Don't know if this will shed any light, but.....

My son has had rats as pets. One of their more unique traits (which I have
witnessed many times), is how a rat will take tiny bites of a "new" food,
and then wait until "something happens" to deterimne if it is safe to eat.
Whatever happens conditions that rat to either accept or avoid that
particular food in the future. Quite often, if no reactions occur, the rat
will return to the food within a few (maybe 10-15 or so?) minutes and
finish eating with relish (well, as much relish as a rat can muster :)

This would seem to be similar to, but not *exactly* like, what Jean-Claude
is talking about. The rat does not instictively "know" if the food is good,
but does have a built in "food meter" it uses to classify the food for
future reference.

I would think that, if this same ability is present in humans, it is
probably somewhat diminished. After all, a rat is not able to reason as a
human does, and therefore must trust it's own hard-wiring a little bit
more.