Gustatory Responsiveness to Six Bitter Tastants in Three Species of Nonhuman Primates

Sour, but acceptable: Taste responsiveness to five food-associated acids in zoo-housed white-faced sakis, Pithecia pithecia

White-faced sakis (Pithecia pithecia) are commonly considered as frugivores but are unusual among primates as they do not specialize on ripe fruits but rather include a high proportion of unripe fruits into their diet, even during seasons when ripe fruits are available. Using a two-bottle preference test of short duration we therefore assessed whether this dietary specialization affects the taste responsiveness and sour-taste tolerance of four adult white-faced sakis for five food-associated acids. We found taste preference thresholds of the sakis to be 1-10 mM for citric acid, 0.5-20 mM for ascorbic acid, 2-10 mM for malic acid, 0.1-1 mM for tannic acid, and 2-20 mM for acetic acid, respectively. When given the choice between a reference solution of 50 mM sucrose and mixtures containing varying concentrations of sucrose plus citric acid, the sakis displayed a high sour-taste tolerance and required only 100 mM of sucrose (when mixed with 10 mM citric acid) or 200 mM of sucrose (when mixed with 30 or 50 mM citric acid), respectively, to prefer the sweet-sour mixture over the purely sweet 50 mM sucrose reference solution. These results demonstrate that white-faced sakis have a well-developed taste sensitivity for food-associated acids which is not inferior to that of primates specializing on ripe fruits. Compared to other platyrrhine primates, the sakis displayed a markedly higher sour-taste tolerance. These results may therefore reflect an evolutionary adaptation to the dietary specialization of the white-faced sakis to sour-tasting unripe fruits.

The sensory ecology of primate food perception, revisited

Twenty years ago, Dominy and colleagues published "The sensory ecology of primate food perception," an impactful review that brought new perspectives to understanding primate foraging adaptations. Their review synthesized information on primate senses and explored how senses informed feeding behavior. Research on primate sensory ecology has seen explosive growth in the last two decades. Here, we revisit this important topic, focusing on the numerous new discoveries and lines of innovative research. We begin by reviewing each of the five traditionally recognized senses involved in foraging: audition, olfaction, vision, touch, and taste. For each sense, we provide an overview of sensory function and comparative ecology, comment on the state of knowledge at the time of the original review, and highlight advancements and lingering gaps in knowledge. Next, we provide an outline for creative, multidisciplinary, and innovative future research programs that we anticipate will generate exciting new discoveries in the next two decades.

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 preferences and feeding behaviour in the facultative herbivorous fish, Nile tilapia Oreochromis niloticus

Taste preferences in fishes are known mainly for carnivorous species, whereas herbivorous consumers were rarely used in such studies. The main goal of the present study was to evaluate the taste preferences in the herbivorous African cichlid fish, Nile tilapia Oreochromis niloticus. In laboratory settings, the palatability of widely used taste substances (four taste substances that are considered to be sweet, sour, bitter and salty for humans - sucrose, citric acid, calcium chloride and sodium chloride; 21 free L-amino acids; 12 sugars and artificial sweetener Na-saccharin; 0.1-0.0001 M) was evaluated. In each trial, a standard agar pellet flavoured with a substance was offered for fish individually. The consumption of pellet, the number of grasps and the retention time before the pellet was finally ingested or rejected were registered. Overall, 21 of 38 substances were palatable, whereas other substances did not shift consumption of pellets in relation to blank pellets. Pellets containing citric acid, L-cysteine, L-norvaline, L-isoleucine, L-valine, Na-saccharin and D-sorbitol were consumed in >85% of trials. Taste attractiveness of amino acids was highly species-specific and was not associated with the trophic category of the 19 species compared. Moreover, it did not correlate with dietary quantitative requirements of Nile tilapia (r_s  = 0.27; P > 0.05). Palatability of sugars for O. niloticus and their sweetness for humans did not correlate as well (r_s  = 0.21; P > 0.05); nonetheless, Na-saccharin has the most attractive taste for both O. niloticus and humans. The most palatable amino acids lost their effect if the concentration was lowered to 0.01 M for L-cysteine and 0.001 M for L-norvaline (lower than 242.3 μg and 23.4 μg per a pellet, respectively). Single pellet grasp was characteristic of O. niloticus feeding behaviour (>95% of trials), and this pattern may be related to the social lifestyle of this species. Fish spent 4-8 s on average for orosensory evaluation of pellet edibility. The retention time correlated with the palatability of substances and was significantly longer in trials that ended up with pellet swallowing. It is suggested that prolonged orosensory evaluation of food before swallowing provides a reliable and accurate sensory evaluation, which, in turn, can reduce the probability that inadequate food will be consumed.

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.

Exploring individual variation in associative learning abilities through an operant conditioning task in wild baboons

Cognitive abilities underpin many of the behavioural decisions of animals. However, we still have very little understanding of how and why cognitive abilities vary between individuals of the same species in wild populations. In this study, we assessed the associative learning abilities of wild chacma baboons (Papio ursinus) across two troops in Namibia with a simple operant conditioning task. We evaluated the ability of individuals to correctly associate a particular colour of corn kernels with a distasteful flavour through repeated presentations of two small piles of corn dyed different colours, one of which had been treated with a non-toxic bitter substance. We also assessed whether individual variation in learning ability was associated with particular phenotypic traits (sex, social rank and neophilia) and states (age and prior vigilance). We found no evidence of learning the association either within each trial or across trials, nor any variation based on individuals' phenotypes. This appeared to be due to a high tolerance for bitter foods leading to similar acceptance of both palatable and unpalatable kernels. Earlier avoidance of the bitter kernels during pilot trials suggests this higher tolerance may have been largely driven by a drought during the experiments. Overall, our findings highlight the potential influence of current environmental challenges associated with conducting cognitive tests of animals in the wild.

