Sloths are a physiological conundrum tiptoeing a fine, physiological line. As herbivores, sloths get all of their nutrients and calories from a diet of low-nutritional leaves, leaving them with relatively few calories to burn for bodily processes after foraging. (Imagine how much energy you would have after eating nothing but unflavoured rice cakes.) Because sloths do not obtain many nutrients or calories from their diet and they use what little energy they have to gather and digest that poor-quality food, it would make sense that they cut corners on other physiological processes, such as body temperature regulation. Daisuke Muramatsu, with collaborators from Kyoto University, Nara University of Education, Nagoya University and Hokkaido Research Center, Japan, and Universidade Federal do Amazonas, Brazil, investigated the body temperature patterns of wild sloths in the Amazon rainforest of Brazil by monitoring the heart rate and skin temperature in free-ranging pale-throated sloths (Bradypus tridactylus) to find out how cheaply sloths regulate their body temperature on their low energy budget.
The researchers attached heart rate monitors and temperature data loggers to 34 free-ranging sloths. They caught the animals near the Federal University of Amazonas in Brazil by climbing trees and gently taking the relatively undisturbed and unprotesting individuals back to the field lab. There, they used a custom-made harness to attach a modified sloth-friendly Fitbit to monitor each animal's heart rate, a temperature data logger for skin temperature measurements and a VHF transmitter to locate the individual animal to retrieve the precious data loggers. After attaching the harnesses, they released the sloths back to the trees where they had been captured and installed a temperature data logger on the trunk of the tree to measure the temperatures that each sloth experienced. Five days later, the scientists returned to the area and normally the animals had not moved too far.
Analysing the information stored by the skin temperature data loggers, the team discovered that the sloths were always slightly warmer than the trunk temperatures, but those elevated skin temperatures did not appear to be very costly for the animals, as their heart rates did not increase with the temperature. Because the researchers don't know exactly where the sloths were hanging out during each temperature measurement, they assumed that perhaps the sloths were basking in sunny spots when their body temperatures were warmer. The sloths seem to be taking an easy option for maintaining their body temperature. Instead of increasing heart rate or using up expensive fuel to increase or maintain their body temperature, the animals were probably relying more on the sun and environment to do that work for them. This dependence on basking could help explain how sloths are able to maintain their body temperature cheaply instead of constantly burning expensive fuel when on such a tight energy budget.
However, this study, posted online as a preprint at Social Science Research Network, has not yet been peer reviewed, so we should be cautious about some of the researchers’ conclusions. Referring to the sloths’ skin temperatures as body temperatures could lead to some incorrect conclusions about body temperature regulation, as the skin temperatures recorded by Muramatsu and colleagues are likely very different from core body temperatures. Skin temperatures tend to overestimate core temperature during warm environmental temperatures and underestimate it in cool conditions. Regardless, sloths are an exciting study species in which to study body temperature regulation, as they must strike a delicate balance between staying warm enough to keep their fermenting digestive microbiome happy, but doing so in a slow and measured way.