Does the number of functional olfactory receptor genes predict olfactory sensitivity and discrimination performance in mammals?

The number of functional genes coding for olfactory receptors differs markedly between species and has repeatedly been suggested to be predictive of a species' olfactory capabilities. To test this assumption, we compiled a database of all published olfactory detection threshold values in mammals and used three sets of data on olfactory discrimination performance that employed the same structurally related monomolecular odour pairs with different mammal species. We extracted the number of functional olfactory receptor genes of the 20 mammal species for which we found data on olfactory sensitivity and/or olfactory discrimination performance from the Chordata Olfactory Receptor Database. We found that the overall olfactory detection thresholds significantly correlate with the number of functional olfactory receptor genes. Similarly, the overall proportion of successfully discriminated monomolecular odour pairs significantly correlates with the number of functional olfactory receptor genes. These results provide the first statistically robust evidence for the relationship between olfactory capabilities and their genomics correlates. However, when analysed individually, of the 44 monomolecular odourants for which data on olfactory sensitivity from at least five mammal species are available, only five yielded a significant correlation between olfactory detection thresholds and the number of functional olfactory receptors genes. Also, for the olfactory discrimination performance, no significant correlation was found for any of the 74 relationships between the proportion of successfully discriminated monomolecular odour pairs and the number of functional olfactory receptor genes. While only a rather limited amount of data on olfactory detection thresholds and olfactory discrimination scores in a rather limited number of mammal species is available so far, we conclude that the number of functional olfactory receptor genes may be a predictor of olfactory sensitivity and discrimination performance in mammals.

Visual detection and fruit selection by the mantled howler monkey (Alouatta palliata).

Howler monkeys (platyrrhini) have evolved routine trichromatic color vision independently from catarrhines, which presents an opportunity to test hypotheses concerning the adaptive value of distinguishing reddish from greenish hues. A longstanding hypothesis posits that trichromacy aids in the efficient detection of reddish-ripe fruits, which could be an advantage for the detection of the nutritional content of the fruit, such as sugars. In the present study, we assessed fruit visual conspicuity and selection based on color and sucrose content by wild mantled howler monkeys (Alouatta palliata) on Agaltepec Island, Mexico. We used colorimetry to classify dietary fruits as cryptic (greenish) or conspicuous (reddish) against their background leaves. Species-specific color models indicate that trichromatic howler monkeys should be more efficient in discriminating the conspicuous ripe fruits from leaves compared to detecting cryptic ripe fruits from leaves. We found howler monkeys consume more cryptic fruits compared to conspicuous fruits, and that they consume more unripe fruits than ripe fruits. The consumption (acceptance) of fruit was independent of sucrose content, and thus this disaccharide may not play an essential role in mantled howler food selection. Our findings suggest that routine trichromatic color vision may aid in the detection and discrimination of conspicuously colored fruits, but that the final decision whether to accept or reject a fruit probably involves the use of other senses in addition to vision.

Taste Responsiveness of Spider Monkeys to Dietary Ethanol.

Recent studies suggest that frugivorous primates might display a preference for the ethanol produced by microbia in overripe, fermenting fruit as an additional source of calories. We, therefore, assessed the taste responsiveness of 8 spider monkeys (Ateles geoffroyi) to the range of ethanol concentrations found in overripe, fermenting fruit (0.05-3.0%) and determined taste preference thresholds as well as relative taste preferences for ethanol presented in sucrose solutions and in fruit matrices, respectively. Using a 2-bottle preference test of short duration (1 min), we found that spider monkeys are able to detect ethanol concentrations as low as 0.5%, that they prefer ethanol concentrations up to 3% over water, and that they prefer sucrose solutions and pureed fruit spiked with ethanol over equimolar sucrose solutions and pureed fruit without ethanol. However, when presented with an ethanol-spiked sucrose solution and a higher-concentrated sucrose solution without ethanol, the animals clearly preferred the latter, even when the sucrose-ethanol mixture contained 3 times more calories. These results demonstrate that spider monkeys are more sensitive to the taste of ethanol than rats and humans and that they prefer ecologically relevant suprathreshold concentrations of ethanol over water. Tests with sucrose solutions and pureed fruits that were either spiked with ethanol or not suggest that sweetness may be more important for the preferences displayed by the spider monkeys than the calories provided by ethanol. The present results, therefore, do not support the notion that dietary ethanol might be used by frugivorous primates as a supplemental source of calories.

Olfactory sensitivity for mold-associated odorants in CD-1 mice and spider monkeys.

