Re: Forced lethargy in some animal species.

Dear Sebastien,

I think the term you’re looking for is Tonic Immobility. A number of species will enter this state of inactivity when inverted or exposed to some other stimulus.

While this might sound a bit unlikely, I’ve seen Tonic Immobility in action: as a kid, my friend George showed me what he called “chicken hypnosis” by turning a chicken on its back and gently rubbing its sternum. Apparently, one can induce Tonic Immobility in chickens in other ways ( http://www.almanac.com/preview2000/hypnotize.html), some without inverting the animal (check out this video: http://www.youtube.com/watch?v=M268UccYVCE&search=chicken).

This phenomenon has been demonstrated in vertebrates, including sharks, other birds, lizards, and guinea pigs and in invertebrates as well. The argument for its existence is as a last defense to avoid predation; some predators are less likely to attack prey if it’s not struggling. The introduction to this recent article has good references on the topic going back to a study in 1928:

da Silva, L.F.S and L. Menescal de Oliveira. 2006. Cholinergic modulation of tonic immobility and nociception in the NRM of guinea pig. Physiology and Behavior 87 (4): 821-827.

Good luck with your chicken hypnosis or whatever brings up the question.

Cheers,
Jim