Diurnal Changes in Metabolic Rate in Pygmy Marmosets: Implications for Sleep, Torpor, and Basal Metabolism in Primates

Abstract

The pygmy marmoset (Cebuella pygmaea) is the smallest New World Monkey and presents an interesting primate comparison to evolutionary experi- ments in body size evolution. Previous research has been suggested that pygmy marmosets exhibit a lower basal metabolism relative to mammals of similar size, with the adaptive reasons involving dietary constraints. Indeed, this hypometab- olism hypothesis has been evoked as a correlate to the pygmy marmoset’s pro- posed phyletic dwarfism. However, these earlier assessments of basal metabolic rates were conducted on sleeping animals. Pygmy marmosets are capable of quite pronounced diurnal changes in body temperature (4–5°C), with consequent effects on estimates of basal metabolism. Since comparison of a particular species’ metabolism to standard mammalian curves for basal metabolism requires similar assessment parameters, I chose to re-evaluate these measurements in the context of the changes in metabolism that accompany the onset and maintenance of the diurnal inactive period when marmosets typically sleep. V_ o2 estimates at thermo- neutrality in captive raised pygmy marmosets ranged from 34 to 11 ml O2 kg-1 min-1 during active periods to sleep periods. During this same time period, body temperature ranged from 39.2 to 35.5°C. Q10 for metabolism, assessed at similar thermal conductances, during these natural transitions was *4.6, which suggests that the process of sleep and subsequent metabolic adjust- ments are associated with a suppression of metabolism below the normothermic basal value. These results have implications for the understanding of primate metabolism, and reinforce the notion that large diurnal changes in body temper- ature and metabolism are common strategies for energy conservation in small mammals. G.