Gustatory responsiveness to food-associated sugars and ... - CiteSeerX

Physiology & Behavior 70 (2000) 495 ± 504

Gustatory responsiveness to food-associated sugars and acids in pigtail macaques, Macaca nemestrina Matthias Laska* Department of Medical Psychology, University of Munich Medical School, Goethestrasse 31, D-80336 Munich, Germany Received 6 January 2000; received in revised form 3 March 2000; accepted 4 May 2000

Abstract Taste-preference thresholds for five food-associated sugars and acids, respectively, as well as relative sweet-taste preferences were assessed in six pigtail macaques using two-bottle choice tests of brief duration (1 min). In experiment 1, the animals were found to significantly prefer concentrations as low as 10 mM maltose and sucrose, 20 mM fructose and glucose, and 30 mM lactose over tap water. In experiment 2, the monkeys were given a choice between all binary combinations of the same five saccharides presented in equimolar concentrations of 50, 100, 200, and 400 mM. Preferences for individual sugars were stable across the concentrations tested and indicate the following order of relative effectiveness: maltose > sucrose > glucose fructose lactose. In experiment 3, Macaca nemestrina was found to significantly discriminate concentrations as low as 5 mM malic acid, 10 mM ascorbic acid, 20 mM citric acid and acetic acid, and 0.5 mM tannic acid from the alternative stimulus. With the latter substance, the monkeys rejected all suprathreshold concentrations tested, whereas with the former four substances, the animals showed an inverted U-shaped function of preference. The results showed pigtail macaques to be the first primate species tested so far whose taste-preference threshold for maltose is as low as that for sucrose, and which Ð similar to rodents Ð prefers maltose over equimolar concentrations of sucrose and other saccharides. Further, unlike most other primates, pigtail macaques do not generally reject acidic tastants but show a substance- and concentration-dependent change in their behavioral response that may range from rejection to preference. The results support the assumption that the gustatory responsiveness of M. nemestrina to food-associated sugars and acidic tastants might reflect an evolutionary adaptation to its dietary habits. D 2000 Elsevier Science Inc. All rights reserved. Keywords: Pigtail macaques; Macaca nemestrina; Gustatory performance; Taste-preference thresholds; Relative sweetness; Food-associated tastants

Macaques are one of the most widely used primate genera both for studies on gustatory neuroanatomy (e.g., Refs. [32,37]) and electrophysiology (e.g., Refs. [29,30,36, 43,44]). This should not be surprising given their close phylogenetic relationship to man and the apparent similarities in coding of taste quality as inferred from the patterns of neural activity recorded from the gustatory cortex in macaques and reports on human taste quality perception [44]. Surprisingly little, however, is known about the taste performance of macaques at the behavioral level. Most studies have either concentrated on detectability of the four basic taste qualities, usually using sucrose, sodium chloride, hydrochloric acid, and quinine hydrochloride as the only,

* Tel.: +49-89-5996-655; fax: +49-89-5996-615. E-mail address: [email protected] (M. Laska).

prototypic, stimuli [33], or have assessed the ability of macaques to respond to artificial sweeteners [6,27]. There is only sparse information as to the taste responsiveness of macaques for naturally occurring sugars and acids other than sucrose and HCl. Fruits and vegetative parts of plants that make up the food of non-human primates, however, usually contain a variety of substances termed sweet and sour by humans [26], and the food selection behavior of monkeys suggests that they may use gustatory cues, and the relative salience of sweetness and sourness in particular, to assess palatability and nutritional value of a potential food item [3]. It was therefore the aim of the present study to assess the gustatory responsiveness of pigtail macaques to five foodassociated sugars and acids, respectively, that are all important constituents of the natural diet in Macaca nemestrina [2,26]. For this, a two-bottle preference test of brief

