Body temperature and locomotor capacity in a heterothermic rodent.

We quantify the locomotor capacity of the round-tailed ground squirrel (Spermophilus tereticaudus), a mammal that can lower energetic costs by relaxing thermoregulatory limits without becoming inactive. We measured maximum sprint speed, maximum limb cycling frequency and maximum force production in animals at body temperatures ranging from 31 degrees C to 41 degrees C. We found no thermal dependence in any of these parameters of locomotion. Results (means +/- S.E.M.) across this range of body temperatures were: sprint speed = 4.73+/-0.04 m s(-1), limb cycling frequency = 19.4+/-0.1 Hz and maximum force production = 0.012+/-0.0003 N g(-1). The neuro-muscular system of this species may thus be less thermally dependent at these temperatures than that of other mammals, allowing for the maintenance of whole-animal performance across a broader range of body temperatures. The absence of any significant loss of locomotor capabilities associated with either a decrease of 7-8 degrees C or a rise of 3-4 degrees C in body temperature from typical mammalian values raises significant questions regarding our understanding of the evolution and physiology of the mammalian mode of thermoregulation.