Effects of age of pups and removal of existing litter on pup survival during cross-fostering between multiparous outbred mice

Periparturient manipulation of mice is a valuable tool for modern research facilities. Although fostering and Caesarian section frequently are used to eradicate pathogens, an often overlooked use is to rescue poorly breeding strains of mice. Here we characterized the weaning success rates after fostering outbred pups of variable ages (younger than 24 h; 5 to 7 d; 10 to 12 d) with full or partial replacement of litters and multiparous dams. There were no significant differences between most groups when analyzed by full or partial replacement or age of donor pups as compared with control groups, in which pups were manipulated but returned to the birth dam or the birth dam was not disturbed. However, significant differences were associated with fostering of 10- to 12-d-old pups in combination with younger pups. Overall, these findings suggest that limiting fostering to pups that are within 48 h of age and age-matching litters when fostering are unnecessary.