We raised rat's in Florida many year's ago to feed our Indigo's and had a simular problem but were told that our rat's had a sodium deficiency due to the many litters that they were putting out, and that by eating thier young, they were getting the sodium back. So, if they were healthy I guess this could be the problem, but more likely the over population. They wouldnt have happend to have been Chinese rat's and only the female's were missing were they?