These Asian monkeys can’t taste the sweetness of natural sugars

Not all monkeys have a sweet tooth. Some, like the Asian colobine monkeys, whose members live on a diet comprised mainly of leaves, cannot taste natural sugars and show no preference for foods flavored with sugars, a new study has found.

Take the Javan lutung, or Javan langur (Trachypithecus auratus), a colobine monkey found in Indonesia, for example. It mostly eats leaves and unripe fruits — foods that humans typically don’t find very palatable. To find out why the lutung chooses these unsweet foods, a team of researchers from the Primate Research Institute of Kyoto University in Japan looked into the monkey’s genes.

In general, humans and other mammals are able to taste the sweetness of natural sugars because of the sweet-taste receptor genes TAS1R2/TAS1R3 and their associated taste buds on the tongue. The Javan lutung has these receptor genes. But when the researchers expressed these genes in single cells, the cells did not show any response to natural sugars like maltose, sucrose, glucose, fructose, lactose and sorbitol.

The researchers speculate that while the sweet-taste receptors do exist in the Javan lutung, these could have undergone some mutation and stopped functioning. The receptors could also be helping them taste other sweet compounds such as sweet amino acids, said Hiroo Imai, a professor at the Primate Research Institute and co-author of the study published in Primates. “We have not tried to test sweet proteins yet, only some natural sugars,” he said.

The researchers also looked at how two other Asian colobine monkeys responded to sugars via a behavior test. They exposed two silvery lutungs (Trachypithecus cristatus) and one Hanuman langur (Semnopithecus entellus) to two baskets: one containing sugar-flavored jellies, and the other with non-sugar-flavored jellies. Then they compared their jelly intake with those of Japanese macaques (Macaca fuscata) or snow monkeys, previously shown to have a sweet tooth.

They found that while the colobines (the silvery lutung and the hanuman langur) ate all of the jellies without preferring one over the other, the macaques preferred sucrose- and maltose-flavored jellies over bland ones.

In a previous study, the researchers had shown that colobine monkeys are also unable to taste bitterness. This inability to taste either sweet or bitter foods creates a niche for the colobines to exploit, the researchers say.

“If the monkeys can taste sweet and bitter compounds, they will chose sweet foods and avoid bitter foods, similar to human and other mammals,” Imai told Mongabay. “If the monkeys cannot [taste both], they will not choose sweet foods and don’t avoid bitter foods, which is like leaves and unripe fruits. They are niche for the colobines and will not be disrupted by other animals.”

The inability of the colobines’ sweet-taste receptors to detect natural sugars could be related to their easily digestible leafy diets. The monkeys’ guts are better adapted to digesting cellulose and hemicellulose contained in leaves through bacterial fermentation, the researchers say, than sweet-tasting, energy-rich fruits or starch-rich leaves. In fact, these monkeys are known to suffer from diarrhea and other digestive problems if they eat too much carbohydrates and sugars.

“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 animals’ body,” lead researcher Emiko Nishi said in a statement. “Some mammals with specialized feeding habits and less exposure to specific tastes lose sensitivity to particular tastes, as has happened in panda and members of the cat family.”