Variation in aromatase activity in the medial preoptic area and plasma progesterone is associated with the onset of paternal behavior.

The effects of aromatase within the brain on sexual behavior have been studied in a wide variety of species. Relatively few non-mating behaviors have been considered, despite evidence that estrogen affects many social behaviors. Testosterone promotes paternal behavior in California mouse (Peromyscus californicus) fathers, acting primarily via aromatization to estradiol. Virgin male California mice rarely exhibit paternal behavior, so we investigated whether aromatase in the brain changed with the onset of paternal behavior in California mouse fathers. In the medial preoptic area (MPOA), a brain area known to regulate parental behavior in rodents, we found that fathers had significantly more aromatase activity than mated males without pups, suggesting that an increase in estrogen production in this brain area contributes to the onset of paternal behavior. We also found that progesterone (P(4)) levels were lower in fathers compared to sexually inexperienced males and that P(4) was negatively correlated with aromatase activity in the MPOA. These P(4) findings agree with a recent study that found an inhibitory effect of P(4) on paternal behavior. Overall, we found that aromatase activity and P(4) levels change in association with an important life history transition, and may provide a mechanistic basis for plasticity in paternal behavior.