Kyoto, Japan -- As climate change alters the temperatures of animal habitats, it seems natural that endotherms, warm-blooded animals like us, would prefer to hang out in the shade during hot weather. The use of microhabitats in the sun and shade is an important thermoregulatory behavior that has been reported across a wide range of animal species, and researchers are becoming increasingly interested in how animals -- especially those with long lifespans -- flexibly cope with thermal stress.
Japanese macaques, sometimes colloquially called snow monkeys, reside further north than any other non-human primates, and also have the highest hair density, which may make it difficult for them to dissipate heat. While observing some macaques in the field, a researcher at Kyoto University noticed that some of them appeared to choose resting sites that were neither fully sunny, nor fully shaded.
"That observation led me to wonder whether semi-shade might play a more meaningful role in thermoregulation than previously recognized," says corresponding author Yoshiyuki Tabuse.
Semi-shade, an intermediate microhabitat between sun and shade, has been suggested to help lizards maintain optimal body temperatures, but its significance to endotherms has remained unclear. Humidity also likely influences behavior in hot weather, as has been reported in humans, but its role in mammalian thermoregulation is often overlooked.
Basing his study on the island of Yakushima, Tabuse spent about one year continuously observing a group of 24 female macaques aged four years or older. He defined three microhabitats according to the proportion of each macaque's body surface exposed to direct sunlight, classifying 0 to 33 percent as shade, 33 to 67 percent as semi-shade, and 67 to 100 percent as sun. On sunny days when all three microhabitats were present, he recorded the microhabitat selected by each individual when they began to rest, while also measuring the temperature and relative humidity.
The results revealed that humidity influenced the macaques' microhabitat use at high temperatures, but not low temperatures. While they were more likely to select shade under hot and humid conditions, hot and dry conditions prompted them to select semi-shade. This suggests that, rather than simply being a way to avoid direct sunlight, semi-shade may play a thermoregulatory role under hot, dry conditions.
"I find it very interesting that semi-shade is not simply an intermediate microhabitat between sun and shade, but is itself an important thermoregulatory option," says Tabuse.
These findings suggest that this microhabitat may be an overlooked but important thermal refuge for endotherms under heat stress. This study adds to growing evidence that humidity, in addition to temperature, should be considered when assessing how animals cope with heat. Next, Tabuse plans to examine how other types of rest site selection function as thermoregulatory behavior.
