Parturition influences initial pup preferences at later onset of maternal behavior in primiparous rats.

Initial maternal responsiveness as a function of varying pup stimuli was assessed in primiparous Long-Evans rats. Pups were removed during parturition and the dams tested beginning 24 hr later. These dams were most likely to respond maternally towards newborn (0-2-day-old) rat pups (100%) and 6-8-day-old hamsters, which are the size of newborn rats (83.3%). In contrast, dams were significantly less likely to respond maternally towards newborn hamsters (50%) and 8-10-day-old rats (16.7%), pups which are half as large and twice as large, respectively, as newborn rats; indeed, dams were likely to attack these pups (33.3% and 25%, respectively). The maternal response (less than or equal to 1 hr) to dead newborn rats was similar to that towards live newborn rats, except that fewer dams retrieved dead pups rapidly (less than or equal to 1 min). Cesarean-delivered dams did not display higher maternal responsiveness towards 0-2 than towards 8-10-day-old rats. Further, whereas no parturition-experienced dam displayed infanticide towards newborn rats, 21.9% of primiparous Cesarean-delivered dams did. Thus, the exogenous and/or endogenous stimuli associated with parturition enhance selective maternal responsiveness and diminish infanticide towards pups the size of newborn rats.