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M. Laska / Physiology & Behavior 70 (2000) 495±504

duration was employed. This method makes it possible to determine preference thresholds Ð as a first and conservative approximation of the gustatory capacity of a species Ð as well as to directly measure relative preferences for or aversions to tastants. A comparison of such basic measures of taste performance in the pigtail macaque to those of other species also allowed me to address the question of whether the gustatory responsiveness of primates mirror an evolutionary adaptation to their respective dietary habits. 1. Methods 1.1. Animals Testing was carried out using three adult male and three adult female pigtail macaques M. nemestrina maintained as part of an established breeding colony of 10 animals. The colony was housed in a double enclosure comprising a 40m3 home cage joined to a 14-m3 test cage by a sliding door that could be closed to allow the temporary separation of animals for individual testing, and was maintained on a 12:12-h light/dark cycle at 22± 24°C. The six test subjects were trained to enter the test cage voluntarily and were completely accustomed to the procedure. Animals were fed commercial monkey chow, fresh fruits, and vegetables ad libitum, but were deprived of water overnight before testing on the following morning. The experiments reported here comply with the Guide for the Care and Use of Laboratory Animals (National Institutes of Health Publication no. 86-23, revised 1985) and also with current German laws. 1.2. Procedures In all experiments, a two-bottle choice test of brief duration was employed. Twice each day, approximately 2 and 1 h, respectively, before feeding, the animals were allowed 1 min to drink from a pair of simultaneously presented 750-ml cylinders with metal drinking tubes. In experiment 1, the monkeys were given the choice between tap water and defined concentrations of the saccharides sucrose, glucose, fructose, lactose, and maltose (reagent grade, Merck, Darmstadt) dissolved in tap water. For each of these substances testing started at a concentration of 200 mM and proceeded in the following steps (100, 50, 20, 10 mM, etc.) until the animals failed to show a significant preference. To keep up the animals' motivation and willingness to cooperate, testing did not follow a strict descending staircase procedure but followed a pseudo-randomized scheme in which trials with high and thus presumably readily perceptible and attractive concentrations of sugars were alternated with low and thus presumably less attractive concentrations. The order in which the five sugars were tested was the

same for all animals ((1) sucrose, (2) glucose, (3) fructose, (4) lactose, (5) maltose). In this, as well as in the following experiments, sugar solutions were allowed 24 h to mutarotate. In experiment 2, the monkeys were given the choice between all binary combinations of sucrose, fructose, glucose, lactose, and maltose presented in equimolar concentrations. In order to assess whether relative preferences are stable at different concentration levels, four test series were performed: at 50, 100, 200, and 400 mM, respectively. In experiment 3, the monkeys were given a choice between a 50-mM sucrose solution and defined concentrations of the organic acids ascorbic acid, citric acid, acetic acid, malic acid, and tannic acid (reagent grade, Merck) dissolved in a 50-mM sucrose solution. The use of a weak aqueous sucrose solution rather than water both as solvent for the tastants and as alternative stimulus was necessary because M. nemestrina was found to cooperate in tests of liquid consumption only as long as at least one of the bottles contained a sapid solution. A 50-mM sucrose solution is a factor of 5 above this species' preference threshold (cf. results of experiment 1) and thus clearly detectable for the pigtail macaques and sufficiently attractive to induce reliable liquid consumption. At the same time, this concentration is thought to be weak enough to prevent massive masking effects when applied in heterogeneous taste mixtures. With all acids except tannic acid, testing started at a concentration of 100 mM and proceeded in the following steps (50, 20, 10, 5, 2, 1 mM, etc.) until the animals failed to show a significant preference or aversion. With tannic acid, testing started at a concentration of 1 mM and proceeded in the following steps (0.5, 0.2, 0.1, 0.05 mM, etc.). Similar to the procedure adopted in experiment 1, testing did not follow a strict descending staircase procedure but followed a pseudo-randomized scheme in which trials with high and thus presumably readily perceptible concentrations of acids were alternated with low concentrations presumed to be difficult to perceive. The order in which the five acids were tested was the same for all animals ((1) citric acid, (2) ascorbic acid, (3) malic acid, (4) acetic acid, (5) tannic acid). In all three experiments, each pair of stimuli was presented 10 times, and the position of the stimuli was randomized in order to counterbalance possible position preferences. 1.3. Data analysis For each animal, the amount of liquid consumed from each bottle was recorded, summed for the 10 test trials with a given stimulus combination, converted to percentages (relative to the total amount of liquid consumed from both bottles), and 66.7% (i.e., 2/3 of the total amount of liquid consumed) was taken as criterion of preference. This rather conservative criterion was chosen for reasons of comparability of data as the same criterion had been used in


