Sunbeam snakes are not often captive bred, so they are heavily imported. This is rather unfortunate as most imported animals die within the first six months of capture. Wild-caught sunbeam snakes stress very easily, this combined with improper environmental conditions encountered during transport and holding at wholesale facilities, is just too much for many of these snakes.
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As sunbeams stress easily, do not handle them unless it is absolutely necessary. So success with sunbeams amounts to four things; keep them humid, buried, warm, and leave them alone.