Incidence of pup killing and parental behavior in virgin female and male rats (Rattus norvegicus): differences between Wistar and Sprague-Dawley stocks.

The responses toward young shown by males and nulliparous females differed substantially between two outbred stocks of laboratory rats. Sprague-Dawley females showed maternal behavior either spontaneously (35% of the naive rats) or through concaveation (92% of the initially neutral virgins). Of the Wistar females, however, only 10% showed maternal behavior spontaneously, and only 29% of the neutral virgins came to behave maternally during 15 days of concaveation. Prepubertal cohabitation with lactating rats did not facilitate maternal responsiveness in adulthood in the Wistar virgin females. Of the Sprague-Dawley males, 50% showed paternal behavior spontaneously, and only 4% killed the young. Among the Wistar males, however, only 4% showed paternal behavior spontaneously, and 76% killed pups. Such profound differences between outbred stocks of rats may be a source of discrepancies between the results of studies dealing with the induction of parental behavior in nonlactating rats.