The proboscis monkey, nicknamed the “long-nosed monkey” due to its huge, protruding nose, now has another claim to fame: it regularly regurgitates and rechews its food.
The proboscis monkey demonstrates the first naturally occurring, ongoing instances of this behavior in a primate, conclude the authors of the paper, published in the latest Royal Society Biology Letters.
Gorillas and humans will also sometimes upchuck, rechew, and swallow, but if done on even a semi-regular basis, the actions are considered to be pathological.
“The digestive tract of the proboscis monkey is drastically different from that of humans and great apes,” lead author Ikki Matsuda told Discovery News. “The proboscis monkey has a distinct sacculated (chambered) forestomach where bacterial digestion occurs prior to the glandular stomach.”
Matsuda, a scientist at Kyoto University’s Primate Research Institute, and his colleagues recorded the behaviors of proboscis monkeys living along a tributary of Kinabatangan River, Malaysia. The researchers collected their data from a boat on the river during early mornings and late afternoons from January 2000 to March 2001.
At least 23 different monkeys were videotaped regurgitating and rechewing. When this happened, the monkey’s abdomen would contract and the primate would stick its tongue outside its pursed mouth.
The regurgitated material stayed in the mouth, but could be seen at times on camera. The monkey then puffed out its cheeks as it rechewed and swallowed the food for the second time.
Matsuda said he and his colleagues “speculate that the behavior served to allow for an increased food intake under yet-to-be-specified conditions.”
He explained the behavior likely allows the monkey to digest larger particles of food faster. This, he theorized, “means the monkey can eat more sooner, because the bacteria do not need as long to digest the material.”
The proboscis monkey’s diet consists of various proportions of leaves and fruits. Some can be quite fibrous. The researchers, however, were not able to associate any particular type of food to the regurgitation/rechewing behavior.
Ruminants, such as cows, digest in a similar way all the time. They tear off plant materials and swallow them. After some processing, a contraction sends the cud material back up to the mouth where it is chewed for a long time before being swallowed again.
Peter Langer, a professor in the Institute of Anatomy and Cell Biology at Justus-Liebig University, is one of the world’s leading experts on ruminants and their feeding processes. He is the author of the book “Mammalian Herbivore Stomach: Comparative Anatomy, Function and Evolution.”
Langer told Discovery news that it’s important to make it “clear that rumination in the Artiodactyla (cows and other hoofed animals) and regurgitation and remastication in other mammalian orders, including primates, are different physiological processes.”
Koalas, for example, are not ruminants, but like the proboscis monkey, they too have been observed regurgitating and rechewing their food. They are only believed to do this under certain circumstances, such as when their teeth wear out due to old age, or when lactating females need to consume more food.
Matsuda said that if he and his team had observed the behavior in the monkeys more regularly, “say 10 minutes after waking in the morning, we might have called it rumination.” He added that this study only focused on one population of the monkeys, so the behavior might even be a learned tradition.
He explained, “Traditions, especially related to feeding, have been reported in primates -- like the macaques that wash food and even season it with salt water. We simply cannot exclude such a tradition.”