Why is my rat biting her tail?

Brief：
She keeps biting her tail, not hard, but holding it in her mouth, and walking around in circles. Like a dog chasing its tail, but I havent heard of ra.. 

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She keeps biting her tail, not hard, but holding it in her mouth, and walking around in circles. Like a dog chasing its tail, but I haven't heard of rats doing this before. Some website said walking in circles is a sign of a tumor.

:[

She seems fine though, so I'm hoping it's something else. I've seen her do it before, a few months back, and shes still fine So I dont think its a tumor.

I know of only one other rat who would carry it's tail in it's mouth and walk with it. This was a male rat who was obsessed with nest building and mistook his tail as building material.

The only tumor in female rats that causes the walking in circles is a pituitary tumor.

http://ratguide.com/health/neoplasia/pit...

but the first classic sign of a pit tumor is the inability to hold food in the front paws.

It could be the pain of an inner ear infection and this is her way of trying to relieve the discomfort.

http://ratguide.com/health/auricle_ear/o...

It could be hormones acting on her maternal instinct as in a false pregnancy. She thinks her tail is a baby rat.

http://ratguide.com/breeding/postpartum/...

It could be a sign of dementia in the old rat.

There could be pain in the tail as in a broken tail or arthritis and she is trying to get to the source of the pain in order to relieve it.

There could be pain/itch in the tail as in mites having burrowed under the scales of the tail and she is trying to get to the source of the pain/itch in order to relieve it. There would be tiny sores on the tail as proof of mites.