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Jean-Baptiste Leca, Ph.D.

Associate Professor

Department of Psychology

University of Lethbridge


Dental flossing in Japanese macaques: Implications for the determinants of behavioral innovation and the constraints on social transmission

We described the appearance of a particular tool-use behavior in the free-ranging provisioned group of Japanese macaques living in the mountains on the northwest outskirts of city of Kyoto, near the suburb of Arashiyama.

This behavior, consisting of using hair as dental floss, had never been reported in Japanese macaques before.

So far, it is an idiosyncratic behavior, i.e. performed by only one individual at Arashiyama: An adult female named Chonpe-69-85-94 (photo on the left).



This female used 3 different techniques to floss her teeth:

1) The “stretching with mouth” technique consisted of stretching its own hair or another monkey’s hair by clenching its lips on to the basal part of the hair, inserting the hair between the front teeth by slightly pulling its head downwards, and pulling the head backwards while gradually moving the lips to the distal end of the hair and performing repeated teeth-chattering.

2) The “stretching with hand” technique consisted of stretching its own hair or another monkey’s hair by grasping and pulling the tip(s) of the hair between the thumb and forefinger of one hand, moving the mouth to the hair, and inserting the hair between the front teeth by performing repeated teeth-chattering.

3) The “plucking” technique consisted of pulling out its own hair with one hand, holding the hair horizontally by grasping and pulling the tips of the hair between the thumb and forefinger of both hands, taking the hair to the mouth, and inserting the hair between the front teeth by performing repeated teeth-chattering.

The "plucking" technique is illustrated in the following video:

The floss was always created from one or a few long monkey hairpieces. The innovator female could use either her own hair or another monkey’s hair to floss her teeth.

We used this example to discuss the determinants of behavioral innovation and the various constraints on the early diffusion of behavioral traditions.



Because chance may account for a good number of behavioral innovations, and dental flossing was always associated with grooming activity, we suggested that the dental flossing innovation was an accidental by-product of grooming.

Thus, the following is a reasonable scenario: during regular grooming episodes, Japanese macaques sometimes bite into hair or pull it through their mouths to remove external parasites. Because of particular anatomical constraints such as diastema (i.e. gaps between incisors), pieces of hair may accidentally have stuck between this female’s teeth, and as she drew them out, she may have noticed the presence of food remains attached to them. The immediate reward of licking the food remains off the hair may have encouraged her to repeat the behavior for the same effect in the future, by actively inserting the hair between her teeth.

Therefore, the dental flossing innovation could be a transformation of grooming patterns via the running of hair between the teeth to remove louse eggs.

Among the various factors that may facilitate or restrict the diffusion of behavioral innovations within social groups of animals in general, and non-human primates in particular, we discussed in our research paper how group size, kinship, and dominance relationships are likely to limit the opportunities for any group member to observe the innovator, and thus constrain the diffusion of potential candidates as new behavioral traditions.

For example, in our study, the dental flossing innovator had only two close kin: her mother and only one sibling, a 2 year-old brother. She had no offspring. Since the pathways of diffusion of most behavioral innovations by Japanese macaques involve at some point the spread among siblings and the downward vertical transmission from mother to offspring, this paucity in individuals closely related to the innovator may have limited opportunities for diffusion of dental flossing behavior.

Finally, depending on their form, function, or context of performance, some types of behavior may be more or less prone to spread within a group.

First, from our detailed analysis of the dental flossing behavior, and as currently practiced by its innovator, we suggested that this innovation was not conspicuous enough to be reliably noticed and learnt by naive group members.

Second, although dental flossing is a form of tool use, we doubted that this innovation had significant, or even any, survival value, through the very small amount of extra food the flosser can obtain from its behavior.

Moreover, like most other “comfort innovations”, such object manipulation has a narrow window of applicability.

All these structural and functional aspects could partly explain why this behavior has not spread to other individuals.

Further reading: Newspaper articles and website links about the dental flossing innovation at Arashiyama:


“From flossing to . . . philosophy?”

By Rowan Hooper (Sunday, Aug. 9, 2009)


“Tidy Monkey Flosses Teeth”

By Jennifer Viegas (Tuesday, Dec. 29, 2009)