"Out of suffering have emerged the strongest souls; the most massive characters are seared with scars".- Edwin H. Chapin
"When a man is pushed, tormented, defeated, he has a chance to learn something; he has been put on his wits ... he has gained facts, learned his ignorance, is cured of the insanity of conceit, has got moderation and real skill".
- Ralph Waldo Emerson
Don't know your setup at all, but if it is a bite, what is in his habitat that would be biting him? If it's a paper cut, you use newspaper as a substrate? If so, I doubt he could get a cut by a piece of paper sitting on the ground unless you have a water dish right beside the edge of a piece of paper which would keep it from moving. Looks more line a scrape to me, do you have anything with a rough edge that he could have caught it one while moving about?
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Trochu Motor League
If you feed him live, it almost looks to me like a scratch from the claws of the back foot of a rat that might have been thrashing about during the constriction. Could that have been a possibility?