Researchers studied a type of mouse that did not have progesterone and found that, unlike their counterparts with normal levels of progesterone, they did not act aggressively towards their young. The study appears in this week's issue of the Proceedings of the National Academy of Sciences.
"The most important part of this study is showing that male behavior towards infants is biologically based and that the receptors of the hormone progesterone, when it is activated, appear to increase aggressive behavior," says Jon E. Levine, senior author of the paper and a professor of neurobiology and physiology at Northwestern University in Evanston, Ill.
"When [progesterone receptors are] inhibited, it appears to decrease aggressive behavior and increase paternal nurturing behavior," Levine adds.
Levine's team studied "knockout" mice, ones that had been altered to lack the genes that encode progesterone receptors, making them unaffected by the presence of the hormone.
"This is very new," Levine adds. "We had no idea that the progesterone receptor was even remotely connected with paternal behavior, much less nurturing behavior."
According to Levine, "No one's had a clue about what mediates the expression of paternal behavior. As far as aggression [among animal fathers], there are conflicting findings. But everyone thought that since testosterone enhances some other forms of aggression, it would enhance aggression toward infants."
The team studied male mice with both the knockout gene and without it. In the 19 control mice, 74 percent killed their young, a common practice, in the first litter and 58 percent did so with the second litter. The 60 progesterone receptor knockout mice did not kill their young.
Both the knockouts and the controls exhibited similar aggression to other adult males, suggesting that progesterone may be a key hormone just in controlling aggression towards infants in male mice.
Levine says they do not know whether the same findings would apply to humans. "We hope to study this in humans at some point," he says.
"Here is what I am speculating," Levine says. "Nature may have adopted or co-opted the same neuroendrocrine mechanism that may govern maternal behavior and use it to regulate paternal behavior. In females, in most mammals, progesterone levels are high during pregnancy, serving to maintain it, and with labor and childbirth, progesterone comes crashing down [when a maternal response is needed]. This is a situation we are experimentally mimicking in the male with the progesterone receptor knockout animals."
"If the same mechanism were to operate in humans -- and we don't know that yet -- it might help to explain why it is that some men exhibit more nurturing paternal behavior than others," he says.
Another expert praises the new study. "This paper is quite exciting and groundbreaking," says Jeff Blaustein, a professor at the Center for Neuroendocrine Studies at the University of Massachusetts.
"Besides demonstrating the importance of particular genes to behavior, this study continues to make the case that the brain's progesterone receptor gene has a central position in the regulation of a variety of behaviors and physiological events," he adds.