loss of hair

Question
I have a mother and daughter guinea pig who have been living together always. The mother is very laid back and fat(not overweight though)but the daughter is quite thin. The daughter has been getting thinner recently and has a bald patch in the middle of her back, but I have seen her eating some hay, grass and vegetables. Also the daughter sometimes irritates the mother by biting her on the bum and sort of head-butting her. Should we be keeping them together and is the daughter ill? We have been to the vet but she just said it might be hormones and wasn't sure.

Answer
Hi Cat.  I am slightly concerned about this bald patch you say.  It could be due to a parasitic infestation as this can cause weight loss and well as hair loss.  The fact that she is irritating her mum by nibbling or biting at her suggests that she herself is indeed irritated.  Pigs can behave like this toward other pigs or us humans to vent their frustration or to show their inability to cope with intense irritation such as itchy skin.  Are her droppings okay, or do you notice them being smaller, or softer these days?  
I would recommend you return to your vet and suggest parasites being a possibility to treat for; they may opt to use Ivomectin to do this which would be good.  In the meantime if you are particularly concerned about the little one's weight you could ask your vet for some Oxbow Critical Care or Supreme Science Recovery to additionally feed her to ensure she still gets important nutrients.
I hope this helps.