Social environmental influences on the development of fighting behavior in male mice were studied with respect to two types of factors: the necessary conspecific interactions; and the critical age of such interactions. Males housed with females fought as much as isolated males, but males housed with males or with a group of males and females fought much less, indicating that the lack of male-male interactions causes the increased "isolation-induced" fighting in the test situation. Males isolated or housed with females for 1 mo showed similar fighting tendencies whether the isolation occurred during the second month of age or during the third month of age. This suggests that the early juvenile age is not the critical point in development of fighting, rather that isolation from other males can increase fighting of male mice at juvenile as well as at adult ages.
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