Disputing established scientific beliefs, recent experiments by a team of Northwestern professors, researchers and graduate students have revealed a new cause for aggressive behavior in male mice.

The experiments, led by neurobiology and physiology Prof. Jon Levine and fifth-year graduate student Johanna Schneider, showed that the paternal behavior of male mice toward their pups is affected by the level of progesterone, a hormone associated with female maternal behavior. Past scientific knowledge has assumed testosterone was the hormone responsible for the aggressive behavior patterns in male mice.

Levine and Schneider said they were investigating progesterone receptors in mice reproduction two years ago when they noticed a difference in the behavior of male mice that had been genetically modified to produce less progesterone.

“Johanna noticed that the males that were genetically engineered for the progesterone hormone were much more gentle in the nurturing of the mouse pups,” Levine said. “Normally male mice are rotten dads. These mice were not, and it was very clear to us.”

The team followed-up on its discovery with two experiments that confirmed progesterone led to heightened paternal behavior in male mice, Levine said.

“The bottom line is that the activation of the progesterone hormone in mice increases aggressive behavior towards baby mice,” Levine said. “Decreasing progesterone tends to increase paternal behavior towards baby mice.”

Neena Schwartz, professor emeritus of the department of neurobiology and physiology, said the experiments’ conclusions surprised her.

“It’s a totally new kind of discovery because people haven’t really looked at what progesterone does in males,” Schwartz said. “I think that is really exciting because progesterone causes real behavior changes in these mice and . . . (shows) that the progesterone receptor is as important in males as it is in females.”

Levine said the experiment also yielded another interesting discovery: The pacifism male mice exhibited toward mice pups did not apply to other adult males. Male mice still were aggressive with other males, even with decreased levels of the hormone.

Schneider said creating the experiments and working on the research was difficult because of the groundbreaking nature of the discovery.

“It was very exciting, but a little hard for a student because there was no precedent of what to expect next,” she said.

Levine said the discovery currently applies only to mice. Scientists would need to conduct further research to see if the hormone affects humans in the same way, he said.

“At least in male mice, this appears to be a molecular switch that prompts more aggressive behavior on one hand or paternal behavior on the other,” Levine said. “We must see if this physiological mechanism occurs in naturally paternal species, and humans are one of those more naturally paternal species.”

In addition to investigating progesterone’s effects on humans, Levine and Schneider said they want to research the molecular mechanisms of progesterone in mice brains to draw a clearer picture of their current discovery.

“We have a big black box,” Levine said. “We know how to get into it, but we don’t know what happens inside of it.”