Free-ranging primates are confronted with the challenge of maintaining an optimal range of body temperatures within a thermally dynamic environment that changes daily, seasonally, and annually. While many laboratory studies have been conducted on primate thermoregulation, we know comparatively little about the thermal pressures primates face in their natural , evolutionarily relevant environment. Such knowledge is critical to understanding the evolution of thermal adaptations in primates and

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... comparative evaluation of humans' unique thermal adaptations. We examined temperature and thermal environment in free-ranging, mantled howling monkeys (Alouatta palliata) in a tropical dry forest in Guanacaste, Costa Rica. We recorded subcutaneous (T sc) and near-animal ambient temperatures (T a) from 11 animals over 1586.5 sample hours during wet and dry seasons. Howlers displayed considerable variation in T sc , which was largely attributable to circadian effects. Despite significant seasonal changes in the ambient thermal environment, howlers showed relatively little evidence for seasonal changes in T sc. Howlers experienced warm thermal conditions which led to body cooling relative to the environment, and plateaus in T sc at increasingly warm T a. They also frequently faced cool thermal conditions (T a < T sc) in which T sc was markedly elevated compared with T a. These data add to a growing body of evidence that non-human primates have more labile body temperatures than humans. Our data additionally support a hypothesis that, despite inhabiting a dry tropical environment, howling monkeys experience both warm and cool thermal pressures. This suggests that thermal challenges may be more prevalent for primates than previously thought, even for species living in nonextreme thermal environments. Am J Phys Anthropol 000:000-000,