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Costs of parental care can include increased physical, physiological, and behavioral demands and reduced prospects for future reproduction. Therefore, animals may benefit from recognizing and discriminating their own offspring during the period of infant care to ensure that they provide care only to their own young. Differences in offspring discrimination between mothers and fathers may arise due to differences in the amount of parental care they provide. In mammals, mothers bear the costs of gestation and lactation, while fathers do not; therefore, we might expect that in mammal species in which both parents provide parental care, mothers will show greater levels of offspring discrimination than fathers. In the present study, we examined possible sex differences in offspring discrimination in the biparental, monogamous California mouse (Peromyscus californicus) across the lactational period. On postpartum days (PPD) 3, 7, 16, and 28, parents were housed individually in a test cage and presented simultaneously with two wire mesh balls, one containing their own pup and the other containing an unrelated pup. On individual test days, parents behaved similarly toward the two balls. Fathers, but not mothers, showed significant changes in latencies to approach and interact with the non-kin ball across test days, but no clear pattern was observed. Fathers also approached the non-kin ball more frequently than mothers when pups reached weaning age (PPD 28). Thus, fathers may show increased interest in unrelated young as their offspring become independent. [ABSTRACT FROM AUTHOR]