Why We Touch Our Mouths So Much: Evidence From Ants

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.

Ants have a similar early-warning system, says a new study described here.

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.

2 Replies to “Why We Touch Our Mouths So Much: Evidence From Ants”

  1. Maybe eye rubbing close to bedtime is similar ?

    Maybe even nail biting is similar ?
    (combined with precious nutrient recycling in that case)

  2. What I noticed in law school, where you really don’t want to get called upon and then ritually humiliated for an hour, that if you didn’t touch your face it looked as if you understood the material — and were less likely to be the victim of the day.

    Did not work when the prof used an alphabetical chart.

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