previous studies using the same method with other primate species [14 ±22], and in order to avoid misinterpretation of data due to a too liberal criterion. Additionally, two-tailed binomial tests [45] were performed and an animal was only regarded as significantly preferring one of the two alternative stimuli if it reached the criterion of 66.7%, and consumed more from the bottle containing the preferred stimulus in at least 8 out of 10 trials (binomial test, p < 0.05). Preliminary analysis of the data indicated that there were no reliable differences in choice behavior and liquid consumption between the male and the female subjects and between the first and the second presentation of the day. Intraindividual variability in the amount of liquid consumed across the 10 test trials with a given stimulus combination was low and averaged less than 20%. Thus, a theoretically possible bias in the overall preference score due to excessive drinking in aberrant trials did not occur. Therefore, the data for the males and the females obtained in the 10 test trials were combined and are reported as group means and standard deviations. Comparisons across tasks were made using the Friedman two-way analysis of variance (ANOVA). When ANOVA detected differences between tasks, this was then followed by separate pairwise Wilcoxon signed-rank tests for related samples to evaluate which tasks were involved. Preliminary tests showed that the animals rejected solutions containing high concentrations of the acids. Following convention, the results are nevertheless expressed as percentage preference for the tastant and not for the solvent. Accordingly, 33.3% (i.e., 1/3 of the total amount of liquid consumed) was taken as criterion of aversion. 2. Experiment 1: Taste-preference thresholds for foodassociated sugars 2.1. Results Fig. 1 shows the taste-preference thresholds of M. nemestrina to be 10 mM for sucrose and maltose, 20 mM for fructose and glucose, and 30 mM for lactose. All six monkeys significantly discriminated these concentrations from tap water, and in some cases single individuals even scored slightly lower preference threshold values. All animals, however, failed to show a significant preference for the lowest concentrations presented, suggesting that the preference for higher concentrations was indeed based on the chemical nature of the stimuli. In most cases, interindividual variability of scores was low for both sub- and suprathreshold concentrations tested (cf. SDs in Fig. 1). 2.2. Discussion Table 1 compares the taste-preference threshold values obtained in the present study with those obtained with the

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same or a similar method in other primate species, and with detection threshold values obtained for humans, using psychophysical procedures. For all five sugars tested M. nemestrina responded to slightly or even moderately lower concentrations compared to most other non-human primate species, and thus pigtail macaques appear to rank among the most sugar-sensitive primates investigated so far. The limited data available on sugar taste-preference thresholds in other members of the genus Macaca showed the pigtail macaque to take an intermediate position between the rhesus macaque [5] and the long-tailed macaque [33]. The taste-preference thresholds of M. nemestrina were also found to be as low as or even lower than detection thresholds established for humans [1]. This is remarkable considering that while the taste detection thresholds for humans have been established using sophisticated signal detection methods, the two-bottle preference test used in this study provides only a conservative approximation of a species' limits of gustatory capacity. This suggests that the sweet-taste sensitivity of pigtail macaques might indeed exceed that of humans as taste-preference thresholds tend to be higher than detection thresholds obtained with operant conditioning procedures [34] or with taste-aversion conditioning procedures [12]. The results also showed pigtail macaques to be the first primate species tested so far whose taste-preference threshold value for maltose is as low as that for sucrose that usually is the best detected food-associated saccharide in both human and non-human primates [5,15,18,22]. It is interesting to note that at all suprathreshold concentrations tested the animals' responsiveness to maltose (in terms of percentage of preference) was significantly higher compared to that for sucrose (Wilcoxon, p < 0.05) suggesting that the former might have a higher stimulating efficiency to M. nemestrina than the latter. A similar pattern of responsiveness has been reported in rodents such as rats, hamsters, gerbils, and spiny mice [4,42]. 3. Experiment 2: Relative taste preferences for foodassociated sugars 3.1. Results Fig. 2 shows the mean preference ( SD) of the six pigtail macaques given a choice between two saccharides presented at equimolar concentrations of 50, 100, 200, and 400 mM, respectively. At all four concentrations tested the monkeys significantly preferred maltose over all other sugars, and sucrose over fructose, glucose, and lactose. Choice tests between the latter three sugars did not yield significant preferences. However, at the two highest concentrations tested (200 and 400 mM) the animals showed a non-significant tendency to prefer fructose and glucose over lactose, and to prefer glucose over fructose. Interindi-

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Fig. 1. Taste responsiveness of six pigtail macaques to aqueous solutions of sucrose, maltose, glucose, fructose, and lactose tested against tap water. Each data point represents the mean value ( SD) of 10 test trials of 1 min per animal.

