Serial visual reversal learning in captive black-handed spider monkeys, Ateles geoffroyi.

Recent research suggests that socio-ecological factors such as dietary specialization and social complexity may be drivers of advanced cognitive skills among primates. Therefore, we assessed the ability of 12 black-handed spider monkeys (Ateles geoffroyi), a highly frugivorous platyrrhine primate with strong fission-fusion dynamics, to succeed in a serial visual reversal learning task. Using a two-alternative choice paradigm we first trained the animals to reliably choose a rewarded visual stimulus over a non-rewarded one. Upon reaching a pre-set learning criterion we then switched the reward values of the two stimuli and assessed if and how quickly the animals learned to reverse their choices, again to a pre-set learning criterion. This stimulus reversal procedure was then continued for a total of 80 sessions of 10 trials each. We found that the spider monkeys quickly learned to reliably discriminate between two simultaneously presented visual stimuli, that they succeeded in a visual reversal learning task, and that they displayed an increase in learning speed across consecutive reversals, suggesting that they are capable of serial reversal learning-set formation with visual cues. The fastest-learning individual completed five reversals within the 80 sessions. The spider monkeys outperformed most other primate and nonprimate mammal species tested so far on this type of cognitive task, including chimpanzees, with regard to their learning speed in both the initial learning task and in the first reversal task, suggesting a high degree of behavioral flexibility and inhibitory control. Our findings support the notion that socio-ecological factors such as dietary specialization and social complexity foster advanced cognitive skills in primates.

Howler Monkey Die-Off in Southern Mexico.

In May and June 2024, a die-off of Mexican mantled howler monkeys (Alouatta palliata mexicana) occurred in southern Mexico. This commentary documents the event, attributing it to extreme heatwaves, drought, wildfires, and habitat impoverishment. Despite their reported resilience to habitat disturbances, mantled howler monkey mortality rate in some areas reached 31%. Key evidence points to heatstroke as the primary cause of death, exacerbated by limited hydration and reduced dietary diversity in disturbed habitats. Immediate responses included community-led rescues (e.g., hydrating the monkeys), coordination of rescue activities by nongovernmental organizations (NGOs) (e.g., managing donations), involvement of scientists (e.g., monitoring of primate populations), and assistance from government officials (e.g., providing legal support for animal management). This event underscores the urgency of developing action plans to prevent and attend future crises. Among other actions, we highlight (i) establishing primate care infrastructure with medical and rehabilitation centers; (ii) developing protocols and training programs to ensure rapid crisis response; (iii) fostering collaboration among government, NGOs, and academic institutions for effective crisis management; and (iv) developing targeted research on climate change impacts, predictive models, and long-term health monitoring. We emphasize the critical need for coordinated conservation efforts to protect wild primates and maintain natural ecosystem resilience in the face of escalating climate challenges.

Anatomy and dietary specialization influence sensory behaviour among sympatric primates.

Senses form the interface between animals and environments, and provide a window into the ecology of past and present species. However, research on sensory behaviours by wild frugivores is sparse. Here, we examine fruit assessment by three sympatric primates (Alouatta palliata, Ateles geoffroyi and Cebus imitator) to test the hypothesis that dietary and sensory specialization shape foraging behaviours. Ateles and Cebus groups are comprised of dichromats and trichromats, while all Alouatta are trichomats. We use anatomical proxies to examine smell, taste and manual touch, and opsin genotyping to assess colour vision. We find that the frugivorous spider monkeys (Ateles geoffroyi) sniff fruits most often, omnivorous capuchins (Cebus imitator), the species with the highest manual dexterity, use manual touch most often, and that main olfactory bulb volume is a better predictor of sniffing behaviour than nasal turbinate surface area. We also identify an interaction between colour vision phenotype and use of other senses. Controlling for species, dichromats sniff and bite fruits more often than trichromats, and trichromats use manual touch to evaluate cryptic fruits more often than dichromats. Our findings reveal new relationships among dietary specialization, anatomical variation and foraging behaviour, and promote understanding of sensory system evolution.

