Is there something wrong with my under-sized gerbil?

I have an 18-month-old gerbil which is only about half the size of its

gerbil companion.

I have noticed she is permanently holding her head on one

side and has trouble balancing.

She even has to spin up her tubes as she

can't climb or jump properly.

However she seems happy otherwise and is still

eating/drinking/chewing etc.

I realise that she is fairly aged in gerbil

terms but can she be helped?