Shrew metabolic rates and thyroxine utilization.

Elevated basal metabolic rates (BMR) in shrews have been observed for over 40 years, yet no one has investigated the physiological basis for this phenomenon. This paper reports on investigations of thyroid function in a shrew (Sorex vagrans), and in three rodent species for comparison. The utilization rate of thyroxine in shrews is found to be much higher than for any of the rodents, and is correlated with BMR expressed as a percentage of the predicted BMR. It is concluded that the unusually high thyroxine utilization rates of Sorex are an important, if not the sole, determinant of their elevated metabolic rates.