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

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.

Systematic analysis reveals novel insight into the molecular determinants of function, diversity and evolution of sweet taste receptors T1R2/T1R3 in primates

Sweet taste is a primary sensation for the preference and adaption of primates to diet, which is crucial for their survival and fitness. It is clear now that the sweet perception is mediated by a G protein-coupled receptor (GPCR)-sweet taste receptor T1R2/T1R3, and many behavioral or physiological experiments have described the diverse sweet taste sensitivities in primates. However, the structure-function relationship of T1R2s/T1R3s in primates, especially the molecular basis for their species-dependent sweet taste, has not been well understood until now. In this study, we performed a comprehensive sequence, structural and functional analysis of sweet taste receptors in primates to elucidate the molecular determinants mediating their species-dependent sweet taste recognition. Our results reveal distinct taxonomic distribution and significant characteristics (interaction, coevolution and epistasis) of specific key function-related residues, which could partly account for the previously reported behavioral results of taste perception in primates. Moreover, the prosimians Lemuriformes species, which were reported to have no sensitivity to aspartame, could be proposed to be aspartame tasters based on the present analysis. Collectively, our study provides new insights and promotes a better understanding for the diversity, function and evolution of sweet taste receptors in primates.

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.

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.

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.