Sometime, especially in colonies the rats will adopt each other's young. 1.3 colonies I find, spend a lot of time stealing pinks from each other, and/or working together to raise the babies. If you already have an established colony you may find a female who may have recently had a litter may adopt a different species. If it's still alive and looks relatively healthy you may have successfully fostered by the female you're referring to.

That's my 2 cents, but I hadn't spent a lot of time breeding (8 months before I closed down my breeding project) - And I never had to foster babies, but that's something I remember reading somewhere.

Cheers.