Testosterone is the hormone most often linked to male dominance and aggression, but a new study indicates that the hormone progesterone may play a bigger role in how males react to their offspring.
Adult male mice rarely show nurturing behavior toward their offspring, and frequently attack and kill them shortly after the babies are born. Studies have shown that testosterone levels, correlated with adult intermale aggression in animals, do not correlate reliably with paternal nurturing or aggression in mice. Looking for another explanation, Johanna Schneider et al. investigated the effects of progesterone—a hormone produced in large amounts by females, and in much smaller amounts by males—on the parenting behavior of male mice.
The researchers created progesterone receptor "knock-out" (PRKO) mice, missing the gene that encodes these receptors (meaning that the mice are not affected by progesterone). Lead researcher Jon Levine reports, "In male knockout mice we noticed something quite startling. They behaved differently, and the most obvious changes were a complete lack of aggression toward infants and the emergence of active paternal care. These animals are terrific dads."
While 74 percent of male control mice killed their offspring, none of the PRKO mice committed infanticide. Moreover, the PRKO mice were more nurturing, frequently touching their pups and returning them to their nests.
In another experiment, the researchers administered a drug that blocked progesterone receptors in normal mice. These mice, they found, behaved much like the knock-out mice, becoming more nurturing and less aggressive toward their offspring.
Interestingly, both the progesterone knock-out mice and the mice receiving the progesterone-blocking drug continued to show typical levels of aggression toward other adult male mice. Thus, the aggression- enhancing effects of progesterone appear limited to behavior toward infant mice. While Levine notes that further research is needed to determine if these results are applicable to other species including humans, he says, "At least in the case of mice, this appears to be an important neurochemical switch that can increase paternal behavior and decrease aggressive behavior toward infants."
The researchers note that while females have higher levels of progesterone than males, studies show that "female responses to progesterone are both qualitatively and quantitatively different from those observed in males." In females, high progesterone appears to inhibit responsiveness toward infants and to increase aggression toward adult male intruders, but not to increase violence against infants.
"Progesterone receptors mediate male aggression toward infants," Johanna S. Schneider, Marielle K. Stone, Katherine E. Wynne-Edwards, Teresa H. Horton, John Lydon, Bert O'Malley, and Jon E. Levine, Proceedings of the National Academy of Sciences USA, Vol. 100, No. 5, March 4, 2003, 2951-2956. Address: firstname.lastname@example.org.
"Progesterone regulates male mouse behavior," press release, Northwestern University, February 24, 2003.