The 'natural diet' discussion comes up from time to time, and it does not seem that anyone has done the work to give a definitive answer. Should garters only be fed what that subspecies eats in the wild? The garter I work with most is the eastern blackneck, which I'd put money does not eat mice in the wild. Yet, it's not so hard to switch them to mice using frog-scenting. Am I depriving them of essential nutrients or loading them up with too much of other nutrients? The only answer I can offer is that mine eat a rodent diet and seem quite healthy. And ... lots of people keep garters on a completely mouse diet with good results. I think even those species that take some mice in the wild do so opportunistically, so that a 100% mouse diet is probably "unnatural." That does not necessarily mean it is unhealthy.

I'd offer this speculation: could it be that in the course of speciation, many members of the genus Thamnophis may have evolved certain behavioral prey preferences, but perhaps their digestive systems have not changed in such a way that they (most, anyway) cannot thrive on a mouse diet. That would seem to be in line with the experiences of garter keepers that feed mice. Not that I 'know' this to be the case, but it seems like an interesting guess.

It would be nice if someone were able to do some controlled experimentation with one group being fed mice, and the other matched group being fed a diet based on whatever is known of their wild diet (from habitat, observation, stomach contents studies). Growth, mortality, reproduction, and other measures could be recorded and compared, to see what differences there may be.