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.

Salivary tannin-binding proteins are a pervasive strategy used by the folivorous/frugivorous black howler monkey.

Dietary tannins can affect protein digestion and absorption, be toxic, and influence food selection by being astringent and bitter tasting. Animals that usually ingest tannins may regularly secrete tannin-binding salivary proteins (TBSPs) to counteract the negative effects of tannins or TBSPs production can be induced by a tannin-rich diet. In the wild, many primates regularly eat a diet that contains tannin-rich leaves and unripe fruit and it has been speculated that they have the physiological ability to cope with dietary tannins; however, details of their strategy remains unclear. Our research details the salivary protein composition of wild and zoo-living black howler monkeys (Alouatta pigra) feeding on natural versus manufactured low-tannin diets, and examines differences in TBSPs, mainly proline-rich proteins (PRPs), to determine whether production of these proteins is dependent on the tannin content of their food. We measured the pH, flow rate, and concentration of total protein and trichloroacetic acid soluble proteins (an index of PRPs) in saliva. Howler monkeys produced slightly alkaline saliva that may aid in the binding interaction between tannin and salivary proteins. We used gel electrophoresis to describe the salivary protein profile and this analysis along with a tannin-binding assay allowed us to detect several TBSPs in all individuals. We found no differences in the characteristics of saliva between wild and zoo-living monkeys. Our results suggest that black howler monkeys always secrete TBSPs even when fed on foods low in tannins. This strategy of constantly using this salivary anti-tannin defense enables them to obtain nutrients from plants that sometimes contain high levels of tannins and may help immediately to overcome the astringent sensation of their food allowing howler monkeys to eat tanniferous plants.

Variation in the Diet of Hatchling Morelet's Crocodile (Crocodylus moreletii) in the Wild.

The relationship between diet and behavior is essential to understanding an animal's strategies to obtain food, considering ontogenical changes. In reptiles, there is a relationship between the length of the individual and the size of the prey it consumes. Studies have focused on the ontogenetic changes in reptile diets from hatchling to adult, but only a few studies have focused on the transition from hatchling to juvenile. We aimed to describe and analyze the composition, variation, diversity, and overlap in the diet of hatchling Morelet's crocodiles (Crocodylus moreletii) for three size intervals during the hatchling-juvenile transition. We captured 31 hatchling Morelet's crocodiles in an urbanized lagoon in Tabasco. We performed stomach-flushing to determine the diet. Additionally, we estimated the volume, frequency of occurrence, and relative importance of diet items and analyzed the relationship between prey type and the total length of the individuals. The diversity of the hatchling prey suggests a generalist diet. We observed two items not previously described in the diet of hatchling crocodiles. In addition, we found differences in diet between the initial and final size intervals, as increases in the length of prey appeared that they did not consume when they were hatchlings. Our results contribute new information to the dietary changes that occur during the hatchling-juvenile transition.

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.

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.

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.

Forgetting curve in spider monkeys (Ateles geoffroyi): Effects of delay but not the amount of information.

The forgetting curve is a phenomenon in which animals fail to retrieve information about an event as time passes. We tested this effect in spider monkeys (Ateles geoffroyi) in a task where they had to retrieve a reward hidden in a container after a delay. We combined different delays (0, 5, 10, 20, and 30 s) and different numbers of containers in which the reward could be hidden (2-6) and tested their effects on the accuracy in retrieving the reward. Our data suggest that the longer the delay, the more errors the spider monkeys made. However, a greater number of containers meant only marginally lower accuracy. An analysis of the errors showed that the spider monkeys used spatial information and showed a generalization of responses that depended on the delay; the longer the delay, the greater the generalization and the dispersion of errors. In a second experiment, we tested whether spider monkeys could use visual cues to increase their accuracy. We used the same task with an apparatus with 6 containers and 30 s of delay. Three colors were used as cues, with each color associated with 1 pair of containers. The data showed that the colors did not increase the accuracy of responses but reduced the generalization of the errors. The findings resembled previous findings supporting dynamic field theory, but more investigation is necessary to understand the brain areas and the neural dynamics of spider monkeys to explain and predict their behavior. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

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.

