When size matters: energy allocation and thermogenic mechanisms in very small mammals, African pygmy mice

Thermoregulation is a major challenge for extremely small mammals such as the African pygmy mice Mus mattheyi and Mus minutoides, weighing less than 12 g. A previous study showed that these tiny mice exhibit different mitochondrial energy efficiency for ATP synthesis, with a higher efficiency in M. mattheyi (∼6 g) than in M. minutoides (∼10 g). This result suggests a lower mitochondrial heat production at rest in M. mattheyi, despite its lower body surface and inevitably greater heat loss from its body surface, than in M. minutoides. Consequently, a compensatory thermoregulatory strategy should exist in M. mattheyi to maintain homeothermy. The present study aimed to assess whether M. mattheyi uses non-shivering thermogenesis in brown adipose tissue (BAT) and/or activity-thermoregulatory heat substitution. For this purpose, multidisciplinary approaches involving behavioral, physiological, biochemical and molecular analyses were used. Mus mattheyi showed higher daily mass-specific energy expenditure and food intake per unit mass and allocated more daily energy to vital function, spent less time moving in their cage during the daytime but exhibited higher non-locomotor activity and higher movement-related energy cost compared with M. minutoides. BAT of M. mattheyi was metabolically more active, exhibiting higher mitochondrial respiration rates and citrate synthase activity than in M. minutoides, but lower uncoupling protein 1 content. Altogether, these results suggest that the tiny M. mattheyi mainly uses non-exercise activity thermogenesis, increased cost of movement and, to a lesser extent, BAT non-shivering thermogenesis for remaining warm.