Using operant conditioning procedures, we assessed the olfactory sensitivity of six CD-1 mice and three spider monkeys for mold-associated odorants. We found that with all eight stimuli, the mice detected concentrations as low as 0.1 ppm (parts per million), and with two of them individual animals even detected concentrations as low as 1 ppt (parts per trillion). The spider monkeys detected concentrations as low as 4 ppm with all eight stimuli, and with four of them individual animals even detected concentrations as low as 4 ppb (parts per billion). Between-species comparisons showed that with all eight odorants, the mice displayed significantly lower threshold values, that is, a higher sensitivity than the spider monkeys, but not than human subjects tested in previous studies. Analysis of odor structure-activity relationships showed that in both species, the type of oxygen-containing functional group and the presence versus absence of a double bond as well as the length of the carbon backbone of the odor stimuli had a systematic effect on detectability. We conclude that both mice and spider monkeys are clearly able to detect the presence of molds and thus to assess the palatability of potential food using the volatiles produced by molds during putrefaction.

Olfactory Sensitivity for the Mammalian Blood Odor Component Trans-4,5-epoxy-(E)-2-decenal in CD-1 Mice.

Using a conditioning paradigm and an automated olfactometer, we investigated the olfactory sensitivity of CD-1 mice for the mammalian blood odor component trans-4,5-epoxy-(E)-2-decenal. We found that two of the animals significantly discriminated concentrations down to 3.0 ppt (parts per trillion) from the solvent, and three animals even successfully detected dilutions as low as 0.3 ppt. Intraspecific comparisons between the olfactory detection thresholds obtained here with those obtained in earlier studies with other odorants show that mice are extraordinarily sensitive to this blood odor component. Interspecific comparisons of olfactory detection thresholds show that human subjects are even more sensitive to trans-4,5-epoxy-(E)-2-decenal than the mice tested here. Both intra- and inter-specific comparisons suggest that neither neuroanatomical properties such as the size of the olfactory epithelium, the total number of olfactory receptor neurons, or the size of olfactory brain structures, nor genetic properties such as the number of functional olfactory receptor genes or the proportion of functional relative to the total number of olfactory receptor genes allow us to reliably predict a species' olfactory sensitivity. In contrast, the results support the notion that the behavioral relevance of an odorant rather than neuroanatomical or genetic properties may determine a species' olfactory sensitivity.

Behavioral Responses of CD-1 Mice to Six Predator Odor Components.

Mammalian prey species are able to detect predator odors and to display appropriate defensive behavior. However, there is only limited knowledge about whether single compounds of predator odors are sufficient to elicit such behavior. Therefore, we assessed if predator-naïve CD-1 mice (n = 60) avoid sulfur-containing compounds that are characteristic components of natural predator odors and/or display other indicators of anxiety. A 2-compartment test arena was used to assess approach/avoidance behavior, general motor activity, and the number of fecal pellets excreted when the animals were presented with 1 of 6 predator odor components in one compartment and a blank control in the other compartment. We found that 2 of the 6 predator odor components (2-propylthietane and 3-methyl-1-butanethiol) were significantly avoided by the mice. The remaining 4 predator odor components (2,2-dimethylthietane, 3-mercapto-3-methylbutan-1-ol, 3-mercapto-3-methylbutyl-1-formate, and methyl-2-phenylethyl sulphide) as well as a nonpredator-associated fruity odor (n-pentyl acetate) were not avoided. Neither the general motor activity nor the number of excreted fecal pellets, both widely used measures of stress- or anxiety-related behavior, were systematically affected by any of the odorants tested. Further, we found that small changes in the molecular structure of a predator odor component can have a marked effect on its behavioral significance as 2-propylthietane was significantly avoided by the mice whereas the structurally related 2,2-dimethylthietane was not. We conclude that sulfur-containing volatiles identified as characteristic components of the urine, feces, and anal gland secretions of mammalian predators can be, but are not necessarily sufficient to elicit defensive behaviors in a mammalian prey species.

Olfactory Discrimination Learning in an Outbred and an Inbred Strain of Mice.

