Oxytocin changes in males over the reproductive cycle in the monogamous, biparental California mouse, Peromyscus californicus.

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.