Why is my rat suddenly getting a lot of gray hair?

I'm a proud owner of four rats. One of these, a female, has suddenly started developing a lot of gray and white hair behind her ears and around her shoulders. I have two of her siblings who are the same age, but they both lack the grey hair. The female in question has given birth before (back in October of '05). But she currently does not seem to be stressed. Could somebody please provide me with an explaination for this?Why is my rat suddenly getting a lot of gray hair?
Having had many rats as pets and love them as a group, it seems they tend to age, as we do, after they give birth. The best thing to do is keep her from any stress she doesn't need. They are very lovable, and very sweet, but they only live about 3 years, and any added stress such as birth tends to age them. Good luck; she is precious, and the grey shows her age, and stress level just like it does to people.Why is my rat suddenly getting a lot of gray hair?
Lack of nutrient in body