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Levels of plasma oxytocin in male Peromyscus californicus were assessed across the reproductive cycle to provide a foundation for subsequent studies of the hormonal basis of paternal behavior and pair bonds. In Experiment 1, plasma oxytocin levels were significantly higher in expectant fathers than in virgin males or fathers. Plasma oxytocin increased in expectant fathers 1 day postcopulation and remained elevated for the first 15 days of gestation. Plasma oxytocin declined by Day 20 of gestation and oxytocin levels remained low in fathers throughout the lactational period. Oxytocin levels among expectant fathers did not differ between parental and nonparental males or between infanticidal and noninfanticidal males, suggesting that plasma oxytocin may not be involved in the prepartum onset of paternal behavior or the inhibition of infanticide. In Experiment 2, plasma oxytocin levels also did not differ between males that were parental or non-parental prepartum or postpartum. However, males separated from their mate and pups on the day of birth had elevated oxytocin levels on Day 3 postpartum compared to males that remained with their family, suggesting that disruption of the pair bond and/or absence of the young affects plasma oxytocin levels in males. This possibility remains to be determined.

Relatively little is known about hormonal mechanisms underlying paternal behavior in mammals. Male California mice, Peromyscus californicus, display extensive parental care toward their young. Parental behavior of fathers, expectant fathers (males living with their pregnant partner), and virgin males was assessed in a 10-min test with a 1- to 3-day-old alien pup. Few virgin males acted parental (19%) compared to fathers one day postpartum (80%) and expectant fathers (56%). Plasma prolactin levels were significantly elevated in fathers 2 days postpartum compared to expectant fathers and virgin males. Paternal prolactin levels were similar to those of mothers. There were no differences between groups in levels of plasma testosterone. These data suggest, contrary to other reports, that prolactin is a likely correlate of paternal behavior in rodents.