if its always in the same place- i've seen kings and corns do it before. once i saw a king bite itself where its heart was while it was dieing- probably had a heart attack and was biting what was hurting it. it could be a million and 1 reasons.... if its a male it might smell a female and think itself is a female too(seen that before too)
~kin
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~Sober Serpents~
Corns, Creamsicles, A Black Rat, Thayeri, Cal Kings, A Jungle Corn(Just A Pet), A Bearded dragon, Leopard Geckos, Green Anoles, a Snapping turtle, and a white cheeked mud turtle