My rat Gherkin does this thing where when she touches the bars of her enclosure sometimes she'd immediately start biting and chewing at her fingers after only to put her hands on the bars and repeat. theres never any blood she just kinda cleans them. I think her hands are sensitive to the texture of the bars and she hates it and so she constantly cleans them but she also loves climbing up for attention so she forgets that "bar texture bad" and puts her hands back on the bars anyway and remembers "ew bar texture gross"??? Idk tho. went to a vet once and they said that they'd never seen that behaviour in a rat before.