The present study compared olfactory discrimination learning in CD-1 mice, a widely used outbred strain of mice with that of C57BL/6J mice, one of the most widely used inbred mouse strains. Using an automated olfactometer and a standard operant conditioning procedure, I found that CD-1 mice needed 60 trials to reach learning criterion in an initial 2-odor discrimination task. They improved in learning speed in subsequent discrimination tasks in which either the rewarded or the unrewarded stimulus was replaced for a new stimulus. C57BL/6J mice, in contrast, needed 120 trials to reach learning criterion in an initial 2-odor discrimination task and also needed significantly more trials than the CD-1 mice in 3 of the 4 subsequent discrimination tasks. Further, the results showed that discrimination learning performance of both mouse strains was largely unaffected by the odor stimuli used. The results of the present study demonstrate differences between an outbred and an inbred strain of mice with regard to odor discrimination learning, a classical measure of cognitive performance in comparative psychology. Thus, they emphasize the need to be careful with generalizing statements as to cognitive or sensory abilities of Mus musculus when inbred strains of mice are used.

Olfactory sensitivity and odor structure-activity relationships for aliphatic ketones in CD-1 mice.

Using a conditioning paradigm, the olfactory sensitivity of CD-1 mice for a homologous series of aliphatic 2-ketones (2-butanone to 2-nonanone) and several of their isomeric forms was investigated. With all 11 odorants, the animals significantly discriminated concentrations as low as 0.01 ppm (parts per million) from the solvent, and with two odorants (2-octanone and 5-nonanone), the best-scoring animals even detected concentrations as low as 3 ppt (parts per trillion). Analysis of odor structure-activity relationships showed that the correlation between olfactory detection thresholds of the mice for the 2-ketones and carbon chain length can best be described as a U-shaped function with the lowest threshold values at 2-octanone. Similarly, the correlation between olfactory sensitivity and carbon chain length of symmetrical ketones (3-pentanone to 6-undecanone) can best be described as a U-shaped function. In contrast, no significant correlation was found between olfactory detection thresholds of the mice and position of the functional carbonyl group attached to a C7 backbone. A comparison between the olfactory detection thresholds obtained here with those obtained in earlier studies suggests that mice are significantly more sensitive for 2-ketones than for n-carboxylic acids of the same carbon chain length. Across-species comparisons suggest that mice are significantly more sensitive for aliphatic ketones than squirrel monkeys and pigtail macaques, whereas the ranges of human olfactory detection threshold values overlap with those of the mice with seven of the 11 ketones tested. Further comparisons suggest that odor structure-activity relationships are both substance class and species specific.

Geosmin, a Food- and Water-Deteriorating Sesquiterpenoid and Ambivalent Semiochemical, Activates Evolutionary Conserved Receptor OR11A1.

Geosmin, a ubiquitous volatile sesquiterpenoid of microbiological origin, is causative for deteriorating the quality of many foods, beverages, and drinking water, by eliciting an undesirable "earthy/musty" off-flavor. Moreover, and across species from worm to human, geosmin is a volatile, chemosensory trigger of both avoidance and attraction behaviors, suggesting its role as semiochemical. Volatiles typically are detected by chemosensory receptors of the nose, which have evolved to best detect ecologically relevant food-related odorants and semiochemicals. An insect receptor for geosmin was recently identified in flies. A human geosmin-selective receptor, however, has been elusive. Here, we report on the identification and characterization of a human odorant receptor for geosmin, with its function being conserved in orthologs across six mammalian species. Notably, the receptor from the desert-dwelling kangaroo rat showed a more than 100-fold higher sensitivity compared to its human ortholog and detected geosmin at low nmol/L concentrations in extracts from geosmin-producing actinomycetes.

Food preferences and nutrient composition in captive Southern brown howler monkeys, Alouatta guariba clamitans.

Studies of food preferences in captive primates have so far mainly been restricted to frugivorous species. It was therefore the aim of the present study to assess the occurrence of spontaneous food preferences in a mainly folivorous primate, the captive Southern brown howler monkey, and to analyze whether these preferences correlate with nutrient composition. Using a two-alternative choice test, we presented ten male and five female adult Alouatta guariba clamitans with all possible binary combinations of ten types of food that are part of their diet in captivity and recorded their choice behavior. We found the howler monkeys to display the following rank order of preference: banana > mango > watermelon > papaya > beetroot > apple > pear > orange > cucumber > tomato. This preference ranking significantly and positively correlated with the total carbohydrate content and with the sucrose content of the food items. We also found significant positive correlations between the food preference ranking and the content of the minerals copper and magnesium. Male and female howler monkeys did not differ significantly in their food preference rankings. These results suggest this howler monkeys under human care are not opportunistic, but selective feeders with regard to maximizing their net gain of energy as only the content of carbohydrates, but not the contents of total energy, proteins, or lipids significantly correlated with the displayed food preferences. Thus, the food preferences of this primate are similar to those reported in several species of frugivorous primates tested with cultivated fruits and vegetables.

