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Why We Touch Our Mouths So Much: Evidence From Ants

Posted Apr 20 2012 12:00am

In a recent post I proposed that we touch our mouths so much to transfer germs from our hands to our immune system. It’s an early warning system. The full sequence is: 1. Hands. 2. Skin around mouth. 3. Tongue (lick lips). 4. Tonsils (immune system). Forewarned is forearmed: exposure to a tiny amount of Germ X makes you much more likely to survive exposure to a large amount of Germ X.

Cremer and her colleagues began by investigating how nestmates encountering an infected ant acted. They infected Lasius neglectus ants with Metarhizium anisopliae, a fungus that sticks to the insects’ outer cuticles and causes infection only after it has worked its way into the body, which takes a day or more. The researchers then placed infected or non-infected ants in a box with five nestmates, and watched what happened. . . .  Ants without the spores were groomed at a constant rate over 5 days, while Cremer saw a spike in grooming of the fungus-infected ants in the first day or two of infection, suggesting that the pathogen was prompting a behavior change in the nestmates.

The grooming was protective:

But even though they’d been exposed, only 2 percent of nestmates died from fungal infections, even though half of the initially infected ants, which had been dipped in solvent with M. anisopliae spores, died within 5 days. When ants were exposed to a dose of fungus expected to cause a 2 percent death rate, Cremer’s group saw an increase in antifungal activity, suggesting that this low level of infection was indeed enough to stimulate a protective immune response.

Earlier studies had shown what is called “social immunization” (“a protection of naive individuals of a colony after social contact to exposed individuals”) among insects. This study was about how social immunization happens.

After I thought of this explanation of mouth touching, I became much less concerned about contact with sick people. I hadn’t known about social immunization.

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