May 05 2013

Suicidal mice

Evolution by natural selection says that those characteristics that enable organisms to survive and reproduce more than others will tend to end up dominating the population. In that model, organisms seek to propagate their genes as much as possible. Suicide as a biological instinct is clearly not advantageous and should be selected against and disappear over time. So what are we to make of some mice that seem to commit suicide by actually running towards cats and being killed and eaten by them?

Ed Yong explains that it is caused by a parasite that infects many animals but can only complete its life cycle in the guts of a cat. So how does the parasite do that? By invading the brains of mice and causing permanent changes in its neuronal structure, causing them to lose their fear of cats and run towards the source of cat urine. So basically this parasite ends up controlling the brain of a mouse, at least as far as this particular behavior is concerned.

This example because shows the danger of taking a superficial view of behavior and that the organism for which we should apply natural selection may not be the one we see. Traits that seem maladaptive on the surface when looked at in terms of one organism (the mouse in this case) turn out to be the byproduct of a deeper mechanism that is beneficial for a different organism (the parasite).

It struck me that to the extent that we assign consciousness to a mouse, as far as the mouse is concerned it might think that it was acting of its own volition in going towards the cat. It had essentially ‘chosen’ to commit suicide. This has implications for free will in that it reinforces the argument that those actions that we think of as freely chosen are really the result of brain processes over which we have no control.


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  1. 1

    Oh, it’s worse than that. Humans don’t need physical parasites, we have ideas to die for.

  2. 2
    Marcus Ranum

    brain processes over which we have no control.

    You mean: all of them?

    Brain processes control us, not the other way around. Because they are “us”!! What we think of as control is a feedback loop, not a command/control system. It’s that model of command/control that leads us to erroneously posit the existence of “souls.”

  3. 3
    Marcus Ranum

    Oh, it’s worse than that. Humans don’t need physical parasites, we have ideas to die for.

    Ideas promulgated and propagated by parasitic humans (aka: “the ruling elite”) because they are benificial to their lifecycle.

  4. 4

    A number of years ago a friend of mine showed me pictures of his young cat that had “befriended” a mouse.
    The cat had no interest in killing the mouse as per usual cat behavior from lack of instinct and/or the way it was raised.
    The mouse (not of the domesticated variety) was behaving apparently socially towards the cat, which was puzzling to me at the time. This phenomena apparently explains the oddity.

  5. 5
    other dave

    “Suicide as a biological instinct is clearly not advantageous and should be selected against and disappear over time.”

    Depends on the age and conditions of the environment surrounding the suicide.

    If in an environment of limited resources, suicide after reproductive age might be advantageous to the species as a whole, making suicide a special case of altruism and advantageous that way.

    (With all due respect, ask a young job seeking Ph.D about the advantages of suicide in the elder Ph.Ds to the entire population of scientists in that field.)

  6. 6

    And all this time, I thought “Cheese Chasers” (1951) was just another ridiculous Looney Tunes cartoon. Who knew?



  7. 7

    http://en.wikipedia.org/wiki/Dicrocoelium_dendriticum for a liver fluke which inspires similar behaviour in ants

  8. 8

    I read somewhere that many car drivers who suffered accidents have been infected by Toxoplasma Gondii, more than average. So reckless behaviours can be the result by infection in non-rodent species, too.

    I would love to spray some political hypocrites with an aerosol containing the organism, thus increasing the probabability that they would get careless and caught out.

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