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 conspecifics. In order to determine if gonadal steroid can also modulate fear reactions in nonsocial situations, testosterone-treated heifers were exposed to various events reported to elicit fear in cattle. The experimental subjects received daily im injections of testosterone propionate (0.60 mg/kg body wt) for 100 days while controls received the same volume of the vehicle. In a first experiment, the influence of testosterone 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 effects of androgen treatment on cardiac and adrenal responses were evaluated in another group of subjects placed in the same situations. Whereas heart rates after the fear-eliciting events never differed between groups, the increase in cortisol levels was always lower in treated heifers than that in controls in response to human approach, to a surprising event, and to fear conditioning. Furthermore, after stimulation of the adrenal cortex by ACTH administration, the increase in cortisol levels was twice as great in controls than in treated heifers. Thus, prolonged androgen treatment reduces fearfulness in cattle, at least in some situations. Possible mechanisms by which testosterone influences fear-related behaviors are proposed.