Taste detection threshold of human (Homo sapiens) subjects and taste preference threshold of black-handed spider monkeys (Ateles geoffroyi) for the sugar substitute isomalt.

The artificial sweetener isomalt is widely used due to its low caloric, non-diabetogenic and non-cariogenic properties. Although the sweetening potency of isomalt has been reported to be lower than that of sucrose, no data on the sensitivity of humans for this polyol are available. Using an up-down, two-alternative forced choice staircase procedure we therefore determined taste detection thresholds for isomalt in human subjects (n = 10; five females and five males) and compared them to taste preference thresholds, determined using a two-bottle preference test of short duration, in a highly frugivorous nonhuman primate, the spider monkey (n = 4; one female, three males). We found that both species detected concentrations of isomalt as low as 20 mM. Both humans and spider monkeys are less sensitive to isomalt than to sucrose, which is consistent with the notion of the former being a low-potency sweetener. The spider monkeys clearly preferred all suprathreshold concentrations tested over water, suggesting that, similar to humans, they perceive isomalt as having a purely sweet taste that is indistinguishable from that of sucrose. As isomalt, like most sweet-tasting polyols, may elicit gastric distress when consumed in large quantities, the present findings may contribute to the choice of appropriate amounts and concentrations of this sweetener when it is employed as a sugar substitute or food additive for human consumption. Similarly, the taste preference threshold values of spider monkeys for isomalt reported here may be useful for determining how much of it should be used when it is employed as a low-caloric sweetener for frugivorous primates kept on a vegetable-based diet, or when medication needs to be administered orally.

Taste responsiveness of chimpanzees (Pan troglodytes) and black-handed spider monkeys (Ateles geoffroyi) to eight substances tasting sweet to humans.

Using a two-bottle choice test of short duration, we determined taste preference thresholds for eight substances tasting sweet to humans in three chimpanzees (Pan troglodytes) and four black-handed spider monkeys (Ateles geoffroyi). We found that the chimpanzees significantly preferred concentrations as low as 100-500 mM galactose, 250 mM sorbitol, 0.5-2 mM acesulfame K, 0.5-2.5 mM alitame, 0.5 mM aspartame, 0.2-2 mM sodium saccharin, 0.001-0.2 mM thaumatin, and 0.0025-0.005 mM monellin over tap water. The spider monkeys displayed lower taste preference threshold values, and thus a higher sensitivity than the chimpanzees, with five of the eight substances (2-20 mM galactose, 20-50 mM sorbitol, 0.2-1 mM acesulfame K, 0.002-0.005 mM alitame, and 0.002-0.5 mM sodium saccharin), but were generally unable to perceive the sweetness of the remaining three substances (aspartame, thaumatin, and monellin). The ranking order of sweetening potency of the eight taste substances used here correlates significantly between chimpanzees and humans, but not between spider monkeys and humans. This is in line with genetic findings reporting a higher degree of sequence identity in the Tas1r2 and the Tas1r3 genes coding for the mammalian heterodimer sweet-taste receptor between chimpanzees and humans compared to spider monkeys and humans. Taken together, the findings of the present study support the notion that taste responsiveness for substances tasting sweet to humans may correlate positively with phylogenetic relatedness. At the same time, they are also consistent with the notion that co-evolution between fruit-bearing plants and the sense of taste in animals that serve as their seed dispersers may explain between-species differences in sweet-taste perception.

Gustatory responsiveness to six bitter tastants in three species of nonhuman primates.

