Experimental procedure to favor an otherwise undetectable potential rat muricidal behavior

We hypothesized that variation in mouse aggressiveness could, at least in part, affect muricidal attitude of rats, therefore we introduced a sedated (diazepam 50 mg/kg/PO) instead of a normal (nonsedated) mouse into a cage with a rat. Rats were challenged with sedated or nonsedated mice for two-four days. It was found that a) the percentage (40-60%) of rats which killed a sedated mouse was higher than that (about 10%) of those which killed a nonsedated mouse; b) rats trained to kill a sedated mouse did not kill a nonsedated mouse and c) rats which did not kill a nonsedated mouse did not kill a sedated mouse. The present results indicate that the first experience in the presence of a sedated mouse uncovers a muricidal behavior which persists upon exposure to subsequent similar experimental conditions. This behavior undergoes seasonal variations, reaching a maximum in September-October and a minimum in February-March.