Pantherophis alleghaniensis routinely forages several metres above ground, raiding bird nests and tree cavities. Reaching them means climbing surfaces with no footholds at all.
The snake uses concertina locomotion. It presses a stretch of the body against the trunk in tight lateral bends, generating friction on both sides at once, then anchors that section and extends the head end upward before drawing the tail section up in turn. The keeled belly scales, slightly angled where they meet the flanks, increase purchase on bark. Movement proceeds in discrete pulses rather than as a smooth glide.
The cost is high. Laboratory measurements of snakes climbing show energy expenditure far above that of level slithering, and the animals fatigue quickly, which is one reason a climbing rat snake pauses often. Ratsnakes have also been recorded climbing smooth artificial poles by the same method, which is why purpose-built predator baffles on nest boxes need to be wide and smooth-sided rather than merely tall.