Berislav Horvatic wrote:Lay flat and motionless and irresponsive... "Feigning death" is something quite different from that...
I agree with this statement, that´s why some author make the distinction between tonic immobility (what you observed with your V.berus) and a more complex death feigning strategy which involves for instance body flipping, tongue hanging, defecation, etc.
Ilian Velikov wrote: I only know about the N. natrix and N. tessellata doing this (any others?)
In my short note I cite papers reporting on death feigning for most of "our" colubrids (but this behaviour is indeed more common in the 2 species you mention). If you look beyond Europe, then hognose snakes are also well-known examples regarding death feigning.
Ilian Velikov wrote:Also from all the N.natrix I've cought (which are a lot) only one individual did feign death
This proves you are a gentle man, Ilian I personally experienced more death feigning N.natrix over the last years. In their article (DOI: 10.1037/0735-7036.121.2.123), Gregory and Isaac report a pretty high number for N.natrix: "Death feigning was seen in 66% of wild-caught snakes". Also, I think you will be interested in their conclusion: " so far as we can ascertain, death feigning in grass snakes has been observed only in response to humans (...) What is required to complete this story for grass snakes and most other species is convincing demonstration of successful death feigning in response to natural predators in the field and consequent fitness benefits compared with non-death-feigners.".
This study is already from 2007, I have no idea if this last point was investigated since then...