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Most parents will tell you how common it is for children to eat their own mucus. This strange behavior has a special name: mucophagy. However, data on how many children actually do this are scarce. We do not have exact numbers. Scientists have not conducted large-scale surveys to count every child who does this. But everyone agrees it happens often. It is a normal part of growing up for many kids.
Researchers have found that mucophagy is not just a human habit. It is shared by at least 12 other primate species. This discovery began with a specific type of monkey called the aye-aye. Evolutionary biologist Anne-Claire Fabre first noticed this when she was watching the aye-aye. This lemur species is known for its very long middle finger. The finger is about 3 inches (8 centimeters) long. They use it to pry insects out of hard-to-reach crevices in trees.
But when Fabre was watching a captive aye-aye in 2015, she was surprised. She saw the animal stick that long, thin digit into its nostrils. It extracted mucus and then licked its finger clean. "It was hilarious and disgusting at the same time," recalled Fabre. She is an associate professor at the University of Bern in Switzerland. "It seemed that it was really enjoying what it was doing. It's something that they do pretty often." Fabre noted that the captive aye-aye might have been unusual in this habit, but there is no reason to assume it does not happen in wild aye-ayes either.
This observation made Fabre wonder if other primates eat their mucus too. When she carried out a literature review that included her own observations, she found strong evidence. Gorillas, bonobos, chimpanzees, macaques, capuchins, and other primates also pick their noses and eat the mucus. Most species used their fingers, but some used sticks to pry out the spoils. Some primates even extended the favor by picking the noses of others, the research found.
Fabre explained that we must understand what mucus actually is. "When you see the composition of mucus, it's mostly water, at more than 98%," she said. The remainder is composed of a protein-carbohydrate ingredient called mucins, along with salts. It is possible that animals reap some benefit from consuming these ingredients. This is similar to how some species will eat their own feces to digest the remaining nutrients there, Fabre explained.
This idea raises the question of whether there may be a deeper evolutionary basis for mucophagy in humans. Mucus creates a protective layer that traps dust, spores, and disease-causing microorganisms. It catches these things as we inhale them, before they reach our lungs. In 2013, a biochemist shared a hypothesis. They suggested that eating boogers could therefore expose children to small doses of pathogens. This might train the immune system to identify these molecules and help trigger an immune response.
However, this idea was not ultimately tested in empirical research. It remains a hypothesis. Dr. Chittaranjan Andrade is skeptical. He is the author of a 2001 study on nose-picking in teenagers. He is wary about such theories. "I am skeptical. Any immune substance that survives drying in the mucus is likely to be very small in quantity, and it is also likely to be digested after ingestion," he explained. He is a senior professor emeritus at the National Institute of Mental Health and Neurosciences in Bangalore, India. He noted that this likely has a limited effect.