The role of body weight in resident‐intruder aggression

The behavior of male NIH Swiss mice of various body weights in the resident‐intruder test of aggression was investigated. Mice were housed individually for 10 days prior to the test, and allocated to six groups. In the first three groups body weights of residents and group‐housed intruders were matched, and the animals were divided into light, average, or heavy groups. In the last three groups weights of the intruder mice were either matched with the residents, or intruders were lighter or heavier than the residents. We found that light residents spent significantly less time in aggressive behaviors and longer time in defensive behaviors than the other two groups. The heavy mice showed most social investigation. The body weights of intruders were also shown to affect the behavior in the test: those residents which had light opponents spent a longer time in aggressive behavior than those which had matched or heavy opponents. The resident mice with heavy opponents showed most defensive behaviors. To study whether pharmacological manipulation may have different effects on behavior in the resident‐intruder test in mice having different weights, animals of light, average, and heavy body weight received a low dose (0.8 g/kg) of ethanol 30 min prior to the test. We did not note any effect of ethanol on aggressive behavior in the three groups. The results suggest that body weight plays a significant role in determining the level of aggression and defensive behaviors in the resident‐intruder test.