vidual variability was remarkably low as can be inferred from the small SDs, and with only few exceptions all six animals either reached the criterion of preference in a given task or all six animals failed to do so. Preferences for a given stimulus combination were rather stable across the range of concentrations tested. Accordingly, ANOVA only detected significant changes in the strength of preferences across concentrations for maltose vs. sucrose, sucrose vs. fructose, and fructose vs. glucose. The stability of prefer-

ences for individual saccharides is also reflected by the finding that no animal showed a change from preference to avoidance or vice versa between concentrations with a given stimulus combination. 3.2. Discussion Table 2 compares the relative preferences for individual sugars found in the present study with those obtained with

M. Laska / Physiology & Behavior 70 (2000) 495±504 Table 1 Taste-preference thresholds (in mM) for food-associated sugars in pigtail macaques and other primate species Species

Sucrose Fructose Glucose Lactose Maltose a

M. nemestrina P. hamadryas anubisb At. geoffroyic Saimiri sciureusd Cebuella pygmaeae Sa. midas nigere M. mulattae M. fascicularisf Cercopithecus pygerythruse Ce. nictitanse A. trivirgatuse Callithrix jacchuse Lemur mongoze Cheirogaleus mediuse Microcebus murinuse Loris tardigraduse Nycticebus coucange Galago senegalensise Homo sapiensg

10 10 3 10 33 66 6 30 11 11 17 25 125 143 167 50 330 66 10

20 20 15 40 50 66

40

20 25 20 90 100 330

80

30 20 10 100 125 >250

72

10 20 20 90

38

Study [1] established detection thresholds rather than preference thresholds. a Present study. b Ref. [21]. c Ref. [18]. d Ref. [15]. e Ref. [5]. f Ref. [33]. g Ref. [1].

the same or a similar method in other non-human primates and the rat, and with findings on relative sweetness obtained in humans, using psychophysical procedures. The results of this experiment confirm the conclusion drawn from experiment 1 that maltose has a higher stimulating efficiency for pigtail macaques than sucrose, glucose, fructose, and lactose, and that sucrose, in turn, is a behaviorally more potent stimulus than the remaining three saccharides. This pattern differs strikingly from the one found in squirrel monkeys [16], spider monkeys [19], and in humans [28] that all have been shown to prefer sucrose over (or to perceive it as sweeter than) equimolar concentrations of other food-associated saccharides. The superiority of maltose over sucrose in stimulating efficiency or attractiveness as found in the present study has repeatedly been observed in rats [4,35,41] and other rodent species such as hamsters, gerbils, and spiny mice [4]. The explanation proposed to underlie this phenomenon is that rodents are presumed to have additional taste receptors for starch-derived polysaccharides that are presumably responsive to maltose but not to sucrose, and that non-rodent species are thought to lack [41]. However, in line with our findings, another member of the genus Macaca, the bonnet macaque, has been reported to prefer maltose and polycose as strongly as sucrose in solution vs. water tests [48]. Although the authors measured relative preferences for these sugars indirectly, i.e., by

499

comparing relative amounts of consumption, they concluded that M. radiata, like rats, might have the above-mentioned additional taste receptors for starch-derived polysaccharides. Summated chorda tympani responses to stimulation with saccharides in M. fuscata yielded the following order of relative effectiveness in terms of magnitude of integrated responses: maltose > sucrose > glucose > fructose > lactose [40]. This order corresponds perfectly with our behavioral data in M. nemestrina whereas other primate species such as the squirrel monkey have been reported to show a higher responsiveness to sucrose compared to maltose both electrophysiologically [46] and behaviorally [16]. Thus, our findings lend further support to the assumption that members of the genus Macaca in general might differ from other primate species with regard to relative effectiveness of saccharides. 4. Experiment 3: Taste-preference thresholds for foodassociated acids 4.1. Results Fig. 3 shows the mean performance of five pigtail macaques in the two-bottle preference tests with a 50-mM sucrose solution used both as solvent for the acids and as the alternative stimulus. At the group level, the animals significantly discriminated concentrations as low as 5 mM malic acid, 10 mM ascorbic acid, 20 mM citric acid and acetic acid, and 0.5 mM tannic acid from the alternative stimulus. In most cases, interindividual variability of scores was low as can be inferred from the SDs in Fig. 3. Thus, at the individual level, the threshold values for a given substance only differed by a factor of 2, if at all, between subjects. With tannic acid, the monkeys failed to show a preference at the lowest concentrations tested and clearly rejected the higher concentrations. With ascorbic acid, citric acid, acetic acid, and malic acid, however, the animals showed an inverted U-shaped function of preference, i.e., they rejected high concentrations (100 mM), but significantly preferred low but detectable concentrations (between 5 and 10 or 20 mM) of these acidic tastants over the alternative sweet stimulus. With all five acids, the lowest concentrations presented were consumed at equal amounts compared to the alternative stimulus, suggesting that the avoidance of high concentrations as well as the preference for low but detectable concentrations of four of the acids was indeed based on the chemical nature of the stimuli. 4.2. Discussion Table 3 compares the taste-preference threshold values obtained in the present study with those obtained with the same or a similar method in other primate species, and with detection threshold values obtained for humans, using psychophysical procedures.

