A hormone long recognized for its function in the preparation of the mammalian female body for pregnancy and childbirth may play a considerably darker role in the male brain, suppressing paternal instincts and even triggering infanticidal behavior.

For females, progesterone is involved in the maintenance of the uterine wall in anticipation of the attachment of a fertilized egg; later, during the course of pregnancy, placental progesterone maintains the pregnant physiological state and promotes lactation.

“The functions of progesterone in men are not well characterized,” according to Jon Levine of Northwestern University (Evanston, IL). “Progesterone is present in the serum of men in amounts that are less than females, but certainly not negligible. Male brains and pituitary glands, moreover, express progesterone receptors in patterns and amounts that parallel those in females.”

Levine's group explored a potential link between progesterone activity in male mice and infant-directed aggression, a behavior commonly seen in most laboratory strains (Proc. Natl. Acad. Sci. USA, 4 March). They used a transgenic mouse line lacking expression of the progesterone receptor (PRKO mice) and monitored the reactions of these mice toward pups and other adult males.

The results were striking. Wild-type C57BL/6 mice committed infanticide against their own young with a frequency of 74% for their first litter and 58% for their second. This behavior was absent in the PRKO mice. Relative to other strains, the PRKO mice showed substantially stronger paternal behavior, with only 8% of PRKO males exhibiting aggression upon presentation with a strange pup, as compared with nearly 30% for isogenic wild-type mice. Notably, PRKO male aggression against adult males was not substantially altered, suggesting that different mechanisms govern the two behavior patterns.

These results open a number of potential avenues for future study, including examination of the male-specific molecular and cellular mechanisms of progesterone action, and the manner in which progesterone pathways influence paternal behavior.

“There is little information that has been published...regarding fluctuations in progesterone levels in males in response to female pregnancy or childbirth,” Levine tells Lab Animal. “We are exploring the idea that fluctuations in progesterone [activity] during gestation, parturition, and lactation of a mate may be responsible for the induction of paternal behavior in those animals that normally exhibit paternal responsiveness—including men.”