Physiology & Behavior, Vol. 36, pp. 223-228.Copyright©PergamonPress Ltd., 1986.Printedin the U.S.A. 0031-9384/86$3.00 + .00 Behavioural, Physiological and Immunological Consequences of Social Status and Aggression in Chronically Coexisting Resident-Intruder Dyads of Male Rats A. RAAB, R. DANTZER, 1 B. MICHAUD, P. MORMEDE, K. TAGHZOUTI, H. SIMON AND M. LE MOAL INRA, Psychobiologie des Comportements Adaptatifs, INSERM U.259, Rue Camille Saint-SaYns F-33077 Bordeaux Cedex, France Received 3 May 1985 RAAB, A., R. DANTZER, B. MICHAUD, P. MORMEDE, K. TAGHZOUTI, H. SIMON AND M. LE MOAL. Be- havioural, physiological and immunological consequences of social status and aggression in chronically coexisting resident-intruder dyads of male rats. PHYSIOL BEHAV 36(2) 223-228, 1986.--The behavioural and physiological conse- quences of social status and reciprocal fighting in resident-intruderdyads of Long Evans male rats were evaluated. Before a chronic cohabitation of 10 days, residents and intruders were individually housed for one month to increase their aggres- siveness. Control animals included isolates, i.e., animals kept individually housed throughout the experiment and pair- housed rats, i.e., pairs of rats housed together from their rats in the laboratory. In 19 out of 20 dyads, a clear dominance relationship developed with an advantage to the resident in 68% of the cases. Dominants showed more exploratory activity than subordinates in an open-field test at the end of the cohabitation period; subordinates groomed longer than animals from other experimental groups. Dominants had lower pain thresholds than individually and pair-housed animals. Both dominants and subordinates had higher tyrosine hydroxylase enzymatic activities in the left adrenal than isolated and pair-housed rats. Subordinates lost body weight and had higher plasma corticosteroid concentrationsthan animals from the other experimental groups. In addition, they had smaller thymus glands and reduced spleen lymphocyte responses to mitogenic stimulation in vitro, in comparison to dominant animals. These results show that subordination in the dyadic resident-intruderparadigm leads to a complex syndrome of behavioural and physiological changes, some of which may be modulated by the intensity of aggressive interactions. Rat Social status Resident-intruder Aggression Open-field Physiology Plasma corticosterone Tyrosine hydroxylase Cellular immune response Analgesia ALTHOUGH much interest has been devoted to the study of behavioural and physiological correlates of social be- haviour in laboratory animals, using models such as over- crowding [10], disruption of dominance orders [17] and in- trusion of new animals in preexisting groups [4], there is still an unsufficient knowledge of the psychosocial factors re- sponsible for the observed changes. This is easily apparent from the large literature on endocrine correlates of dominance-subordinationin animals. Although defeat and submissiveness are in general accom- panied by enhanced pituitary-adrenal function [5,6], there are many examples of a lack of relationship between social status and pituitary-adrenal activity [18,30] and even some cases of higher plasma or urine glucocorticoid levels in dominant than in subordinate animals [12,22]. Such diver- gent data indicate that the concept of dominance alone has no explanatory nor predictive value for the physiological re- sponse to social situations. There is evidence that the influence of social status is modulated by at least two additional factors, the frequency and intensity of agonistic interactions and the possibility of control. The role of agonistic interactions'is exemplified by the positive correlations observed between the amount of aggressive behaviour and the plasma corticosterone re- sponse to paired encounter tests in rats [33] or between the number of bites received and post-defeat analgesia in de- feated mice [24]. The possibility of a significant role of con- trol is suggested by studies in which the subordinate is able to protect himself from the dominant's threats and attacks because of spatial arrangements of the cage [28]. However, ~Requests for reprints should be addressed to Dr. R. Dantzer. 223