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Fig. 2. Relative taste preferences of six pigtail macaques given a choice between two saccharides presented at equimolar concentrations of 50, 100, 200, and 400 mM, respectively. Each bar represents the mean preference ( SD) for the sugar typed horizontally, relative to the sugar typed slopingly.

For the majority of acids tested M. nemestrina showed slightly or even moderately higher preference threshold values compared to most other non-human primate species, and thus pigtail macaques do not appear to rank among the most acid-sensitive primates investigated so far. With the exception of a study by Pritchard et al. [33] who reported that long-tailed macaques responded indifferently to concentrations of HCl ranging from 0.3 to 30 mM in a single-bottle preference test, there are no behavioral data available on acid taste responsiveness in other members of the genus Macaca. Single cell recordings from the gustatory cortex in M. fascicularis, however, showed the long-tailed macaque to reliably respond to 10 mM solutions of all five acids used in the present study, but no response threshold values were determined [30]. The most remarkable finding of this experiment, however, is that, unlike most other primate species tested so far [5,17,20], M. nemestrina did not generally reject acidic tastants but showed a concentration-dependent change in responsiveness to ascorbic acid, citric acid, acetic acid, and malic acid that ranged from rejection to preference. So far,

only two New World primate species, the owl monkey, Aotus trivirgatus, and the spider monkey, Ateles geoffroyi, have been reported to prefer citric acid and acetic acid Ð and thus the same substances as found here Ð over an Table 2 Relative taste preferences for food-associated sugars in pigtail macaques and other species M. nemestrinaa S. sciureusb At. geoffroyic H. sapiensd Rattus norvegicuse R. norvegicusf

maltose > sucrose > glucose fructose lactose sucrose > fructose > glucose maltose lactose sucrose > fructose > glucose lactose maltose sucrose > fructose > maltose glucose lactose maltose > sucrose = glucose > lactose maltose > sucrose > glucose = fructose

Study [28] established relative sweetness of sugars rather than relative taste preferences. a Present study. b Ref. [16]. c Ref. [19]. d Ref. [28]. e Ref. [35]. f Ref. [41].


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Fig. 3. Taste responsiveness of five pigtail macaques to ascorbic acid, citric acid, acetic acid, malic acid, and tannic acid dissolved in a 50-mM sucrose solution and tested against a 50-mM sucrose solution as alternative stimulus. Each data point represents the mean value ( SD) of 10 test trials of 1 min per animal.

alternative stimulus at low but detectable concentrations in two-bottle choice tests [7,20]. The possibility that the use of a 50-mM sucrose solution both as the solvent for the tastants and as the alternative stimulus might have contributed to this unusual pattern of performance has to be taken into consideration, particularly as it is well established that masking effects or other types of qualitative interactions may occur when using heterogeneous taste mixtures [11,28]. However, in a

comprehensive human psychophysical study on detection of tastes in heterogeneous binary mixtures, Stevens [47] showed that sucrose concentrations of a factor of 10, respectively, 100, above threshold elevated the detection threshold for citric acid in a mixture only by a factor of 2, respectively, 4. Further, he reported that tastants termed sweet and sour by humans mask each other much less efficiently compared to other combinations of tastants. This latter finding is supported by findings from electro-

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Table 3 Taste-preference thresholds (in mM) for food-associated acids in pigtail macaques and other primate species Species M. nemestrinaa At. geoffroyib S. sciureusc Mi. murinusd L. tardigradusd N. coucangd G. senegalensisd C. pygmaead Sa. midas nigerd A. trivirgatusd H. sapiense

