Postpartum estrus in Norway rats. I. Behaviour

Female rats in the wild appear to mate and conceive during a brief period of postpartum estrus, usually within the first 24 h following parturition. This shortens the interval between successive litters and can thereby greatly increase the female's overall reproductive output. This study presents a quantitative analysis of the time of onset, duration, and quality of the receptive behavior of females during this time. Females were tested for receptivity every 3 h following the delivery of the first pup. The percentage of females displaying lordosis rose rapidly from 30% at 6 h to 100% at 9 h and at 12 h. Receptivity declined steadily after 12 h, with 20% showing lordosis at 21 h. The quality of lordosis followed a similar curve. The postpartum estrus appeared to depend more upon the onset of parturition than it did upon the ambient photoperiod.