New research suggests that males may be like autos with their accelerators welded to the floor, their aggressive and sexual urges held in check by constant application of the brakes.
Scientists at Johns Hopkins University reported today in the British journal Nature that male mice with a single gene switched off - so their brains can’t produce the chemical nitric oxide - exhibit a startling pattern of behavior.
They appear fearless when hanging upside down from miniature tightropes, lingering for long periods while normal mice scramble to right themselves. They chase squealing female mice around their cages for hours, even when the females are not in heat. Male mice typically quickly stop trying to mate with females that are not in estrus.
Most strikingly, genetically altered males fight other males to the death, continuing their assaults even when their victim’s adopt a submissive posture.
“We’ve never seen it before,” said Dr. Randy J. Nelson, professor of psychology at Hopkins. “Mice are very docile creatures.” The animals fight only to establish dominance, he said, and rarely, or never, kill each other.
Critics of efforts to find biological causes for violence say the Hopkins scientists have been too quick to interpret the actions of the altered mice as aggression, and to suggest that the work may apply to humans.
The Hopkins scientists say, meanwhile, that their study is the most dramatic demonstration so far of the link between brain chemistry and apparently impulsive behavior.
“These animals were very, very aggressive - dramatically so,” Nelson said. “They don’t seem to recognize social cues which would normally turn off” reckless, impulsive or violent behavior.
Female mice with the inactivated gene, meanwhile, did not display any increase in aggression.
In other respects, the genetically altered mice - called “knockouts,” because they have had a gene “knocked out” - seem to see, smell and behave normally, Nelson said. That suggests the loss of the gene doesn’t seem to cause brain damage or other neurological problems that might explain their behavior.
The research does not establish a link between an absence of nitric oxide in the human brain with vicious behavior. But the study’s authors said that their work hints that such a link might exist, at least in some cases.
The brains of rodents and people are very similar, said Dr. Solomon H. Snyder, a Hopkins neurobiologist and a co-investigator in the study.