Gustatory responsiveness of six adult squirrel monkeys, four spider monkeys, and five pigtail macaques to six bitter tastants was assessed in two-bottle preference tests of brief duration (2 min). Animals were given the choice between a 30-mM sucrose solution and defined concentrations of a bitter tastant dissolved in a 30-mM sucrose solution. With this procedure, Saimiri sciureus, Ateles geoffroyi, and Macaca nemestrina were found to significantly discriminate concentrations as low as 0.2, 0.05, and 0.1 mM quinine hydrochloride; 1, 1, and 0.05 mM caffeine; 20, 5, and 1 mM naringin; 5, 2, and 1 mM salicin; 0.01, 0.001, and 0.02 mM sucrose octaacetate; and 0.05, 0.01, and 0.5 mM denatonium benzoate, from the alternative stimulus. With the exception of naringin in the pigtail macaques, all three species rejected all suprathreshold concentrations of all bitter tastants tested. The spider monkeys and the pigtail macaques displayed the lowest taste avoidance thresholds with three of the six tastants each; in contrast, the squirrel monkeys displayed the highest taste avoidance thresholds with four of the six tastants. The across-tastant patterns of taste avoidance thresholds were identical in spider monkeys and squirrel monkeys; both species displayed the following order of sensitivity: sucrose octaacetate > denatonium benzoate > quinine hydrochloride > caffeine > salicin > naringin. All three primate species were more sensitive to the two artificial tastants (sucrose octaacetate and denatonium benzoate) compared to the four naturally occurring tastants. However, the concentrations detected by all three primate species with the four naturally occurring tastants are well below those reported in plants or arthropods consumed by these species suggesting that they may use bitterness as a criterion for food selection.

Traditional nutritional analyses of figs overestimates intake of most nutrient fractions: a study of ficus perforata consumed by howler monkeys (Alouatta palliata mexicana).

There continue to remain uncertainty concerning the nutritional importance of figs (Ficus spp.) in the primate diet. Although studies have been performed on the nutritional analysis of fig pulp and seed fractions separately, there has not been an attempt to quantify the contribution of animal matter within figs. Here we report nutritional values of figs (Ficus perforata) (Urostigma) consumed by a troop of howler monkeys (Alouatta palliata mexicana). Separate nutritional assays were performed on the pulp, seed, and animal fraction. Whole-fig analyses significantly exaggerate the concentrations of all nutrients (lipids especially) as seeds, which represent a large proportion of the fig (45%), cannot be digested. Animal matter only represents 1% of the fig, and augments fig protein and lipid content by 0.44 and 0.30%, respectively. This represents the 11 and 9% of the fraction available for digestion. Differences in fig consumption were observed between age and sex classes (P=0.04) and periods of the day (P=0.001); females consumed more figs than males and the highest consumption of figs was observed in the afternoon. F. perforata figs may be an important component of the howler monkeys diet owing to their high content of water and calcium.

Taste difference thresholds for monosodium glutamate and sodium chloride in pigtail macaques (Macaca nemestrina) and spider monkeys (Ateles geoffroyi).

The purpose of this study was to determine taste difference thresholds for monosodium glutamate (MSG) and sodium chloride (NaCl) in pigtail macaques (Macaca nemestrina) and spider monkeys (Ateles geoffroyi). Using a two-bottle preference test of brief duration, three animals of each species were presented with four different reference concentrations of 50, 100, 200, and 400 mM of a tastant and tested for their ability to discriminate these from lower concentrations of the same tastant. The just noticeable differences (JNDs), expressed as Weber ratios (DeltaI/I), were found to range from 0.1 to 0.5 for MSG and 0.2 to 0.45 for NaCl in the pigtail macaques, with a significant tendency for higher Weber ratios with higher reference concentrations. In the spider monkeys, JNDs ranged from 0.15 to 0.4 for MSG and 0.1 to 0.25 for NaCl, with Weber ratios staying fairly constant across the reference concentrations tested. Thus, the JNDs were found to be generally similar in both species and to be at least as low as those found in humans for MSG and NaCl, as well as those found in spider monkeys for sucrose. The results support the assumption that both pigtail macaques and spider monkeys may use differences in perceived intensity of MSG and NaCl as a criterion for food selection.

Hand preferences in two unimanual and two bimanual coordinated tasks in the black-handed spider monkey (Ateles geoffroyi).

