Problem Rat

Question
I bought two sisters 8 months ago. (I fear that they came from a poor breeder
with hindsight) When i first got them home they were completely scared of
any and all human contact. I had done extensive reading and understood the
importance of contact so i played with them as much as possible. No matter
what i did they would both try and run off and hide and were constantly
scared of anything moving around them. Over the course of the next 6
months I kept trying every day to make friends and bond with them. The first
rat (Jess) became much happier and although she shied away from me she
would let me pick her up and would sit on my shoulder, I was never happy
enough to let her roam around on her own however. the second (Tara) has
never gotten any better and it is nearly impossible to hold her. She will come
out when i call her and is happy to take food from me but as soon as i try and
remove her from her cage she runs away. Sadly Jess dies suddenly a week ago
and keen not to let Tara live on her own i have bought 2 more babies to go
with her. I followed all the advice given to me and allowed them to meet in
neutral areas. All seemed to be going well as there was allot of sniffing etc
but then for no obvious reason Tara grabbed one of the new babies and
pinned her down by the neck. She bit the baby so hard that she drew blood
and now if i put my hand anywhere near Tara she tries to bite me. I'm really
concerned they will not be able to live together. I have them in a large 3
teared cage  and i have separated them off so that they can see each other
and smell each other but also so that Tara can not attack the new babies.

Is there anything that i can do to try and get Tara to be more friendly and
what can i do to stop her attacking the others? It is very noticeable  how
happy the new babies are and they are already happy to run around out of the
cage and are very affectionate towards me.

Answer
 Some females, though more rare, detest living with other rats.  It is possible that Tara simply doesn't want any new rats living with her.  Since she has an aggressive personality, she will be a bit harder on the babies when it comes to asserting her dominance, and if the babies are really little she can easily hurt them even if she doesn't mean to.  I would keep them separated for now, perhaps waiting until the babies are a bit larger and can hold their own if she fights with them.

 You may have to try introducing them all over again several times.  Some rats just need more time to get used to a new cagemate.  I would wait a week or two then try again.  If you try several times over the course of a month or two and she still doesn't accept them, then she will be happier (and the babies will be safer) living alone.  It is true that rats highly enjoy company, but some rats choose to live alone and if a rat makes that decision by attacking any potential company then their decision should be respected.

 Hope that helps, good luck!