My Rat “Casper” was born on the 28th October 1998. When she was about 3 or 4 weeks old she and two of her brothers (from a litter of ten) were noticably smaller than their siblings.
Sadly the two little males died at 3 and 3 1/2 weeks old, respectively. By the time the rest of the babies were 5 weeks old, it was clear that Casper was always going to be quite tiny. The remaining 7 babies went to new homes, but I decided to keep Casperwith her mother a bit longer.