Olfactory discrimination ability of Asian elephants (Elephas maximus) for structurally related odorants.

Using a food-rewarded two-choice instrumental conditioning paradigm, we assessed the ability of Asian elephants, Elephas maximus, to discriminate between 2 sets of structurally related odorants. We found that the animals successfully discriminated between all 12 odor pairs involving members of homologous series of aliphatic 1-alcohols, n-aldehydes, 2-ketones, and n-carboxylic acids even when the stimuli differed from each other by only 1 carbon. With all 4 chemical classes, the elephants displayed a positive correlation between discrimination performance and structural similarity of odorants in terms of differences in carbon chain length. The animals also successfully discriminated between all 12 enantiomeric odor pairs tested. An analysis of odor structure-activity relationships suggests that a combination of molecular structural properties rather than a single molecular feature may be responsible for the discriminability of enantiomers. Compared with other species tested previously on the same sets of odor pairs (or on subsets thereof), the Asian elephants performed at least as well as mice and clearly better than human subjects, squirrel monkeys, pigtail macaques, South African fur seals, and honeybees. Further comparisons suggest that neither the relative nor the absolute size of the olfactory bulbs appear to be reliable predictors of between-species differences in olfactory discrimination capabilities. In contrast, we found a positive correlation between the number of functional olfactory receptor genes and the proportion of discriminable enantiomeric odor pairs. Taken together, the results of the present study support the notion that the sense of smell may play an important role in regulating the behavior of Asian elephants.

Olfactory discrimination ability of South African fur seals (Arctocephalus pusillus) for enantiomers.

Using a food-rewarded two-choice instrumental conditioning paradigm we assessed the ability of South African fur seals, Arctocephalus pusillus, to discriminate between 12 enantiomeric odor pairs. The results demonstrate that the fur seals as a group were able to discriminate between the optical isomers of carvone, dihydrocarvone, dihydrocarveol, menthol, limonene oxide, α-pinene, fenchone (all p < 0.01), and ß-citronellol (p < 0.05), whereas they failed to distinguish between the (+)- and (-)-forms of limonene, isopulegol, rose oxide, and camphor (all p > 0.05). An analysis of odor structure-activity relationships suggests that a combination of molecular structural properties rather than a single molecular feature may be responsible for the discriminability of enantiomeric odor pairs. A comparison between the discrimination performance of the fur seals and that of other species tested previously on the same set of enantiomers (or subsets thereof) suggests that the olfactory discrimination capabilities of this marine mammal are surprisingly well developed and not generally inferior to that of terrestrial mammals such as human subjects and non-human primates. Further, comparisons suggest that neither the relative nor the absolute size of the olfactory bulbs appear to be reliable predictors of between-species differences in olfactory discrimination capabilities. Taken together, the results of the present study support the notion that the sense of smell may play an important and hitherto underestimated role in regulating the behavior of fur seals.

Is sugar as sweet to the palate as seeds are appetizing to the belly? Taste responsiveness to five food-associated carbohydrates in zoo-housed white-faced sakis, Pithecia pithecia.

Differences in taste perception between species are thought to reflect evolutionary adaptations to dietary specialization. White-faced sakis (Pithecia pithecia) are commonly considered as frugivores but are unusual among primates as they do not serve as seed dispersers but rather prey upon the seeds of the fruits they consume and are thought to exploit the lipids and proteins that these seeds contain in high amounts. Using a two-bottle preference test of short duration we therefore assessed whether this dietary specialization affects the taste responsiveness of four adult white-faced sakis for five food-associated carbohydrates. We found that the sakis significantly preferred concentrations as low as 10 mM sucrose, 10-40 mM fructose, 20-30 mM glucose and maltose, and 30-40 mM lactose over tap water. When given the choice between all binary combinations of these five saccharides presented at equimolar concentrations of 100, 200, and 300 mM, respectively, the sakis displayed significant preferences for individual saccharides in the following order: sucrose > fructose > glucose ≥ maltose = lactose. These results demonstrate that seed-predating white-faced sakis have a well-developed taste sensitivity for food-associated carbohydrates which is not inferior to that of most other primates including seed-dispersing frugivores, but rather ranks among the more sweet-taste sensitive species. Further, they show that their pattern of relative preference for the five carbohydrates is similar to that found in other frugivorous primate species. These findings may represent an example of Liem's paradox as the sakis' morphological adaptations to efficiently predate on and exploit the lipid- and protein-rich hard-shelled seeds of fruits does not compromise their ability to detect the carbohydrates found in the pulp of fruits at low concentrations.

