Suppression of postpartum fertility in pairs of female rats sharing the same nesting environment

The probability that a successful pregnancy will follow postpartum estrus in the rat is significantly reduced when the postpartum female is housed with another female that is both pregnant and lactating. We studied 16 pairs of females, each pair maintained in a separate nesting environment containing a sexually active male. In 12 of the pairs, the female that gave birth in the presence of a lactating-pregnant partner either did not mate, mated but failed to implant, resorbed her fetuses or delivered non-viable young. Several mechanisms that might underlie this evident suppression of fertility are discussed.