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Tidy Monkeys Shed Light on Human Cleanliness

Monkeys that are revolted by often disease-carrying things such as poop suggest that disgust drives cleanliness - and health.

Japanese macaques are so tidy that they even wash their food in salt water, and now a new study finds that these monkeys have fewer parasites than other primates that are not nearly as careful.

Female Japanese macaques, in particular, are grossed out by sometimes disease bearing things like poop, suggesting that feelings of disgust help to fuel cleanliness, and thereby healthiness, among all primates, including humans.

The findings, published in the journal Biology Letters, could carry over to other animals with tidy tendencies too.

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"There are a few other accounts of animals washing food in water, like captive chimpanzees and capuchins, which both seem to wash specifically to remove debris from food items," co-author Andrew MacIntosh of Kyoto University's Primate Research Institute and Wildlife Research Center told Discovery News.

"There was even a recent study with zoo-based European wild boars that showed the animals capable of carrying food to water to wash it, a behavior that was restricted to those food items experimentally covered in debris," he added.

The new study, however, focused on Japanese macaques, which not only wash sweet potatoes in salt water, but also spend much of their days grooming and preening each other.

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In addition to monitoring cleanliness behaviors and associated parasite loads, MacIntosh and co-author Cecile Sarabian placed both actual and fake feces in the monkeys' wild habitat. To attract the macaques, the researchers place either a grain of wheat or a nutrient-packed peanut on both the faux and real poo.

Just seeing both types of poo turned off the monkeys, and especially the females. Although hungry, they passed up the wheat.

The monkeys did, however, go for the peanuts, revealing that even a primate known for cleanliness must constantly weigh the risks versus benefits of exposing themselves to something that could pose a health threat.

"You have to remember that these and other terrestrial animals are basically living in the dirt," MacIntosh said.

Nevertheless, the monkeys seem to go to a lot of trouble to clean off their food by either washing it, as mentioned, when the edibles are covered with sand, or vigorously rubbing and rolling it before consumption.

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Sarabian explained that Japanese macaques live in very humid and warm conditions for much of the year, "conditions which are suitable for the development and transmission of parasites. Animals that do display more cautious behaviors toward potential contaminants may thereby gain a reproductive advantage."

This could help to explain why females appear to engage in more cleanliness-associated behaviors than males. She quickly added that hygiene is also shaped by experience and culture, "but there is an overall biological pattern to our revulsions." Good hygiene, as a result, may be due to both nature and nurture.

"What I think is important about this study is that it shows that the tendency to avoid obviously contaminated or disgust-inducing objects is linked somehow with lower parasite levels, at least in adults. The big question is what is the mechanism for this," said Michael Huffman of Kyoto University.

Valerie Curtis of the London School of Hygiene and Tropical Medicine's Hygiene Center told Discovery News, "I'm really excited to see my theory that disgust evolved to drive behavior that prevents infection born out in experimental work in primates."

"There have been many anecdotal observations of hygienic behavior (in animals), but this is the first experimental demonstration of its likely function in primates, and so it is very exciting."

Japanese macaques eating washed sweet potatoes.

All chimpanzees, including this young female, have culture. Chimpanzees living in different colonies have invented 8 effective -- albeit sometimes messy -- ways of opening a hard-peeled fruit that is a delicacy in the chimp world, a new study has found. The study, published in the latest issue of Animal Cognition, supports the idea that chimpanzees have culture. Just as human cultures are associated with unique ways of preparing food, chimpanzees can now be associated with unique methods of preparing their edibles. In this case, the food is the green monkey orange. "Cultural behaviors are those that are established within a social group, are passed on through social learning (learning through observing others)," lead author Bruce Rawlings, a University of Portsmouth comparative and evolutionary psychologist, told Discovery News. "For many decades, humans were thought to be unique in their ability for culture...A growing body of research has begun to challenge the idea of human exclusivity for culture."

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Chimps go to a lot of trouble to eat yummy green monkey oranges. "In this study, we found that three chimpanzee groups showed differences in the way they open the hard-shelled fruit of the strychnos (green monkey orange)," Rawlings said.

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He and other researchers made the determination after conducting field observations of the chimp colonies at the Chimfunshi Wildlife Orphanage in Zambia. Some of the noted orange-opening techniques were present in certain colonies but were entirely absent in others, he said.

Technique #1: Bang the fruit against a tree. Rawlings and his team video-recorded how the chimps managed to open and eat green monkey oranges. The images here are stills from those videos. A juvenile male demonstrates Technique #1, which involves grasping the fruit and hitting it against a tree. The force cracks the shell-like peel, exposing the soft, juicy flesh of the fruit within.

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Technique #2: Bang the fruit against a rock. Here, "A juvenile male attempts to break the hard shell of the fruit by striking it against a rock," Rawlings explained. This method might be less tiresome than the tree-hitting one, since the downward force would require slightly less effort.

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Technique #3: Throw the fruit. While the outcome isn't always guaranteed, chimps have figured out that if they just throw the fruit, it will often split open upon hitting the ground.

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Technique #4: Nibble the fruit until there's a hole in the exterior. "An adult female displays the half-bite technique," Rawlings said. For this method, she and other chimps nibble a small hole in the fruit's exterior. They then use their hands, and sometimes even their feet, to pull off the hard outer layer to expose the soft fruit inside.

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Technique #5: Chomp into the fruit. Rawlings explained that here, "An adult male uses both hands to support the fruit while using the full bite technique." Humans sometimes do this too. Instead of cutting into an orange, a person might bite forcefully into the peel. If the fruit is small enough, or if the individual's mouth is big enough, he or she might be able to cut the rind in half this way.

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Technique #6: Hit two fruits against each other. "An adult male strikes two fruits against each other to open, known as fruit cracking," Rawlings said. Unlike supermarket oranges, green monkey oranges have an exterior that isn't so flexible. The fruits will usually break apart when struck together in such a way. It could be that humans performing a similar move first invented fire. Fire can result when flint, chert or other materials are struck together, causing sparks.

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Technique #7: Stomp on the fruit. "An adult male stomps on the fruit to crack the shell," Rawlings said, adding that he was "one of only two chimpanzees observed using this technique."

Technique #8: Peel the fruit. Adult chimps were seen peeling away the hard exterior of the green monkey orange. This is usually the preferred method used by humans when peeling fruits with softer rinds.

Chimps learn by watching each other. How does a cultural behavior originate? Usually one very smart or experimental individual comes up with an idea. If the idea works, others will try to copy it. In this image, Darwin the chimp closely watches another chimp named Pan, who is fixated on a tasty green monkey orange. Darwin, wanting a piece of the fruit action, may learn from Darwin's preparation technique and later enjoy a snack of his own. Rawlings said a case can be made that that these chimpanzees and the others in the study "may be showing rudimentary forms of human culture."