Olfactory detectability of L-amino acids in the European honeybee (Apis mellifera).

The honeybee is one of several insect model systems for the study of olfaction, yet our knowledge regarding the spectrum of odorants detectable by Apis mellifera is limited. One class of odorants that has never been tested so far are the amino acids, which are important constituents of floral nectar. Using the proboscis extension response paradigm, we assessed whether the odor of amino acids is detectable for honeybees and determined olfactory detection thresholds for those amino acids that were detectable. We found that honeybees are able to detect the odor of 5 of the 20 proteinogenic amino acids when presented at a concentration of 50 or 100 mM. Median olfactory detection thresholds for these 5 amino acids were 12.5 mM with L-tyrosine and L-cysteine, 50 mM with L-tryptophan and L-asparagine, and 100 mM with L-proline. All detection thresholds were much higher than reported concentrations of amino acids in floral nectars. We conclude that in the foraging and feeding context, honeybees are likely to detect amino acids through taste rather than olfaction. Across-species comparisons of the detectability of and sensitivity to amino acids suggest that the number of functional genes coding for olfactory receptors may affect both a species' sensitivity for odorants and the breadth of its spectrum of detectable odorants.

Olfactory sensitivity for six amino acids: a comparative study in CD-1 mice and spider monkeys.

Using a conditioning paradigm, the olfactory sensitivity of five CD-1 mice for the L- and D-forms of cysteine, methionine, and proline was investigated. With all six stimuli, the animals discriminated concentrations ≤ 0.1 ppm (parts per million) from the odorless solvent, and with three of the six stimuli the best-scoring animals were even able to detect concentrations <0.1 ppb (parts per billion). Three spider monkeys tested in parallel were found to detect the same six stimuli at concentrations <1 ppm, and with four of the six stimuli the best-scoring animals detected concentrations ≤ 1 ppb. Both CD-1 mice and spider monkeys displayed a higher olfactory sensitivity with the L- and D-forms of cysteine and methionine than with the prolines, suggesting an important role of the sulfur-containing functional groups for detectability. Accordingly, the across-odorant patterns of detection thresholds obtained with mice and spider monkeys showed a significant positive correlation. A comparison of the detection thresholds between the two species tested here and those obtained in human subjects suggests that neither the number of functional olfactory receptor genes nor the absolute or the relative size of the olfactory bulbs reliably predicts a species' olfactory sensitivity for amino acids.

Numerical cognition in black-handed spider monkeys (Ateles geoffroyi).

We assessed two aspects of numerical cognition in a group of nine captive spider monkeys (Ateles geoffroyi). Petri dishes with varying amounts of food were used to assess relative quantity discrimination, and boxes fitted with dotted cards were used to assess discrete number discrimination with equally-sized dots and various-sized dots, respectively. We found that all animals succeeded in all three tasks and, as a group, reached the learning criterion of 70% correct responses within 110 trials in the quantity discrimination task, 160 trials in the numerosity task with equally-sized dots, and 30 trials in the numerosity task with various-sized dots. In all three tasks, the animals displayed a significant correlation between performance in terms of success rate and task difficulty in terms of numerical similarity of the stimuli and thus a ratio effect. The spider monkeys performed clearly better compared to strepsirrhine, catarrhine, and other platyrrhine primates tested previously on both types of numerical cognition tasks and at the same level as chimpanzees, bonobos, and orangutans. Our results support the notion that ecological traits such as a high degree of frugivory and/or social traits such as a high degree of fission-fusion dynamics may underlie between-species differences in cognitive abilities.

Non-visual senses in fruit selection by the mantled howler monkey (Alouatta palliata).

