 'descending concentration run' although the variation was slightly greater in the descending results. The preference thresholds were the same in both cases.

Short term preference tests. The results obtained from an 'ascending concentration run' using this method were very similar to those obtained using the long term test (Fig. 3). A preference threshold was determined between 0.005 and 0.0! M and a strong preference was displayed at all concentrations above 0-1 M. This level is higher than that found in the long term preference test and this may be associated with the greater variation between pigs which was observed in this experiment.

From the percentage preference graph (Fig. 3) it would appear that an absolute preference was exhibited for all concentrations from 0.1 to 1.0 M but examination of the volumes consumed (Fig. 4) reveals that a maximum preference was displayed at 0.1 M and that above this concentration the intake of sucrose slightly decreased. Although the fluid consumed in the short term tests was well above baseline level (<0-5 litres) it never exceeded 3.5 litres in the l-hr period.

Glucose Long term preference tests. Pigs display a

strong preference for glucose solutions as may be seen in Fig. 5. A preference threshold between 0.01 and 0.03 ~ was obtained and a strong preference was exhibited for all concentrations above 0-12 M, even up to 0-6 M. However, the volumes of glucose consumed did diminish at very high concentrations (Fig. 2). The volumes of glucose solution drunk reached a maximum at 0.12 M and above this concentration the intake fell. At the optimum concentration the pigs were drinking up to 14 litres in the 12-hr test period. As with the sucrose solutions there was no statistically significant difference between the results obtained in the ascending and descending series.

Short term preference tests. The results obtained using an ascending run were very similar to those found using the long term tests (Fig. 6). A preference threshold between 0.01 and 0.05 M was obtained and a strong preference was shown for all concentrations above 0-05 M including the 1.0 M level. Examination of the volumes of glucose consumed, however, revealed that there was no decline in intake at the higher concentrations. This was in marked contrast to the findings in the long term test (Figs 2 and 4). In the short term test the intake of glucose solution increased consistently throughout the

100

90

80

70

~ 60

5O g ~ 40

~. 3o

/

I ....

0"001

0"001

Molarity (Log. scale)

O.Ot 0,1

, I . . . . . . . , I - 0"005 0-01 0-05 0"t

Sucrose test s o l u t i o l ~ (Molar i ty)

Fig. 3. Sucrose--short term preference test. Mean • SD of results from four pigs. Based on an ascending series of concentrations.

i 0.5

1.0

I 1'0

K E N N E D Y & B A L D W I N : TASTE P R E F E R E N C E S IN PIGS 711

3-5

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1-5

1+0.

0.0

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~ 0 ~

Water CS)

0.005 ~,01 I I 0"05 0.'+ 0'5 1.0

Test solution/Molarity/

Fig. 4. Mean volumes consumed of sucrose, glucose and water in the 1-hr test periods during the short term preference tests.

i ,P

100.

90.

80.

7 0 .

60_

so .

40.

30 .

tO.

0

0-001

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I i

Mofadty(Log.sca|e)

�9 0-01 04 I ' I t I _+

0-.20 0-0 . 0-m ~.- ~, ~.,2 ~,3

Glucose tes t solution(Molarity)

Fig, 5, Glucose---long term preference test. Mean • SD of results f rom twelve pigs. Based on ascending and descending series of concentrations.

1-0 I

712 A N I M A L B E H A V I O U R , 2 0 , 4

100_

BO

70

60

40_

~. 3o_

20_

10_

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0001 0.005 o-oi 0.05 0.1 0.5

Glucose test solutionlMolarity)

Log. Scale

Fig. 6. Glucose--short term preference test. Mean, =k sD, of results from four pigs. Based on an ascending series of concentrations.

range 0.01 to 1.0 ~ reaching a maximum of 3.5 litres in 1 hr at 1.0 M.

Sodium Saccharin Long term preference tests. The pigs displayed

a strong preference for saccharin solutions at concentrations of 0.01 and 0. i M (Fig. 7). The preference threshold would appear to be

t t'0

between 0-005 and 0.01 M but even at the most preferred concentration the percentage preference only rose to 90 per cent. Above 0.1 M the preference rapidly fell and the pigs rejected 1.0 r~ ~accharin, the rejection threshold lying between 0.1 and 1.0 M. The volumes consumed of saccharin and water confirm this general picture, with a sharp peak in the volume of saccharin

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~. 30_

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Saccharin test solution (Melarity) Log. scale

Fig. 7. Sodium saccharin--long term preference test. Mean, -t- SD of results from twelve pigs. Based on ascending and descending series of concentrations.

