WOODEN, K.M.; WALSBERG, G.E.; Arizona State University; Arizona State University: Temperature independent muscle function in a heterothermic mammal

Most birds and mammals must maintain a high and stable body temperature to remain fully functional. A change of more than 2�C in the body temperature of endothermic homeotherms usually leads to a loss of coordinated behavioral and locomotory function, as well as the impairment of physiological function. While the ability to shift to heterothermy is a fairly common response of some endothermic homeotherms to decreased ambient temperatures and food supplies, it also entails a shift to dormancy by these species. There are some species of birds and mammals that can, however, maintain normal activities and behaviors while allowing body temperature to vary by as much as 14�C. These species lower their energetic requirements by as much as 50% while remaining fully functional. How do these species maintain functionality over such a broad range of body temperatures? The round-tailed ground squirrel, Spermophilus tereticaudus, maintains physical performance at body temperatures ranging from 28 – 41�C. In this study, we test the hypothesis that the motor system of these species differs from that of other endothermic homeotherms and is capable of useful function over a broad range of temperatures. We compare to those of other mammals both isometric and isotonic characteristics of the extensor digitorum longus and the soleus muscles, over temperatures ranging from 10 – 40�C. Our results demonstrate that the performance of these muscles is much less temperature dependent than those of other endothermic homeotherms and that functionality is maintained over this entire range. The basis for this extended range of function is as yet unknown.