Ascorbic Citric acid acid

Malic acid

Acetic acid

Tannic acid

10 5 10

5 10 5

20 5 10 8 17 6 4 6 4 8 0.8

0.5 0.1 0.1

1

20 5 5

6 4 4 0.4

0.4

effects. Nevertheless, it would be interesting to directly compare water as solvent and sucrose as solvent in future experiments with pigtail macaques. This, however, would require to find an experimental means of testing both conditions with the same individual animals Ð which was not feasible with the method and animals employed here. Yet the present results can be taken as a first and conservative approximation of the responsiveness of pigtail macaques to acidic tastants. 5. General discussion

0.006

The present study and Ref. [20] used an aqueous sugar solution rather than water as solvent. Ref. [17] used both water and an aqueous sugar solution as solvent, both yielding the same preference threshold values for all acids tested. a Present study. b Ref. [20]. c Ref. [17]. d Ref. [5]. e Ref. [1].

physiological recordings in the gustatory cortex of M. fascicularis; Plata-Salaman et al. [31] showed that in binary taste mixtures sourness induced by HCl was suppressed least among the four basic taste qualities. They concluded that sour-best cells in primates are less susceptible to mixture suppression than other taste cell subtypes. Given that the concentration of sucrose used in the present study was only a factor of 5 above this species' preference threshold (cf. experiment 1) it seems reasonable to assume that masking or suppression effects may have distorted both the preference threshold values and the form of the suprathreshold preference functions reported here only little, if at all. This latter supposition is supported by findings from Gregson and McCowen [10] who investigated changes in the perception of sweetness and sourness with series of sucrose ± citric acid mixtures at near-threshold intensities. They reported that changes in citric acid concentration were generally perceived by some tasters to slightly increase sweetness and by other tasters to slightly decrease sweetness. Similarly, changes in sucrose concentration slightly increased or decreased perceived sourness. The authors concluded that mixture interactions between sucrose and citric acid were generally small when using near-threshold concentrations. Further, a previous study has shown preference threshold values and suprathreshold preference functions in squirrel monkeys using the same acidic tastants and method as in the present study to be identical to those obtained in corresponding tests with the same individual animals and using water as the solvent [17]. There is no reason to believe that squirrel monkeys differ fundamentally from pigtail macaques with regard to theoretically possible taste interaction

The present study sought to assess the gustatory responsiveness of pigtail macaques to food-associated sugars and acids. Three main findings emerged from the two-alternative choice tests employed. First, pigtail macaques are the only primate species tested so far whose taste-preference threshold for maltose is as low as that for sucrose. Second, M. nemestrina differs from all other primate species tested so far in that they actually prefer maltose over equimolar concentrations of sucrose. Third, unlike most other primates, pigtail macaques do not generally reject acidic tastants but show a substance- and concentration-dependent change in their behavioral response that may range from rejection to preference. The question arises as to possible reasons that might underlie this rather uncommon pattern of taste responsiveness. Although there is common agreement that macaques include an extraordinary variety of foods in their diet and may thus be regarded as feeding generalists [39], there are also reports that the proportion of fruits in the diet of M. nemestrina may be as high as 74% [2] that would make pigtail macaques not only the most highly frugivorous member of the genus Macaca but also rank them among the most frugivorous primates investigated so far [38]. The concentrations of sucrose, fructose, and glucose present in ripe tropical fruits have been found to be considerably higher in general than the preference thresholds found here [26] and thus they should contribute substantially to the taste sensation while feeding on fruits. Jenness and Sloan [13] reported the lactose content of the milk of macaques to range from 6.2 to 7.9 g/100 ml, corresponding to a concentration of 170 to 220 mM, which according to the findings of this study is likewise well above threshold and thus easily perceptible for M. nemestrina. The remarkably high sensitivity of pigtail macaques for maltose found here suggests that this carbohydrate is very likely to add to the taste sensation while feeding on starchy plants. As a considerable proportion of the 104 genera of plants that are known to be part of this species' diet in the wild [2] contains high amounts of starch, it seems reasonable to assume that both the high sensitivity of M. nemestrina for maltose and its marked preference for this disaccharide may represent an evolutionary adaptation of its gustatory system to the nutritional assessment of starchy plants. This supposition is