1.0


IO

A $ =

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~0001

consumed at 0.1 u (Fig. 8). However, even at this most preferred concentration, the volume consumed did not exceed 11 litres in the 12-hr test period.

There were no statistically significant differences between the results obtained using ascending and descending concentrations.

Short term preference tests. The results obtained were very similar to those f rom the

�9 . _ ~Saccharin I , I , I I

00005 0001 0005 001 0.l 1"0

Test ~utlon (M~larlty) Log . scale

Fig. 8. Mean volumes consumed of saccharin, cyclamate and water during the 12-hr test periods in the long term preference tests.

long term test (Fig. 9). A preference threshold around 0.01 M was obtained, and a rejection threshold between 0.1 and 0.5 M. The maximum preference was for the 0.1 M solutions of saccharin. However, the percentage preference at this concentration was only 80 per cent which was lower than that found in the long term tests.

The volumes gave similar results, with a maximum intake of 1.8 litres in a 1-hr exposure

100_

90_

80_

8 7o_ Go_

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~ 40_

~30_

-*' 20_

10_

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00001

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I I , 1 o.ooi OOl oos 0.1

Seccharin test sotut ion(Mo|arity) Log. scale

Fig. 9. Sodium saccharin--short term preference test. Mean, • SD of results from four pigs. Based on an ascending series of concentrations.

0.5 10

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A N I M A L B E H A V I O U R , 2 0 , 4

m 1.5

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0"5

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ter

Saccharin

J I I , I , I 00001 0~301 001 04)5 0-I 0 5 1"0

Saccharin test solution (Molarity) Log. scale

Fig. 10. Mean volumes consumed of saccharin and water in the l -hr test periods in the short term preference tests.

to 0.1 M. At 0.5 and 1.0 M concentrations the volume of saccharin consumed fell below 0.25 litres in the 1-hr period (Fig. 10). On both long term and short term preference tests the variation between individual pigs was very wide.

Sodium Cyclamate Long term preference tests. The behaviour of

the pigs towards solutions of cyclamate differed considerably from their reactions to sugars or saccharin (Fig. 11). They seemed to display indifference towards any concentration up to

0.01 M and a rejection of 0-1 M cyclamate. At no concentration did the pigs exhibit a preference although the variation between individual pigs was very wide particularly at concentrations below the rejection threshold.

The rejection of 0.1 cyclamate was confirmed when the volumes consumed were examined (Fig. 8).

Operant Conditioning Sucrose--variable interval schedule. On vari-

able interval (V.I.) schedules of reinforcement

I00.

gO.

80.

70. ID

60.

~.50. QI

~_ 20,

10/

' I 0 I I , i I 000001 0.0001 00005 0.001 0.005 001 0.1

C y c l a m a t e test so iu t ion(Mo la r i t y )

Log. scale

Fig. 1 t. Sodium cyc lamate- long term preference test. Mean, 4- SD of the results from six pigs. Based on an ascending series of concentrations.


(up to mean V.I. of 30 s were used), no significant differences were found in the rate of responding, if all pauses were excluded, for reinforcement with 1.0, 0-5, 0.225 and 0.03 M solutions. However, it was apparent that at the 0.03 M concentration the overall pattern of responding suggested that it was not a strongly reinforcing solution. Evidence of this was revealed by the pigs ceasing to respond before the end of the 20-min session and by frequent pauses during the first 10 min. If the results are examined in terms of responses emitted during the first 10 min of a 20-min session (Fig. 12) it can clearly be seen that the performance

800

700

600

soo

"C= 4oo

3oo

'Y 200_

100-

0.

0-03 M 0.225M 0.SM 1-OM

Sucrose test so[ution(Motarity)

Fig. 12. Illustrates the responses emitted during the first 10 rain of an operant conditioning session in which the reinforcement consisted of sucrose solution of various concentrations. The pigs worked on a variable interval schedule of reinforcement (mean V.I. 30 s). Mean results from four pigs each exposed five times to each concentration.

when working for 0.03 M was markedly less than for the other concentrations. It was apparent that although during the bursts of responding there was no difference in rate, the measure which included pauses revealed a significant difference.