The Taste of Caffeine

Many people avidly consume foods and drinks containing caffeine, despite its bitter taste. Here, we review what is known about caffeine as a bitter taste stimulus. Topics include caffeine's action on the canonical bitter taste receptor pathway and caffeine's action on noncanonical receptor-dependent and -independent pathways in taste cells. Two conclusions are that (1) caffeine is a poor prototypical bitter taste stimulus because it acts on bitter taste receptor-independent pathways, and (2) caffeinated products most likely stimulate "taste" receptors in nongustatory cells. This review is relevant for taste researchers, manufacturers of caffeinated products, and caffeine consumers.

Antimalarial activity of the myxobacterial macrolide chlorotonil a

Myxobacteria are Gram-negative soil-dwelling bacteria belonging to the phylum Proteobacteria. They are a rich source of promising compounds for clinical application, such as epothilones for cancer therapy and several new antibiotics. In the course of a bioactivity screening program of secondary metabolites produced by Sorangium cellulosum strains, the macrolide chlorotonil A was found to exhibit promising antimalarial activity. Subsequently, we evaluated chlorotonil A against Plasmodium falciparum laboratory strains and clinical isolates from Gabon. Chlorotonil A was highly active, with a 50% inhibitory concentration between 4 and 32 nM; additionally, no correlations between the activities of chlorotonil A and artesunate (rho, 0.208) or chloroquine (rho, -0.046) were observed. Per os treatment of Plasmodium berghei-infected mice with four doses of as little as 36 mg of chlorotonil A per kg of body weight led to the suppression of parasitemia with no obvious signs of toxicity. Chlorotonil A acts against all stages of intraerythrocytic parasite development, including ring-stage parasites and stage IV to V gametocytes, and it requires only a very short exposure to the parasite to exert its antimalarial action. Conclusively, chlorotonil A has an exceptional and unprecedented profile of action and represents an urgently required novel antimalarial chemical scaffold. Therefore, we propose it as a lead structure for further development as an antimalarial chemotherapeutic.

Gustatory responsiveness to the 20 proteinogenic amino acids in the spider monkey (Ateles geoffroyi)

The gustatory responsiveness of four adult spider monkeys to the 20 proteinogenic amino acids was assessed in two-bottle preference tests of brief duration (1min). We found that Ateles geoffroyi responded with significant preferences for seven amino acids (glycine, l-proline, l-alanine, l-serine, l-glutamic acid, l-aspartic acid, and l-lysine) when presented at a concentration of 100mM and/or 200mM and tested against water. At the same concentrations, the animals significantly rejected five amino acids (l-tryptophan, l-tyrosine, l-valine, l-cysteine, and l-isoleucine) and were indifferent to the remaining tastants. Further, the results show that the spider monkeys discriminated concentrations as low as 0.2mM l-lysine, 2mM l-glutamic acid, 10mM l-proline, 20mM l-valine, 40mM glycine, l-serine, and l-aspartic acid, and 80mM l-alanine from the alternative stimulus, with individual animals even scoring lower threshold values. A comparison between the taste qualities of the proteinogenic amino acids as described by humans and the preferences and aversions observed in the spider monkeys suggests a fairly high degree of agreement in the taste quality perception of these tastants between the two species. A comparison between the taste preference thresholds obtained with the spider monkeys and taste detection thresholds reported in human subjects suggests that the taste sensitivity of A. geoffroyi for the amino acids tested here might match that of Homo sapiens. The results support the assumption that the taste responses of spider monkeys to proteinogenic amino acids might reflect an evolutionary adaptation to their frugivorous and thus protein-poor diet.

Effect of bitter compounds on amylase secretion in murine submandibular glands: Signaling pathway mechanisms

BACKGROUND

Amylase is synthesized in submandibular glands (SMG) and released into the oral cavity to degrade carbohydrates in the mouth. Bitter taste receptors (T2R) belong to the G-protein coupled receptor (GPCR) family and are expressed in the taste cells and also in the digestive tract.

METHODS

The activity of amylase secreted by murine SMG was measured, detecting maltose by Bernfeld's method. Amylase and T2R6 were detected by imunohistochemistry and Western blot. The expression of Ggustducin, Gi, and phospholipase Cβ2 was also studied by Western blot. cAMP levels were measured by radioimmunoassay and inositol monophosphate production was quantified by ELISA.

RESULTS

Theophylline, denatonium and cycloheximide exerted a dose-dependent inhibition on amylase secretion. This effect was reverted by preincubating SMG with an anti-Gαi antibody. cAMP production was increased by the same compounds, an effect that was also abrogated by an anti-Gαi antibody. Bitter compounds reduced inositol monophosphate formation in SMG and H-89, a protein kinase A inhibitor, reverted this action, revealing that this protein kinase down regulates phospholipase C activity.

GENERAL SIGNIFICANCE

We demonstrated that theophylline, denatonium and cycloheximide inhibit salivary amylase secretion, activating an intracellular signaling pathway that involves cAMP and phospholipase C, that cross talks via protein kinase A.