supported by the fact that macaque species possess cheek pouches that are used for salivary predigestion of starch [39] and that frugivorous New World primates, such as the spider monkey, lack. However, the olive baboon, Papio hamadryas anubis, another Old World primate species that feeds on fruits as well as on starchy plants and also possesses cheek pouches has been shown to be less sensitive to maltose compared to sucrose [22] and to clearly prefer sucrose over equimolar concentrations of maltose (Laska, unpublished data). This raises the possibility that the high degree of responsiveness to maltose both in terms of preference thresholds and in terms of relative saccharide preferences found here may be restricted to members of the genus Macaca. However, in order to further corroborate this hypothesis it is clearly important to assess the gustatory responsiveness to food-associated sugars in other macaque species. In line with the results found with the sugars tested, the acid concentrations detected by the pigtail macaques are well below those present in most fruits or other parts of plants that are consumed by this species [26]. Thus, it seems likely that the sourness and/or astringency elicited by the acids in the natural diet of M. nemestrina enter into its food selection behavior. The preference shown by the pigtail macaques for low but detectable concentrations of foodassociated acids might also reflect an evolutionary adaptation to their frugivory. Preferring or at least accepting levels of acidic tastants that other species reject could help to avoid competition pressure between sympatric frugivores for carbohydrate-rich fruits or might even give a species an advantage over competitors in the quest for fruits that are nutritionally valuable but still unripe and low in abundance. In their natural habitat, pigtail macaques have been shown to compete for food with white-handed gibbons, Hylobates lar, and with Indonesian leaf monkeys, Presbytis potenziani [39]. Whereas the latter two species do not include unripe fruits into their diets, pigtail macaques have been reported to readily consume unripe Ficus fruits [2]. Interestingly, unripe Ficus fruits have been shown to contain 15 mM of ascorbic acid [25], i.e., a concentration that M. nemestrina clearly preferred in the present study (cf. Fig. 3), whereas species such as the squirrel monkey that specialize on ripe fruits rejected this concentration of ascorbic acid in two-bottle preference tests [17]. Ripe Ficus fruits, on the contrary, contain only 4 mM of ascorbic acid [25], a concentration that is lower than any preference or aversion threshold reported for any non-human primate species so far (cf. Table 3), thus lending support to the idea mentioned above. However, inverted U-shaped functions of preference for food-associated acids have also been described in humans and some non-primate mammal species such as cattle, sheep, and goats that do not specialize on fruits [9], and thus this phenomenon clearly needs further clarification. It should be emphasized that the preference for low but detectable concentrations of some acidic tastants observed here was found in all individual animals tested (cf. SDs in Fig. 3). Thus it seems unlikely that mechanisms such as the

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Schweppes effect, coined by Glendinning [8] to describe the phenomenon that many but not all humans show a preference for the bitter and astringent taste of Schweppes Tonic Water2, may underlie the finding of the present study as interindividual variability in hedonic response of the pigtail macaques to a given concentration of an acidic tastant was very small. A final aspect of the present study is the question as to possible mechanisms that may account for the observed differences in the gustatory responsiveness of M. nemestrina to ascorbic acid, citric acid, acetic acid, and malic acid compared to tannic acid. Plata-Salaman et al. [30] reported that the response profiles of single neurons in the insular cortex of the long-tailed macaque, M. fascicularis, to citric acid, ascorbic acid, acetic acid, and malic acid fell into a cluster in a two-dimensional taste space whereas tannic acid differed sharply from the other acids implying a differing quality. Human psychophysical studies support this view as here, too, tannic acid is reported to elicit a characteristic astringent and Ð at high concentrations Ð bitter taste [23]. Furthermore, tannins have been shown to inhibit feeding in several primate species including M. mulatta [24], probably due to their negative physiological effects on digestibility of proteins. Thus, it seems likely that oral astringency and/or bitterness leads to a rejection response in non-human primates. Taken together, the results of the present study support the assumption that the gustatory responsiveness of M. nemestrina to food-associated sugars and acids might reflect an evolutionary adaptation to its dietary habits. Acknowledgments I would like to thank Christine Aumann, SoÈren Herzfeld, Andreas Russwurm, and Anja Schimana for help in collecting data, and the Deutsche Forschungsgemeinschaft for financial support (La 635/6-2).

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