There is extensive knowledge about the visual system and the implications of the evolution of trichromatic color vision in howler monkeys (genus Alouatta) related to food selection; however, information about the other sensory systems is limited. In this study we assessed the use of touch, sniffing, and taste in fruit evaluation by 20 adult mantled howler monkeys (Alouatta palliata) on Agaltepec Island, Mexico. During 9 months of observation, we recorded the frequency that each monkey used touch, sniffing, and taste in evaluating cryptic fruits (that remain green during their ripening process) and conspicuous fruits (with red, yellow, or orange colorations when they are ripe). Sucrose content and hardness measurements were made to establish the degree of ripeness of the fruits. We found that mantled howler monkeys used long behavioral sequences during conspicuous fruit investigations. Sniffing was used infrequently, but significantly more often in the evaluation of conspicuous-ripe and unripe fruits compared to cryptic-ripe and unripe fruits. During the evaluation of cryptic-ripe fruits, mantled howler monkeys increased the use of touch compared to evaluating cryptic-unripe fruits. We did not find significant differences in the use of taste in the evaluation of cryptic and conspicuous fruits (both ripe and unripe). Our results suggest that the non-visual senses play an essential role in fruit selection by howler monkeys, with differences in the behavioral strategy according to the fruit's conspicuity. The multimodal signals of ripe and unripe fruits allow the howler monkeys to assess their palatability before being consumed.

Influence of tannic acid concentration on the physicochemical characteristics of saliva of spider monkeys (Ateles geoffroyi).

Tannins are a chemical defense mechanism of plants consumed by herbivores. Variations in salivary physicochemical characteristics such as pH, total protein concentration (TP), and presence of proline-rich proteins (PRPs) in animals have been reported as a mechanism to protect the oral cavity when consuming food with variations in pH and tannins. Variations in salivary physiochemistry as adaptations for consuming tannin-rich foods have been found in omnivorous and folivorous primates, but have not yet been reported in frugivorous species such as spider monkeys. We therefore assessed changes in pH using test strips, TP concentration by measuring absorbance at 595 nm in a spectrophotometer and salivary PRPs using the SDS-PAGE electrophoresis technique in the saliva of nine captive spider monkeys in response to the consumption of solutions with different concentrations of tannic acid. The results showed variations in pH, TP concentration and the presence and variation of possible salivary PRPs associated with tannic acid concentration. These findings suggest that spider monkeys may tailor their salivary physicochemical characteristics in response to the ingestion of potentially toxic compounds.

Olfactory sensitivity for sperm-attractant aromatic aldehydes: a comparative study in human subjects and spider monkeys.

Using a three-alternative forced-choice ascending staircase procedure, we determined olfactory detection thresholds in 20 human subjects for seven aromatic aldehydes and compared them to those of four spider monkeys tested in parallel using an operant conditioning paradigm. With all seven odorants, both species detected concentrations <1 ppm, and with several odorants single individuals of both species even discriminated concentrations <1 ppb from the solvent. No generalizable species differences in olfactory sensitivity were found despite marked differences in neuroanatomical and genetic features. The across-odorant patterns of sensitivity correlated significantly between humans and spider monkeys, and both species were more sensitive to bourgeonal than to lilial, cyclamal, canthoxal, helional, lyral, and 3-phenylpropanal. No significant correlation between presence/absence of an oxygen-containing moiety attached to the benzene ring or presence/absence of an additional alkyl group next to the functional aldehyde group, and olfactory sensitivity was found in any of the species. However, the presence of a tertiary butyl group in para position (relative to the functional aldehyde group) combined with a lack of an additional alkyl group next to the functional aldehyde group may be responsible for the finding that both species were most sensitive to bourgeonal.

Taste-induced facial responses in black-handed spider monkeys (Ateles geoffroyi).

Taste-induced facial expressions are thought to reflect the hedonic valence of an animal's gustatory experience. We therefore assessed taste-induced facial responses in six black-handed spider monkeys (Ateles geoffroyi) to water, sucrose, caffeine, citric acid and aspartame, representing the taste qualities sweet, bitter, and sour, respectively. We decided not to include salty-tasting substances as the concentrations of such tastants found in the fruits consumed by spider monkeys are below their taste preference threshold. We found that the monkeys displayed significant differences in their facial responses between substances, with significantly higher frequencies of licking, sucking, closed eyes, tongue protruding, mouth gaping and lip smacking in response to sucrose, a presumably pleasant stimulus. The response to caffeine and citric acid, in contrast, yielded the lowest frequencies of these behaviors, but the highest frequency of withdrawals from the stimulus, suggesting these substances are perceived as unpleasant. Lip stretching, a newly described behavior, was performed significantly more often in response to caffeine than to any other substance, suggesting an association with the response to bitter taste. The facial response to the artificial sweetener aspartame was generally similar to the response to water, corroborating the notion that Platyrrhines may be unable to detect its sweetness. Overall, the present study supports the idea of similarity of taste-induced facial responses in non-hominoid primates and humans, suggesting these displays to be evolutionarily conserved across the primate order.