Nocturnal hyperthermia induced by social stress in male tree shrews: relation to low testosterone and effects of age.

Stress is known to elevate core body temperature (CBT). We recorded CBT in a diurnal animal, the male tree shrew, during a one-week control period and a one-week period of social stress using a telemetric system. During the stress period, when animals were confronted with a dominant male for about 1h daily, CBT was increased throughout the day. We analyzed CBT during the night when animals were left undisturbed and displayed no locomotor activity. To determine whether nocturnal hyperthermia may be related to stress-induced changes in hormonal status, we measured testosterone, noradrenalin and cortisol in the animals' morning urine. The daily social stress increased the mean nocturnal temperature by 0.37 °C. Urinary testosterone was reduced during the stress period, and there was a significant negative correlation between testosterone and the area under the curve (AUC) of the nocturnal CBT. This means that stress-induced hyperthermia was strongest in the animals with the lowest testosterone concentrations. As expected, urinary noradrenalin was elevated during the stress week but a positive correlation with the AUC data was only found for animals younger than 12 months. Cortisol was also increased during the stress week but there were no correlations with nocturnal hyperthermia. However, the stress-induced increases in noradrenalin and cortisol correlated with each other. Furthermore, there were no correlations between the stress-induced increase in nocturnal CBT and body weight reduction or locomotor activity during the light phase. Interestingly, the extent of nocturnal hyperthermia depended on the animals' ages: In animals younger than 12 months, stress increased the AUC by 48%, in animals aged between 12 and 24 months, stress increased the AUC by 36%, and older animals showed only a 7% increase. However, testosterone was not significantly reduced in the older animals. The present data reveal an interrelation between the extent of stress-induced nocturnal hyperthermia, the animals' gonadal hormone status and their ages. The negative correlation between hyperthermia and testosterone indicates that this hormone in particular plays an important role in the regulation of body temperature in male tree shrews.