One of the classic challenges to arguments favoring Batesian mimicry of coralsnakes in tricolor milk snakes is that so many milk snakes and other potential mimics occur outside the range of coralsnakes. This challenge failed to consider that migratory birds encounter coralsnakes while wintering in Central America and retain inhibitions acquired there when they return North. These birds, likely the target predators for this mimetic model, will avoid anything that looks like a coralsnake regardeless of where they are.
I often see antipredator mechanisms discussed in terms of extreme deterrence, but it's not necessary for predators to be completely frightened off, just derailed a bit. I can attest that when trying to catch a wild animal by hand, one must be fully committed to the grab. Any hesitation will result in split-second delays that allow prey to escape, which they are in the process of doing even as the predatory strike begins. Antipredator devices such as tail coiling in Regal ringneck snakes, mimicry in tricolor milk snakes, Long-nosed snakes, even ground and Shovel-nosed snakes, are all quite useful even if all they do is give pause--which is all the prey needs to escape often enough to select for the trait.
The significance of inducing hesitancy in predators is not included in evaluations of antipredator adaptations often enough, IMO. More acknowledgement of the life-and-death value of inducing hesitancy in predatory strikes would allow the public to appreciate why apparent half-measures like the relatively undeveloped resemblance of Long-nosed snakes to coralsnakes are still selectively advantageous.
Mutations that produce any color scheme are random and there is no guarantee that color schemes will refine over time. It's tempting to think of Long-nosed snakes as a work in progress as far as their resemblance of coralsnakes but this may be the best they will ever do. Even the coralsnakes may have been out of luck as venomous animals that could defend themselves until random mutation gave them colors predators could use to identify them on sight. Non-venomous animals would find that being brightly colored in a world with no dangerous models is a one-way ticket to an early death, so any accidental mutations that produced bright colors in prehistoric times, before venomous coralsnakes, were likely quickly snuffed out leaving no descendants. After the development of a reservoir of Mullerian mimics in the form of 50-odd species of coralsnakes, those same random mutations that once made unprotected species more vulnerable would have become beneficial.
I think one of the most thought-provoking snakes is the black milk snake. It begins life as a coralsnake mimic, protecting itself during its most vulnerable time of life. Later, its priorities change as it needs to warm its large body in the cold, cloudy altitudes where it lives. Being black to warm effectively even in cloudy weather is more important to an adult black milk snake than retaining a resemblance to a protected model. If we needed an example of how mimicry is a bona fide value, I think perhaps this is it--protection when it's most critical, then abandoned for the higher adult priority of thermoregulation at altitude. Baby black milks can thermoregulate their smaller mass easily regardless of what color they are, making their higher priority survival by mimicry.
It's unlikely that black milk snakes moved to higher elevations first and then developed this ontogenetic color change. The ancestral black milk snakes would have already been turning black as adults before being able to exploit the thermally challenging habitat they now inhabit. There may have been a geologcially narrow window of time for the melanistic population to move to higher elevations before predators at lower elevations finished off the no-longer mimetic adults. Probably a percentage of the ancestors of black milk snakes expressed adult melanism, leaving a reservoir of founders in their ancestral habitat; the darkest ancestral black milk snakes lucked into a mutation that took a general tendency for later-life melanism to useful levels. In any case, modern black milk snakes are the only ones that undergo such dramatic color change. They are excellent examples of mutations that enable survival in habitats that demand different priorities at different ages.
Any attribute that enhances survival in baby snakes is a powerful selective advantage, and resemblance to potentially lethal snakes encountered during birds' seasonal residence in coralsnake territory has to be one of the all-time lucky rolls of the genetic dice.
