Low-cost thermoregulation of wild sloths revealed by heart rate and temperature loggers

Arboreal herbivores require large digestive tracts for leaf fermentation and detoxification; however, they must also have a low body mass that allows them to reach the foliage. The three-toed sloth, Bradypus tridactylus, experiences this trade-off, as leaves comprise 97.2% of its diet. Their calorie intake is extremely low owing to the low available caloric density of leaves and slow digestive processes related to leaf fibre fermentation and secondary compound detoxification. Sloths may require a high body temperature to assist fermentation; however, thermogenesis is energy-consuming. To investigate how sloths accomplish thermoregulation using marginal energy, we attached heart rate (HR) and temperature loggers to wild B. tridactylus individuals inhabiting the Amazon rainforest and recorded their HR and body surface temperature (T_skin). T_skin changed with ambient temperature (T_a) but was higher than T_a in 99.2% of cases. Increases in T_skin and HR did not coincide, suggesting that the increases were not caused by thermogenesis. Instead, they may passively increase T_skin by selecting warmer microhabitats and sunbathing. Consequently, 90.5% of T_skin were within 27.6-36.0 °C while the T_a fluctuated between 21.5 and 42.9 °C. This low-cost thermoregulation results in a low HR. In this study, the mean HR during observation was approximately 38.4% of the expected value based on the mammalian allometric relationship between body mass and HR. Thus, these properties may contribute to the low metabolic rates of sloths, alleviating their restricted energy intake.