BEHAVIORAL AND NEURAL BIOLOGY 32, 89-103 (1981) Mouse Aggression Increases after 24 Hours of Isolation or Housing with Females L VINCENT O'DONNELL University of Houston, Houston, Texas 77004 AND ROBERT J. BLANCHARD z AND D. CAROLINE BLANCHARD University of Hawaii, Honolulu, Hawaii 96822 Male mice isolated for only 24 hr show an increase in attack on strange intruders relative to the most aggressive animal of groups of three mice. This effect does not depend on the presence of male litterrnates during the intruder tests: it may also be seen in tests in a neutral arena. Housing of a male mouse with a female or females also increases aggression, and such males will attack former littermates as well as strangers. Openfield test scores were not related to aggression changes after 24 hr of isolation or housing with females. In mice, individual housing produces more aggression than is found in group-housed controls (Ginsburg & Allee, 1942; Kahn, 1954; Thiessen, 1963; Cairns & Nakelski, 1971). This "isolation-induced aggression" (Banerjee, 1970) has been interpreted as indicating that the lack of social and physical stimulation for isolated mice results in physiological changes that alter the emotional state of the subjects. Welch and Welch (1969) have suggested that decreased activity of neurotransmitters and adrenal hormones in isolated mice results in increased receptor sensitivity to conspecific stimulation. Similarly, Cairns and Nakelski (1971) have re- ported that fights involving isolated mice arise from the unusually strong reactions of such animals to the normal investigatory behavior of their opponents. An alternative explanation for the increased aggression among isolated mice is that grouping of male mice has the effect of inhibiting fighting. Preparation of this manuscript was supported by National Institutes of Mental Health Grant MH 29163. 2 To whom requests for reprints should be sent at: 2430 Campus Rd., Honolulu, Hawaii 96822. 89 0163-1047/81/050089-15502.00/0 Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.