Why leaf-eating Asian monkeys do not have a sweet tooth

September 06, 2018

Asian colobine monkeys are unable to taste natural sugars, and in fact have a generally poor sense of taste. This is according to research led by Emiko Nishi of the Primate Research Institute of Kyoto University in Japan. Nishi and her colleagues found that the receptors on the tongues of colobine monkeys do not function in the same way as for fruit-eating monkeys, who are sensitive to sweet tastes. The study is published in the Springer Nature branded journal Primates, which is the official journal of the Japan Monkey Centre.

In general, mammals are able to taste sugary flavours thanks to the sweet taste receptor gene TAS1R2/TAS1R3 and related taste buds on the tongue. In a previous study, the same group of researchers showed that colobine monkeys do not pick up bitter tastes. Nishi and her colleagues conducted a series of laboratory and genetic tests to investigate these protein expressing cells reconstructed from leaf-eating Javan lutung monkeys (Trachypithecus auratus), which are part of the Colobinae subfamily of monkeys. These cells showed no reaction when natural sugars such as sucrose contained in sugarcane, fructose in fruit and maltose in fermented foods. Although the receptor genes are present, they seem not to function.

In a further behavioural test, Nishi and her colleagues watched 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 of the jellies, without preferring the sweet type over the bland version.

Nishi and her colleagues speculates that the animals' lack of interest in foods containing natural sugars and the functional decline of TAS1R2/TAS1R3 might be related to their feeding habits. They are primarily leaf eaters and very seldom eat fruits containing many simple sugars. Nishi further explains that the slim bodies of colobines are not geared towards digesting sweet tasting, energy-rich foods. 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 animals' body," Nishi explains.

Colobine monkeys' low sensitivity to maltose or malt sugar further suggests that the leaves they eat do not have to be starch-rich. In fact, eating too many carbohydrates and sugars rather than fibre-rich food is known to give these monkeys diarrhoea and other digestive problems. Javan lutungs for instance are known to prefer leaves that are easily digestible and fibre-rich to ones that are rich in starches.

"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," Nishi says.
Reference: Nishi, E. et al (2018). Functional decline of sweet taste sensitivity of colobine monkeys, Primates DOI: 10.1007/s10329-018-0679-2


Related Taste Articles from Brightsurf:

Re-mapping taste in the brain
A new study from Stony Brook University found that the map of neural responses mediating taste perception does not involve, as previously believed, specialized groups of neurons in the brain, but rather overlapping and spatially distributed populations.

Sweet taste reduces appetite?
To date, very little is known about how sweetness perception contributes to satiety.

Touch and taste? It's all in the tentacles
Scientists identified a novel family of sensors in the first layer of cells inside the suction cups that have adapted to react and detect molecules that don't dissolve well in water.

How octopus suckers "taste by touch"
Imagine if you could taste something simply by touching it.

Fish oil without the fishy smell or taste
A new study, co-led by University of Cincinnati researchers, describes the development of a refining process that scientists deem a superior method to help produce better dietary omega-3 health and dietary supplements containing fish oil.

New type of taste cell discovered in taste buds
Our mouths may be home to a newly discovered set of multi-tasking taste cells that -- unlike most known taste cells, which detect individual tastes -- are capable of detecting sour, sweet, bitter and umami stimuli.

Sweet-taste perception changes as children develop
While adults prefer levels of sweetness similar to typical soft drinks, children and adolescents are less sensitive to the taste and prefer concentrations that are 50% sweeter, according to research by professor of food science and human nutrition M.

Evolution of loss of smell or taste in COVID-19
This survey-based study examines the clinical course of the loss of sense of smell and taste in a case series of mildly symptomatic patients with SARS-CoV-2 infection.

Chanterelle mushrooms as a taste enhancer
Chanterelles give savoury dishes a rich body and a unique complex flavor.

Neuromarketing of taste
Marina Domracheva and Sofya Kulikova, researchers from HSE University's campus in Perm, have discovered a new approach to analyse the perceived similarity of food products, based on electroencephalography (EEG) signals.

Read More: Taste News and Taste Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.