rats fur

Question
I am hoping you can help me. I have 2 female rats who are both roughly 3 months old. I have noticed over the last couple of days both of my rats are losing small amounts of fur in little patches. The skin underneath  on one rat is very healthy and no scabs. The other has a small amount or redness under the bald patch.
I have been watching out for them scratching and neither of them seem to be scratching , and they are both eating and drinking normally.
They are still playing lots, but the smaller of the two rats seems to keep jumping on the bigger ones back.
I was wondering what you thought about the hair loss and on the smaller one seeming to want to play piggy back!!
They have been together for about 10 weeks now.
I hope you can help.
Thanks for your time.

Answer
 If they have curly whiskers it is possible that they are double rexes, which is a type of coat that loses and regrows hair in patches.  If not, it is possible that they have an external parasite such as mites or lice, or they may just be chewing each other's hair off either as a dominance behaviour or in normal play.  I would recommend treating them for parasites just in case, and if that doesn't help then I would take them to the vet for an exam.  The treatment for parasites is one drop of Advantage or Revolution (flea killing products) on the back of the neck against the skin, then another in two weeks, making sure to clean the cage and all toys within really well in the meantime.  If the skin is healthy it is probably them barbering each other, which should stop in time as they get older and figure out their pecking order.  Allergies are also a possibility, though more rare.

 The jumping on each other's backs is normal dominance / play behaviour, though it can also be construed as mating behaviour.  If you're sure they're both females then you have nothing to worry about =)  I hope that helps answer your question, good luck with your rats!