Behavioral thermoregulation in relation to humidity in wild Japanese macaques (Macaca fuscata yakui): the significance of semi-shade.

Recent climate change is increasing thermal stress, highlighting the importance of understanding how wild animals behaviorally buffer these conditions. Although microhabitat use is known to vary with temperature and to function as behavioral thermoregulation, the effect of humidity on this process (i.e., the effect of humidity on temperature-dependent microhabitat use) remains poorly understood. Here, I aimed to examine microhabitat use in wild Japanese macaques (Macaca fuscata yakui) on Yakushima, Japan, across sunlight categories (sun, semi-shade, shade), explicitly accounting for both temperature and humidity. The study spanned from October 2020 to October 2021, excluding March 2021; temperatures ranged from 6.0 °C to 35.1 °C and humidity from 35.7% to 91.2%. I conducted 512 h of focal follows on 24 adult females (≥ 4 years), averaging 21.3 ± SD 4.2 h per subject, and extracted 722 observations of microhabitat use under sunny conditions when macaques were resting or engaged in affiliative behavior, except for social play. At high temperatures, macaques selected shade at high humidity and semi-shade at low humidity, indicating that humidity modulates microhabitat use under heat stress. These results suggest that semi-shade use can serve as a behavioral thermoregulatory strategy for wild Japanese macaques under hot, dry conditions and may inform our understanding of how primates and other endotherms use microhabitats to cope with heat stress that varies with humidity.