AsianScientist (Sep. 18, 2018) – A research group in Japan has found the reason why some Asian monkeys are less able to taste sweetness. Their findings are published in Primates.
In general, mammals are able to taste sugary flavors thanks to the gene TAS1R2/TAS1R3 and related taste buds on the tongue. The ability to taste sugar is considered important because glucose is the most immediately available nutrient after food consumption.
Yet, in this study, researchers at the Primate Research Institute of Kyoto University in Japan discovered that Asian colobine monkeys are unable to taste natural sugars, and in fact have a generally poor sense of taste. Dr. Emiko Nishi and her colleagues conducted a series of laboratory and genetic tests on cells from the tongues of leaf-eating Javan lutung monkeys (Trachypithecus auratus), which belong to the Colobinae subfamily of monkeys.
These cells showed no reaction to natural sugars such as sucrose contained in sugarcane, fructose in fruit and maltose in fermented foods. Although the genes TAS1R2/TAS1R3 are present in the cells examined, they appear to be less sensitive to sweet tastes than those of fruit-eating monkeys.
In a further behavioural test, Nishi and her colleagues observed what happened when silvery lutung (Trachypithecus cristatus) and Hanuman langur (Semnopithecus entellus) in captivity were given the choice between baskets of jellies with or without sugar added to them. The animals ate all the jellies, without preferring the sweet type over the bland version, which provided further evidence of their poor sensitivity to sugar.
Nishi further explained that the slim bodies of Asian colobine monkeys are not geared towards digesting sweet-tasting, energy-rich foods. In fact, eating too many carbohydrates and sugars rather than fiber-rich food is known to give these monkeys diarrhea and other digestive problems.
It is thus unsurprising that Javan lutungs are known to prefer leaves that are easily digestible and fiber-rich to ones that are rich in starches. Their peculiar ruminant-like stomach helps them digest cellulose and hemicellulose contained in leaves through a fermentation process involving bacteria.
“The consumption of too many ripe fruits might contribute to rapid overfermentation and the overproduction of volatile fatty acids, leading to an increase in acid levels in the lutung’s body,” said Nishi. “Some mammals with specialized feeding habits and less exposure to specific tastes lose sensitivity to particular tastes, as has happened in pandas and members of the cat family.”
The article can be found at: Nishi et al. (2018) Functional Decline of Sweet Taste Sensitivity of Colobine Monkeys.
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Source: Springer; Photo: Shutterstock.
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