29 October 1999

Are unfaithful rats less aggressive?

by Kate Melville

Prairie vole's are rodents and the males interest in defending his pups, is an oddity in male rodents kingdom.

Scientists from Johns Hopkins University think this atypical behavior may come from his brain's production of a compound linked to aggressive behavior in mice. The researchers found that the male vole's brain chemistry closely resembles that of nursing female mice. This unexpected result may help researchers start to understand some of the complex biochemical roots of mating-related behaviors.

Stephen Gammie, a postdoctoral fellow at Hopkins said, "By focusing on these specialised behaviors, we're starting to pick up on some important similarities in the ways they may be triggered. "When they and their pups are approached by a stranger, both the male vole and the female mouse with pups experience increased production of a compound called citrulline in the brain," (citrulline is a byproduct of the reaction brain cells use to produce the messenger compound nitric oxide, which plays a role in turning on these forms of aggression).

The scientists had previously investigated nitric oxide's relationship to aggressive behavior and found that a line of genetically engineered mice produced to study brain damage from stroke had suffered an unexpected side effect. The males among the mice were unusually aggressive, relentlessly attacking other males and ignoring female rejection of attempts to mate. The damaged mice had been given a form of the gene for a protein known as nitric oxide synthase, theoretically leaving the mice with little or no nitric oxide in their brains. In another previous study scientist looked at the effects of the modification on female mice. Instead of gaining increased aggressiveness like the males, the females lost their aggressive behavior!

In the new study, scientists used voles that are also rodents, but are more closely related to lemmings and muskrats than to mice. "Voles were interesting to us because, as in humans, the males are monogamous, and help take care of the pups they produce. Also, while voles are relatively non-aggressive, previous research had shown that males experience a dramatic increase in aggression toward intruders after mating," said Gammie.

The new study found consistently higher levels of citrulline in the mated males and females with pups, the animals that would aggressively confront a stranger. The increase was focused on an area of the brain known as the paraventricular nucleus (an area located in the hypothalamus, part of the brain where environmental stimuli are integrated with internal signals from the brain).

The study is set to continue with scientists planning to test if they can suppress nitric oxide production in the voles with drugs and if so, if using this drug in mated male voles reduces their aggression levels. In addition tests may be conducted to assess the possibility of links between nitric oxide and the monogamous behavior (but we don't recommend any women try this on their partner just yet).