you have completely misunderstood evolution. it is not survival of only the most fit, but survival of everything that except the least fit. a small survival advantage is, frankly, enough for the retention of a feature. no net effect is enough for the retention of a feature. and if there is some small advantage, that does not mean that EVERY similar animal would likewise spontaneously evolve a covergent feature or become extinct.
And perhaps it is you who do not realize into depth what you are talking about. Go to Mimicry thread we can discuss "survival advantage" and "fitness" there generally and in particular cases.
in any case, this is not what people are arguing. if you'll notice above, my argument is that it's a throwback to crocodilian practices.
That's more interesting as the reasoning of PaulK. Do you mean that genes for such behaviour were inherited from dinosurian ancestors and were somehow switched on? You know I like frontloading, epigenesis etc.. I suppose this is what you means. Otherwise if such genes has arisen de novo I wouldn't call such obviously innate and fixed behaviour as "throwback".