As I mentioned, stress biology is a very hot topic nowadays.
From a historical perspective, it's interesting that another split in thought appears to be developing between European and American scientists in this area:
Back in the day, Lorenz, Tinbergen and von Frisch led European scientists into the study of animal behavior via an ethological approach (focusing on innateness), while American scientists followed the lead of Watson and Skinner into its study via an experimental approach (focusing on learning); the result was the classic nature vs. nurture debate and its for the most part very clear geographic boundaries. (It's a quite entertaining and instructive story for those interested in such.) That debate has long since been resolved - nature and nurture both
play a role in all
behavior, of course - but it was with us a long time and indeed some less-informed folks still manage to lose themselves in it.
In stress biology, the Europeans (especially the British) are largely approaching the subject from a philosophical perspective born of the animal rights movement, focusing on whether and how much various kinds of creatures are suffering due to livestock or other food animal handling practices and to the captive confinement of wild animals such as in zoos. They appear by far most concerned with the question of whether humans should (be allowed to) treat animals in these ways at all. In contrast, the Americans are largely approaching it from a pragmatic perspective resulting partly from animal welfare concerns and partly from an interest in productivity. If dealing with agricultural practices, for example, the goal from the American orientation is to reduce animal stress in order to produce the highest quality meat most efficiently, and in the zoo world the goal is to reduce animal stress in order to improve health, longevity and reproduction and minimize unwanted behavior. Mind you, I'm not saying that none of the Europeans have pragmatic rather than philosophical ends in mind, nor that the Americans don't care about animal suffering in and of itself; in particular, with the animal rights movement's help the European perspective has made some headway in mainstream North America even if not as much among American scientists, and most zoo folk in America and elsewhere care deeply about their animals' happiness as well as well-being. But these are the general trends.
I suspect this dichotomy will ultimately resolve much as did the nature vs. nurture debate. Both philosophical and pragmatic perspectives are worthwhile.
As much as animal rights advocates might wish otherwise, I don't believe we're ever going to stop using animals for food, clothing or even recreation. Nor, from my personal perspective as a career scientist dedicated to wildlife conservation, should we. We've obviously evolved with a strong tendency to interact in a variety of ways - including ways that at least appear purely recreational - with the various other creatures with which we share the earth. In the long run that tendency is a very good thing, as it keeps us connected to and caring about those creatures and their environment. We care most for that which we know best - and care least for that which we know least.
Returning to our specific topic from the general situation, even if being picked up by a curious human now and then stresses a frog or snake so badly as to significantly shorten its life - which we have no evidence of it happening, by the way - I would still be for curious humans being allowed, even encouraged, to occasionally pick up a frog or snake. The human-animal connection and its ramifications for wildlife and wild lands conservation is much more important overall than is the lifespan of a few individual animals.
Don't forget, too, that all creatures have evolved to experience stress, and all creatures do indeed experience it at both major and minor levels throughout their lives - whether or not they ever encounter a single human being. Never mind stress alone, real suffering of various kinds and both great and small are a very normal and inescapable part of nature. We can't eliminate animal stress or suffering, not even were we to disappear off the face of the earth tomorrow; the best we can hope to accomplish is to reduce it among those animals over whose lives we have any real influence.
These are the reasons why I advocate (as I have a number of times on these message boards
) that people aim to minimize the amount of stress/suffering they inflict on other creatures by using best practices while dealing with them in whatever pursuit (livestock rearing, hunting, pet keeping, herping, etc.), not that they kindheartedly but foolishly try to eliminate the animals' stress/suffering by ceasing in those pursuits althogether. Said another way, go ahead and do what you want with animals (within the law), but do also think about your purpose and your effects, and consider especially how you might reduce the most negative of your effects while still achieving your purpose.