Howler monkey (Alouatta palliata) thermal behavioral strategies and resting time are temperature-dependent.

Primates exhibit thermal behaviors (use of postures in tree stratum) that help regulate body temperature in response to environmental conditions, reducing the energetic and water costs of thermoregulation. This study examined the relationship between ambient temperature and thermal behavioral strategies during resting periods in mantled howler monkeys (Alouatta palliata) across 3 sites in Veracruz, Mexico: Agaltepec (AGA; 28.4 ± 4.7 °C), Mirador Pilapa (MP; 28.8 ± 2.1 °C), and Zapoapan (ZAP; 28.9 ± 2.5 °C). We used focal-animal sampling, randomly selecting individuals and recording the time invested in each posture and the tree stratum in which they rested, and the ambient temperature. All statistical analyses were performed independently between sites. The results indicate that, across the 3 study sites, a consistent pattern of posture adopted within specific tree strata was observed, despite differences in altitude, vegetation, and ambient temperature. We found a strong positive correlation between lower temperatures and the time recorded in the semi-fetal posture in the high strata (Rc > 0.7547), and between higher temperatures and the time recorded in the extended posture in the low strata (Rc > 0.6803). These results suggest howler monkeys combine posture and vertical positioning to optimize thermoregulation. However, as temperatures rise, the preference for extended postures in lower strata-where temperatures may be cooler-leads to longer rest periods. This behavioral shift could reduce feeding and hydration, potentially increasing the risk of dehydration or heat stress under future climate conditions.