When we performed factor analysis of the behavioral responses to an unfamiliar intruder, five orthogonal factors with Eigenvalues greater than 1 were generated: “Agonistic/Highly Aggressive,” “Harm Avoidant,” “Curious/Bold,” “Threatening,” and “Reactive.” These factors explained 68% of the total variance (Table 2). Prior reports from other laboratories have demonstrated there to be both sex and age differences in responses to an unfamiliar intruder 35. In agreement with this, we found that, during social intrusion, sub-adults were more harm-avoidant than adults (F(1,105) = 12.19, P = 0.007) and that males rated higher on both the Curious/Bold (F(1,105) = 6.7, P = 0.01) and Reactive factors (F(1, 105) = 5.6, P = 0.02) (data not shown).