Progressive ratio schedules. Using progressive schedules of reinforcement in which the pig had to respond once for the first reinforcement, twice for the second and three times for the third, etc., it was possible to demonstrate a consistent relation between the highest ratio reached and the concentration of sucrose used as reinforcement (Fig. 13). From Fig. 13 it can be seen that there was a consistent and highly significant trend towards higher ratios at the higher concentrations of sucrose.

50

40

3 0 _

.9

.= 20

10

0"03

I I

0-25 0 '5 1"O

Test solut ion (Molarity) Fig. 13. Illustrates the results obtained using a progressive ratio schedule of reinforcement with various concentrations of sucrose. Mean results from three pigs each tested five times at each concentration. For further details see text.

Discussion There has been relatively little previous work concerned with taste preferences in pigs. The studies on solid food preferences carried out by Diaz et al. (1958) and Aldinger et al. (1959) have indicated the young pigs prefer food sweetened with sugar to unsweetened :food. In relation to the preference of pigs for saccharin it appears that different strains of pigs may differ in their preferences. Aldinger et al. (1959) observed that young pigs preferred diets which contained saccharin while, in con(rast, Grimstead et aL (1960) found that synthetic

716 A N I M A L B E H A V I O U R , 20, 4

sweeteners reduced the acceptance of a ration. These findings have led to speculation that there is a genetic discontinuity in the taste preferences of different strains of pigs corresponding to that existing in man for the taste of phenylthiocar- bamide. This hypothesis has been tested by Kate et al. (1965) who found that while some strains of pigs exhibited a preference for solutions of saccharin, other pigs avoided drinking the solution. In the present study no such effects were observed although there were wide differences in the responses of individual pigs to saccharin solutions (Figs 7 and 9). However, all the pigs used in the present experiments were closely related.

The pigs displayed a strong preference for sugar solutions in both the long term and short term preference tests. The preference threshold for sucrose, as revealed in the long term preference tests, was between 0.005 and 0"01M which is close to that found in goats (0.03 M, Goatcher & Church 1970), rats (0.0146 M, Richter & Campbell 1940), and man (0.012 M, Richter & Campbell 1940). In contrast to rats (Shuford 1959) and also goats (Bell 1959), pigs do not display an aversion to high concentrations of sugars in long term preference tests. The pigs consumed very large volumes of both sucrose and glucose in the long term preference tests (Fig. 2) drinking as much as 18 litres in a 12-hr period. Some of the pigs became ill after con- suming excessive amounts of sugar solutions but after recovering they continued to drink large amounts of sugar. It is possible that the affected animals did not associate the toxic effects with the consumption of sugar but the work of Kalat & Rozin (1970) makes this unlikely. These workers, using rats, measured the relative tendency for various substances to be associated with toxic effects following ingestion. That is to say, if a rat is given a series of different substances and a toxin is introduced into the series the rat will associate the effect of the toxin with the taste of some substances more readily than with others. This factor they termed 'salience' and found the 'salience' of sucrose to be high. It therefore seems likely that the pigs recognized the toxic properties of the sugar solutions but were induced to consume large quantities because of their strong preference for the taste. The preference exhibited by animals for sucrose and glucose is, without doubt, partly based on their sweet taste and partly on their nutritive properties. Smith & Duffy (1957) found that hungry rats learned to run a maze to

obtain sucrose solution faster than satiated rats but only when the amount of sucrose solution was sufficient to substantially reduce their hunger. They concluded that sugar has a duel reinforcing effect, supplying both need reduction and sensory satisfaction, and that the two are additive. However, although the calorific value of sweet substances certainly contributes to their reinforcing properties, it is not essential. It has been shown here that animals show a distinct preference for saccharin solution compared with water and Sheffield & Roby (1950) have demonstrated that such non-nutritive sweeteners can be used as reinforcers of learned instru- mental behaviour.

