Although very closely related phylogenetically and with similar body masses, two small African pygmy mice (Mus mattheyi, ≈ 5 g and Mus minutoides, ≈ 7 g) appear to exhibit different strategies for thermoregulation (maintaining their body temperature at a constant high value). We hypothesized that M. mattheyi uses its locomotor activity, i.e., muscle work, as a possible source of heat at "rest." Therefore, the locomotor activity (by infrared camera) and metabolic rate (by indirect calorimetry) of the two small African mice were studied to determine if M. mattheyi was more active than M. minutoides during the day and thus, if it used muscle work as a possible source of heat. The mitochondrial activities of their skeletal muscle and brown adipose tissue were also evaluated to determine if their role in heat production is more important in M. mattheyi compared to M. minutoides.