Spider monkeys are interesting to study with regard to hand preferences, as they are one of the few primate species that lack a thumb and, thus, are unable to perform a precision grip. Further, being platyrrhine primates, they also largely lack independent motor control of the digits and, thus, have only limited manual dexterity. It was therefore the aim of the present study to assess hand preferences in black-handed spider monkeys (Ateles geoffroyi) in 4 tasks differing in task demand: simple unimanual reaching for food and 3 versions of the widely used tube task, including 2 bimanual versions that differ from each other in the degree of fine motor control needed and a unimanual version that does not require coordinated action of the hands. We found that black-handed spider monkeys display significant hand preferences at the individual, but not at the population, level. This was true both in the 2 bimanual coordinated tasks and in the 2 unimanual tasks. Further, our results show that the majority of animals were consistent in the hand they preferred in these 4 tasks. Our findings only partially support the notion that task demand positively correlates with strength of hand preference. Finally, we found that the index finger was the most frequently used digit in all 3 tube tasks, although the animals also used other digits and 2- and 3-finger combinations to extract food from a tube. We conclude that limited manual dexterity does not prevent spider monkeys from displaying strong and consistent hand preferences at the individual level. (PsycINFO Database Record

Taste responsiveness to two steviol glycosides in three species of nonhuman primates.

Primates have been found to differ widely in their taste perception and studies suggest that a co-evolution between plant species bearing a certain taste substance and primate species feeding on these plants may contribute to such between-species differences. Considering that only platyrrhine primates, but not catarrhine or prosimian primates, share an evolutionary history with the neotropical plant Stevia rebaudiana, we assessed whether members of these three primate taxa differ in their ability to perceive and/or in their sensitivity to its two quantitatively predominant sweet-tasting substances. We found that not only neotropical black-handed spider monkeys, but also paleotropical black-and-white ruffed lemurs and Western chimpanzees are clearly able to perceive stevioside and rebaudioside A. Using a two-bottle preference test of short duration, we found that Ateles geoffroyi preferred concentrations as low as 0.05 mM stevioside and 0.01 mM rebaudioside A over tap water. Taste preference thresholds of Pan troglodytes were similar to those of the spider monkeys, with 0.05 mM for stevioside and 0.03 mM for rebaudioside A, whereas Varecia variegata was slightly less sensitive with a threshold value of 0.1 mM for both substances. Thus, all three primate species are, similar to human subjects, clearly more sensitive to both steviol glycosides compared to sucrose. Only the spider monkeys displayed concentration-response curves with both stevioside and rebaudioside A which can best be described as an inverted U-shaped function suggesting that Ateles geoffroyi, similar to human subjects, may perceive a bitter side taste at higher concentrations of these substances. Taken together, the results of the present study do not support the notion that a co-evolution between plant and primate species may account for between-species differences in taste perception of steviol glycosides.

Detecting danger--or just another odorant? Olfactory sensitivity for the fox odor component 2,4,5-trimethylthiazoline in four species of mammals.

2,4,5-trimethylthiazoline (TMT) is a volatile component of the anal gland secretion of the red fox and elicits behavioral and physiological fear responses in the rat. Using instrumental conditioning paradigms, we determined olfactory detection thresholds for TMT in three rats, a natural prey species of the red fox, and compared their performance to that of three squirrel monkeys, three spider monkeys and four pigtail macaques, all non-prey species of the red fox. We found that the rats were able to discriminate concentrations between 0.04 and 0.10 ppt (parts per trillion) of TMT from the odorless solvent which is by far the lowest olfactory detection threshold for an odorant reported in rats so far. In contrast, the spider monkeys needed 0.14-1.38 ppb (parts per billion), the pigtail macaques 0.41-4.07 ppb, and the squirrel monkeys 4.07-13.80 ppb to detect TMT which does not rank among the lowest olfactory thresholds reported for these three primate species. Thus, these results support the assumption that the behavioral relevance of an odorant may be an important determinant of a species' olfactory sensitivity.