The pigs displayed a strong preference for solutions of saccharin in the range 0.01 to 0.1 M but exhibited aversion to the more concen- trated 1.0 M solutions. This range of preference and aversion is similar to that shown by rats. The factors influencing the aversion to con- centrated solutions of saccharin are complex but it has been shown that saccharin is not metabolized in the body and therefore would not be likely to have post ingestive effects. It seems probable that the aversive properties of strong saccharin solutions are due to the bitter component in the taste of saccharin (Pfaffman 1969). To human subjects saccharin possesses a mixture of sweet and bitter tastes and artificial sweeteners such as cyclamate, which also has a bitter component, are avoided by rats (Murrey et al. 1953; Wagner 1971).

Cyclamate is avoided by rats at all concentrations above 0.01 M and preferred at none, a finding which is similar to that in pigs in the present study.

The discussion so far has been based upon the results obtained from long term preference tests and most of the work on other species has in- volved the use of such tests. However, long term preference tests possess several dis- advantages. Firstly, they involve the consumption of large volumes of fluid which result in significant post ingestion effects during the preference test. Secondly, preference is measured in terms of consumption of test substance relative to water, which can sometimes give mis- leading results. The first disadvantage can be minimized by the use of short term preference tests and in the present study a 1-hr exposure has also been used. In the short term preference tests it is possible to go to high concentrations of highly preferred test substances, such as


sugars, wi thout serious pos t ingest ion effects becoming apparent .

When highly preferred substances such as sucrose are considered i t is appa ren t that operan t condi t ioning techniques can be extremely useful. In the preference tests, i t can be seen tha t a consis tent high preference was exhibi ted for all concentra t ions of sucrose above 0.03 M. These results would seem to indicate tha t the reinforcing value o f 0.03, 0.06, 0.12 and 0.225 M solut ions o f sucrose were equivalent . However , examinat ion o f the volumes consumed (Fig. 2) reveals tha t the a m o u n t of solut ion consumed rose f rom 0.03 to the 0.225 M solutions. The operan t condi t ioning results conf i rmed the im- pression tha t the more highly concent ra ted solut ions were more at t ract ive to the pigs. Using progressive ra t io schedules o f reinforcement there was a significant corre la t ion between the final ra t io reached and the concent ra t ion of sucrose used as the reinforcer (Fig. 13). The results ob ta ined with 0.03 M sucrose are o f par - t icular interest because in the preference tests this concent ra t ion was highly preferred (Fig. 1) while the progressive ra t io exper iments demon- s t ra ted tha t the pigs were not highly mot iva ted to work for this concent ra t ion c o m p a r e d with s t ronger solutions. These results indicate tha t a l though preference tests are useful for es tab- l ishing preference and aversion thresholds and giving an overal l picture o f the behav iour towards a test substance they are no t sensitive enough to detect differences in the mot iva t ion for highly preferred substances.

W h e n operan t condi t ioning methods are used in the s tudy o f taste preferences care mus t be taken in the selection o f the me thod used. A l though G u t t m a n (1953) observed large differences in the response rate o f rats on var iable interval schedules of re inforcement when working for different concentra t ions o f sugar solu- t ions, in our exper iments this me thod was no t as sensitive as the progressive ra t io technique.

Acknowledgments One o f us, J .M.K. , wishes to acknowledge receipt o f an S.R.C. research t ra in ing grant. We are grateful to M r E. Wai ters for carrying ou t the statist ical analysis and to D r K. J. Hi l l of Uni lever for p rov id ing sodium saccharin and sodium cyclamate.

R E F E R E N C E S Aldinger, S. M., Speer, V. C., Hays, V. W. & Catron,

D. V. (1959). Effect of saccharin on consumption of starter rations by baby pigs. J. Anita. Sci., 18, 1350--1355.

Baldwin, B. A. & Ingrain, D. L. (1967). Behavioural thermoregulatioa in pigs. PhysioL Behav., 2, 15-21.

Baldwin, B. A. & Stephens, D. B. (1970). Operant conditioning procedures for producing emotional responses in pigs. J. Physiol., 210, 127-128P.

Bell, F. R. (1959). Preference thresholds for taste dis- criminafion in goats. J. agric. ScL, Camb., 52, 125-128.

Bell, F. R. & Williams, H. L. (1959). Threshold values for taste in monozygotic twin calves. Nature (Lond.), 183, 345-346.

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(Received 13 December 1971; revised 10 May 1972; MS. number: 1113)

taste preferences in pigs for nutritive and non-nutritive sweet solutions

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