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The mouse that didn’t roar: Dormitory housing defuses hardwired male territorial aggression

Stanford neuroscientist Nirao Shah, PhD, has made a career out of focusing on behaviors (such as mating, aggression and nurturing offspring) that innately differ between male and female mammals. He has identified specific nerve circuits that act differently in male and female brains to cause those different behaviors. And he's found specific genes within those nerve circuits that seem to control the execution of these behaviors. I wrote about his work quite recently in an article for our magazine, Stanford Medicine, titled "Two minds: The cognitive differences between men and women."

But it's hard to keep up with Shah's pace. In a new study published in PNAS, he and his colleagues showed that an innately hardwired behavior common among male mice but essentially nonexistent among female mice — territorial aggression: the rabid defense, against any and all male (but not female) intruders, of a chunk of turf a male mouse has been occupying -- is subject to some rather intricate rules of social etiquette.

In short, as I wrote in a news release accompanying the study:

[W]hether a male mouse displays territorial aggression depends on whether it’s recently been the sole occupant of a bachelor pad or living in the mouse equivalent of boarding school. The latter makes for good manners; the former, not so much. ... [Shah's] findings suggest that social forces can override genetically programmed behavior. The findings also could potentially help explain the ill effects of solitary confinement on prisoners, as well as what underlies psychiatric disorders characterized by bursts of violent anger.

Male mice are naturally territorial. In the wild or in the lab, they attack other male mice even if plenty of room, food and females are available. A typical male mouse who’s been dug in for a while as the sole occupant of a piece of territory will normally attack any other male placed into that territory. This behavior is under the control of a small nerve circuit in the male mouse's brain; disabling the circuitry shuts off the defending male's -- or any male's -- fighting spirit.

Contrariwise, the new study demonstrates definitively, “selectively activating just this tiny cluster — about 5,000 nerve cells in a brain with 80 million nerve cells — escalates the level and extent of male mice’s aggressiveness dramatically,” Shah told me when I was interviewing him for the release. Even normally rather meek "intruder" males become fierce fighters when, in the presence of a defending turf-holder, that crucial 5,000-cell cluster is stimulated via sophisticated lab techniques.

But if the male mouse had been housed with other males prior to being tossed into another male mouse's man-cave, it won't attack the aggressive solitary male on the latter’s home turf, even when the researchers rev up the activity in the socially housed male's relevant brain circuit. Something about social housing seems to powerfully temper male mice’s aggressiveness.

As with mice, so with men? We don't know, but it does make one wonder if solitary confinement of aggressive male criminals might be counterproductive.

Previously: Inside the heads of men and women: A look at sex-based cognitive differences, Men's typically taciturn Y chromosomes tell colorful tale of conquests, expansions and Tomayto, Tomahto: Separate genes exert control over differential male and female behaviors
Photo by Robert Orr

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