Male mice are naturally territorial, causing many animal facilities to house them singly to reduce fighting. However, new research published July 27 in Neuron revealed that male mouse living in dormitory-style housing showed reduced aggression compared to those housed by themselves.

The study showed that male mice that were co-housed were less likely to attack other mice, even after cells in the ventromedial hypothalamus were stimulated. Stimulation of these cells cause increased territorial aggression in male lab mice (more on this below).

"In the current study, we showed that experimentally activating these cells is sufficient in males to drive territorial aggression," Nirao Shah, MD, Ph.D., professor of psychiatry and behavioral sciences and of neurobiology at Stanford University, explained to ALN. "In contrast to aggression from a singly housed resident males, activation of these cells in the ventromedial hypothalamus in socially housed males does not elicit aggression when these males enter another male’s territory."

Something about social housing, the researchers conjectured, reduced aggression in male mice.

"Our study shows that housing and social context of the assay matters a great deal in how the animal may behave.  These variables should therefore be taken into consideration when designing and interpreting outcomes of experimental studies," Shah said.

Explaining Aggression
Additionally, the Stanford University School of Medicine research team identified a cluster of nerve cells in the male mouse's brain that triggers territorial rage. Activating this cluster in female mice has no effect.

The researchers found that stimulating cells found in the ventromedial hypothalamus', a section of the brain, of male mice caused fierce displays of territorial aggression in male lab mice housed on their own for over a week. These male mice even attacked females and other species, behaviors typically not seen in mice.

The could help explain violent aggression in people and explore the effects of solitary confinement on people.

"Up to 5% of the population may show intermittent explosive disorder in which the person lashes out violently or in an intimidating manner in situations where the rest of us would not be aggressive.  One could speculate therefore that if such an aggression center also exists in the human brain, then abnormal activation of this center in such disease states could predispose the patient to be aggressive," Shah concluded.