Seasonal Energetics of the Cotton Rat (Sigmon Hispidus) From Southwest Missouri

To determine what strategies the cotton rat (Sigmodon hispidus) may employ to survive harsh winters, monthly (June, 1988 through January, 1990) measurements of metabolic rates, nonshivering thermogenesis (NST), thermal conductance, and body mass were made. In addition, serum thyroxine (T₄) levels (June, 1988 through April, 1989) and triiodothyronine (T₃) serum levels (February, 1989 through January 1990) were analyzed. Oxygen consumption data were measured at temperatures between 5°-36°C and NST was elicited by a subcutaneous injection of norepinephrine. Oxygen consumption data were corrected for differences in animal size by dividing by body mass. Resting metabolic rate (RMR) was found to have monthly differences but did not follow a significant seasonal pattern. NST also differed each month, and was found to follow a seasonal pattern. The maximum RMR and NST both occurred in the second coldest month recorded during the study (February, 1989). Minimum thermal conductance occurred in the lowest temperatures tested (5°-10°C) and varied each month. Body mass followed a seasonal pattern, but it is unclear if this is an actual survival strategy. Both T₄ and T₃ showed monthly differences but did not follow a significant seasonal pattern, although T₄ tended to decline in winter while T₃ displayed elevated winter levels. As an alternative to hibernating, S. hispidus appears to rely on other physiological mechanisms to survive in winter.