This behavior really isn't concerning but more so interesting.
I have noticed that whenever I find my beardie venturing over on the cool side of the tank (which in not very often, he basks a lot), he seems to have a whiter tummy and thus less stress marks.
I've noticed that sometimes, but not always, my beardie has had more stress marks on the hot side. His hands will be flat (not arching up like the surface is too hot). Are the stress marks maybe just a sign he is trying to make himself darker to heat up faster? Does darkening themselves (which I am aware they do) always make stress marks appear? It's never a concerning amount of marks, and there are warmer parts of the rock/branch he could go to if he wasn't warm enough.
I have also noticed he gets slight stress marks right before and during feeding. This makes some sense to me, "stress" is not always bad, it can make us alert. So are these stress marks just caused by heightened arousal when eating? Any ideas?
The dark lines are exactly what you're suspecting. They darken themselves to absorb heat/light. This happens a lot while basking and can also happen if they feel cool. They tend to turn lighter color when they are warm enough or are trying to cool off a bit. The belly lines tend to be common with younger dragons and fade as they get older. While darkening can indicate mood, I find the term stress marks to be misleading since they are so common and seem to be most often related to just temperature regulation rather than indicating a problem.
Thank you! It's good to hear that stress marks don't always mean stress! Of course I'll look out for intense or persistent marks, but this helps me relax a bit