It's a sex thing

Nitric oxide affects aggression in male and female mice in different ways

source: Alison Motluk
New Scientist September 25 1999 p16

Aggression in female mice is affected by nitric oxide in a different way from aggression in male mice. Male mice lacking a gene producing nitric oxide become aggressive, according to Stephen Gammie and team from John Hopkins University, Baltimore, US. The team found that female mice, which are usually only aggressive in defense of their pups, were less aggressive when they lacked nitric oxide. Normal female mice with young pups attack strange males approaching them, whereas females lacking nitric oxide do little to intervene. Female mice appear to need nitric oxide in order to be aggressive, and neurotransmitters appear to have different impacts on males and females. This finding may apply to other mammals, including humans.