The effect of age on two kinds of aggressive behavior in inbred strains of mice.

Shock-induced aggressive behavior is absent or very weak in C57BL/6 mice at the age of 12 weeks, increases at the age of 16 weeks and reaches the highest levels at the age of 20 weeks. This age-dependent increase of aggressive responses is absent in BALB/c and DBA/2 mice at the three ages examined in our experiments. Social isolation induces a clear age-dependent increase of aggressive behavior in BALB/c and DBA/2 mice (the highest level being evident at 20 weeks of age) but not in C57BL/6 mice which are characterized by the lowest or absence of aggressive responses. These results are discussed in terms of the role of developmental and genetic factors in the expression of shock- and isolation-induced aggressive behavior in the laboratory mouse.