EFFECTS OF ANDROGEN TREATMENT ON BEHAVIORAL AND PHYSIOLOGICAL-RESPONSES OF HEIFERS TO FEAR-ELICITING SITUATIONS

Sex steroids are known to influence dominance relationships in cattle. This effect seems due to a reduction of fear in response to conspecif ics. In order to determine if gonadal steroid can also modulate fear r eactions in nonsocial situations, testosterone-treated heifers were ex posed to various events reported to elicit fear in cattle. The experim ental subjects received daily im injections of testosterone propionate (0.60 mg/kg body wt) for 100 days while controls received the same vo lume of the vehicle. In a first experiment, the influence of testoster one treatment on behavioral reactions of animals was studied. Treated heifers were much less fearful than controls: they were less reactive to an unfamiliar environment or to a novel object, and they were also less disturbed by a surprising event. In a second experiment, the effe cts of androgen treatment on cardiac and adrenal responses were evalua ted in another group of subjects placed in the same situations. Wherea s heart rates after the fear-eliciting events never differed between g roups, the increase in cortisol levels was always lower in treated hei fers than that in controls in response to human approach, to a surpris ing event, and to fear conditioning. Furthermore, after stimulation of the adrenal cortex by ACTH administration, the increase in cortisol l evels was twice as great in controls than in treated heifers. Thus, pr olonged androgen treatment reduces fearfulness in cattle, at least in some situations. Possible mechanisms by which testosterone influences fear-related behaviors are proposed. (C) 1994 Academic Press, Inc.