SEASONAL THERMOGENESIS AND CHANGES IN BODY-MASS OF MASKED SHREWS, SOREX CINEREUS

Masked shrews (Sorex cinereus) are the most widely distributed shrews in North America, occurring from Alaska and Canada south into the nort hern one-half of the United States. Favorable winter survivorship of S . cinereus is reported from the Appalachian Mountains of Pennsylvania. In the present study, I examined the role of seasonal changes in rest ing metabolic rate, nonshivering thermogenesis, thermal conductance, a nd body mass of live-trapped shrews as mechanisms that enhance overwin ter survivorship. Shrews were captured monthly from April 1992 to Apri l 1993 in southwestern Pennsylvania. Temperatures at the ground surfac e ranged from -7 to 22 degrees C, in February and April, respectively, and snow occurred from November to March. Mean resting metabolic rate for shrews was highest in autumn (8.30 ml O-2 g(-1) h(-1)) and lowest in spring (5.75 ml O-2 g(-1) h(-1)). Body temperature of shrews avera ged 38.7 degrees C. Thermal conductance averaged 0.79, ranging from a low in spring to a high in autumn. Mean nonshivering thermogenesis was lowest in summer (8.02 ml O-2 g(-1) h(-1)), and peaked in winter (14. 60 ml O-2 g(-1) h(-1)). Capacity for nonshivering heat production in w inter was almost twice that of summer. Nonshivering thermogenesis show ed an inverse relationship to minimum ambient, ground surface, and sub surface temperatures. Body mass averaged 4.15 g and declined 53% from summer to winter. The favorable overwinter survivorship of S. cinereus is due in part to the ability of this species to increase thermogenic capacity by means of nonshivering thermogenesis coupled with energy c onservation in the form of a decline in mass during winter.