Activation of the hypothalamic-pituitary-adrenal axis and the subsequent release of cortisol is considered one of the major components of the physiological stress response in humans (Rose 1984). Stress responses of the pituitary-gonadal axis are not as well known although their sensibility and specifity are impressive.

Earliest studies investigated young military trainees in extremely stressful situations during combat training (Kreuz et al. 1972; Rose et al. 1969) and observed a significant decline of testosterone levels under psychologic and somatic stress. Twenty years later, Opstad (1992) studied Norwegian military cadets during five days of training involving strenuous exercise and almost total deprivation of food and sleep. They confirmed the previous findings of significantly decreased testosterone levels. Opstad attributed the hormonal response to extreme endurance training, sleep deficit and psychic stress (cf. Christiansen et al. 1984; Cortes-Gallegos et al. 1983; Guezennec et al. 1994; Singer and Zumoff 1992). In a similar study design, Bernton et al. (1995) investigated young male soldiers during eight weeks. They lived under extreme psychosomatic stress with long exposure to rough environments, caloric deprivation, four hours of sleep per week and psychologic stressors including constant risk of academic failure and the threat of simulated attack. The soldiers' testosterone levels decreased to clearly hypogonadal levels.

Even under less extreme conditions, psychosomatic stressors exert an influence on gonadal hormones: decreased concentrations of testosterone were found in males after surgery with anaesthesia (Carstensen etal. 1973;Matsumoto etal. 1970; Nakashima et al. 1975), driving heavy goods vehicles (Cullen et al. 1979), and routine flying missions in fighter type aircraft (Leedy and Wilson 1985) as well as significantly higher serum concentrations in female pilots flying a transport plane in comparison to female members of the ground crew (Dongyun and Yumin 1990).

Psychic stressors, e.g., financial difficulties, examinations, serious quarrels, loss of close friends or relatives, dissatisfaction, boredom, or watching a movie with a stressful theme were generally followed by decreasing testosterone levels in males (Christiansen et al. 1985; Francis 1981; Hellhammer et al. 1985; Nilsson et al. 1995). Even anticipation of a stressful event, a final exam at the university, leads to a decrease of salivary testosterone in males and to an increase in females the morning before the stressful situtation (Christiansen and Hars 1995). From the viewpoint of evolutionary biology, the sex-specific response to stress makes sense. While spermatogenesis in males is testosterone-dependent, in females, high levels of androgens are usually associated with anovulatory menstrual cycles. Thus, both reactions suppress fertility in order to increase the chance of survival of the individual and its family or group who are living under threatening or other stressful circumstances, which could be hazardous for pregnancies and infants, and the raising of children.

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