Which factors determine an animals level of aggression? Three hypotheses have been proposed: (1) that levels of aggression increase with population density; (2) that bodyweight is an indicator of an animals aggressivity; (3) that aggression levels vary with external conditions,that is,populations living in the most unfavorable environments demonstrate the highest average aggression levels,and populations living under the most favorable external conditions demonstrate the lowest average aggression level.We tested these three hypotheses by manipulating enclosed populations of root voles (Microtus oeconomus) under different food and predation treatments and observed their aggressive behavior.Aggressive behavior was measured by matching mice in a neutral arena.The experimental results supported hypotheses 2 and 3,but they did not support the first hypothesis.Moreover,we found that reproductively active individuals were more aggressive.We concluded that the growth of population density did not cause or otherwise bring about increased aggressive behavior of root voles,but the external factors (predation and food supply) and physical factors (body weight and reproductive condition) were significantly correlated with aggression levels in a root vole population.